Barian Micas and Exotic Ba-Cr and Ba-V Micas Associated with Metamorphosed Sedimentary Exhalative Baryte Deposits near Aberfeldy, Scotland, UK

Abstract

Regionally metamorphosed, Neoproterozoic stratiform baryte deposits near Aberfeldy in the Grampian Highlands of Scotland, UK, contain barium-poor and barium-rich micas in the host rocks and mineralized strata, respectively. The barium-rich micas include muscovite, biotite, phlogopite, and chromium-bearing muscovite. They occur in schistose metasediments and metabasites, in barium-feldspar rocks, and in small amounts in baryte rock. An extensive study of micas in a range of lithologies using electron-probe micro-analysis found up to 10.86 wt% BaO in muscovite, 5.46 wt% in biotite, and 15.70 wt% in Ba-Cr muscovite, the latter containing up to 9.27 wt% Cr₂O₃. Compositions are comparable with Ba- and Ba-Cr-micas in other metamorphosed Sedimentary Exhalative deposits and barium-rich metasediments worldwide. In one baryte rock sample, disseminated crystals of an exotic Ba-V-Cr mica contain up to 12.33 wt% BaO and 10.82 wt% V₂O₃, compositionally similar to Ba-V micas in the Hemlo lode gold deposit, Ontario. Ba²⁺ incorporation is mainly by coupled substitution with Al³⁺ for K⁺ + Si⁴⁺ in the tetrahedral site. The extent of phengitic (Tschermakitic) substitution is typical of micas in amphibolite-facies metasediments. Similar Fe:Mg ratios in coexisting muscovite and biotite reflect partitioning of iron into sulphides and metamorphic equilibration, with rare exceptions in fine-grained rocks that exhibit millimetre-scale disequilibrium.

1. Introduction

The mica group of phyllosilicate minerals is compositionally diverse due to the accommodating crystal structure and broad physicochemical stability. A general formula for the composition of micas is I M_2-3 □_0-1 T₄ O₁₀ X₂, where I represents interlayer cations (K, Na, Ca, Ba, Cs, NH₄), M represents octahedral layer cations (Al, Mg, Fe²⁺, Fe³⁺, Li, Ti, Mn²⁺, Mn³⁺, Zn, Cr, V, Na), □ is a vacancy in the octahedral layer, T represents tetrahedral cations (Si, Al, Fe³⁺, Be, B), and X represents ligands not bonded to T (OH, F, Cl, O, S) [1]. Rieder et al. (1999) [2] advocated the use of an atomic formula based on cations totalling 22 positive charges, as opposed to older publications in which formulae were based on (O, OH, F) totalling 12. The mica group is subdivided into true micas such as muscovite, brittle micas characterized by a 2:1 layer charge and >50% divalent interlayer (I) cations, and interlayer-deficient micas such as illite. These groups are subdivided into di-octahedral and tri-octahedral micas in which the ideal number of M-site cations is 2 and 3, respectively, although most micas are non-ideal and the di-octahedral/tri-octahedral boundary is placed at 2.5 cations per formula unit [2].

In the modern literature, significant variations from ideal end-member compositions are expressed using adjectival modifiers, such as barian or chromian muscovite, rather than the varietal names such as oellacherite and fuchsite that were used in the older literature. Here, the term ‘barian muscovite’ is used for muscovite containing >0.10 Ba cations per formula unit (based on 22 positive charges), equal to approximately 3.5 wt% BaO. The term ‘chromian’ is here applied to micas containing >0.25 Cr cations apfu, equating to approximately 4 wt% Cr₂O₃. ‘Cr-bearing’ is applied to micas with lower amounts of Cr, between the instrumental detection limit and 0.25 apfu. This nomenclature is also applied in the case of V-bearing and vanadian micas. The tri-octahedral micas are similarly divided into biotites and barian biotites, in which Ba cations account for >0.10 of the formula unit. Foster [3] noted a compositional hiatus between biotite and the more magnesian phlogopite at an Fe/(Fe + Mg) atomic ratio of 0.3, a division chosen here in preference to the Mg:Fe ratio of 2:1 employed by Rieder et al. [2].

Natural micas generally show distortions from the ideal crystal structure, which influence limits on compositional solid solutions [1]. Distortions tend to be larger in di-octahedral micas than in tri-octahedral micas. Barium has a similar ionic radius to potassium, but in contrast to K⁺, Ba²⁺ is divalent and its incorporation into micas influences the tetrahedral–octahedral distortion and accommodation of other cations [1,3]. A variety of cation substitution mechanisms have been proposed (summarised in [1] and discussed below). Ba-rich muscovite is uncommon, though it is known from several localities worldwide, often in association with barium feldspars in metamorphosed hydrothermally-altered clays and Sedimentary Exhalative (SEDEX) ore deposits [1,4]. Ba-rich biotite and phlogopite occur in a wider range of settings, the most studied being metasomatically altered mantle rocks, alkaline and gabbroic plutonic complexes (e.g., [5]), and metamorphosed pelites/carbonates and skarns [1,6,7]. Marble-hosted phlogopites are reported to contain up to 13 wt% TiO₂ and 24 wt% BaO [8]. Ba-rich biotite and phlogopite have very seldom been reported from SEDEX contexts: for example, Jiang et al. [9] report Ba-biotite in this context. Ba-Cr and Ba-V micas are also scarce worldwide. Reported exotic barian-chromian muscovite compositions contain up to 8 wt% BaO and 18 wt% Cr₂O₃ in the Isua and Malene localities, Greenland [10], as well as 10.3 wt% BaO, and 6.4 wt% Cr₂O₃, and 4.4 wt% BaO and 16.4 wt% Cr₂O₃ in the Hemlo gold deposit, Ontario [11]. Barian-vanadian muscovite containing comparable amounts of V₂O₃ is known from only a few localities worldwide (California [12] and Hemlo, Ontario [13]).

This contribution presents electron microprobe analyses to describe several types of mica occurring in metamorphosed SEDEX baryte–chert–sulphide deposits and host metasediments, located near the town of Aberfeldy in the Grampian Highlands of Scotland, UK.

2. Occurrence of Micas Associated with the Aberfeldy SEDEX Mineralization

The Aberfeldy baryte deposits, discovered in the late 1970s by the British Geological Survey (at that time named the Institute of Geological Sciences, IGS), are the largest resource of baryte in the UK [14,15,16]. Drilling-grade baryte rock was extracted for 40 years until 2021 from Foss Mine, both underground and in opencast workings. Since then, extraction has shifted 5 km along strike to a new underground mine, Duntanlich Mine (Figure 1a). The mines are located in an upland area, which is part of the Grampian Highlands, which largely comprise regionally metamorphosed, Neoproterozoic-age, clastic sedimentary rocks of the Dalradian Supergroup [17]. The mineralization is hosted by steeply inclined graphitic quartz mica schists, the Ben Eagach Schist Formation. This stratigraphically overlies a quartzite formation and underlies non-graphitic, calcareous quartz mica schists of the Ben Lawers Schist Formation (Figure 1a,b). In the Foss area, almost all mineralization is on the southern limb of a major fold structure, the Creag na-Iolaire Anticline [18]. Distal stratiform mineralization, comprising a bed <0.5 m thick of pyritic quartz Ba-feldspar chert, outcrops on the northern limb of the anticline in Foss West (Figure 1b).

The stratiform beds of baryte rock and of silica-, barium feldspar-, sulphide-, and carbonate-rich chemical sediments (Figure 2) were deposited in the Ediacaran c. 600 Ma ago [15,16,17,18,19,20,21,22,23,24]. Protoliths of the host rocks were organic-rich shales and siltstones with subordinate sandstones and gritstones. Volumetrically minor components of the host rock sequence are metabasites, of which the protoliths were basaltic tuffs and transgressive sills. In the Ordovician, the sequence was subjected to tectonic deformation and amphibolite-facies regional metamorphism in the Grampian Orogeny [18,20,25], which converted the black shales into graphitic quartz muscovite schists.

Feldspars and micas occurring in the vicinity of the deposits are of particular interest because of the volumetric abundance of barium-rich varieties. These minerals, together with the hydrous barium aluminosilicate, cymrite, were previously described by Coats et al. (1980, 1981) [14,15] and Fortey and Beddoe-Stephens (1982) [26]. Their work is reviewed, and a large number of further analyses are evaluated in the current contribution.

Table 1. Mineralogy of Foss deposit samples mentioned in the text.

Sample Name	Type	Rock Description	Mineralogy †
G100d	outcrop	Quartz celsian chert with sulphate pseudomorphs	CLS, QZ, ms, bt, py, rt, (sp)
G114a	outcrop	Quartz chert with pyrite laminae	QZ, PY, cls, hyal, ms, sp, pyh
429-5	core	Garnet hornblende mica schist (BLS)	QZ, PL, MS, GT, HB, DOL, bt, chl, cal
429-18	core	Baryte rock	BRT, py, cls, qz, bt, hyal, (sp, rt, cal)
BH9-28.6	core * (4330)	Quartz muscovite hyalophane schist	QZ, MS, HYAL, cls, py, sp, (rt, brt)
09-05 [BH9]	core	Muscovite dolomite schist (MB)	DOL, MS, QZ, PYH, (rt, ap)
N81-80	outcrop	Hyalophane chert	HYAL, QZ, bt, ms, py, rt, brt, sp, (ap)
424-8	core	Hyalophane biotite schist	HYAL, DOL, BT, QZ, PY, ms, pyh, rt, (sp)
BH3-27.8	core * (3963)	Banded baryte rock	BRT, QZ, PY, cal, mag, bt, dol
G171	outcrop	Baryte rock	BRT, qz, bt, cls, py, dol
702-11	core	Banded baryte–dolomite rock	BRT, DOL, py, qz, sp, bt, mag, cls, gn, (phy, ap)
207-5	core	Biotite calcite schist (MB)	BT, QZ, CAL, PL, chl, dol, py, rt, ms, ap, (ccp)
410-31	core	Sulphidic baryte rock	BRT, PY, QZ, sp, dol, ms, (gn, cal, ap, ccp)
703-9	core	Quartz chert and sulphide breccia	QZ, PY, CLS, SP, gn, ms, (ccp, ap)
503-23	core	Graphitic dolostone	DOL, sp, phy, qz, bt, ms, py, gn, (chl, rt, ccp, ap)
503-27	core	Graphitic garnet mica schist	QZ, MS, GT, pl, bt, chl, cal, ap, py, rt
705-22	core	Muscovite dolomite schist (metabasite)	DOL, MS, QZ, pyh, (rt, ap, ccp)
705-28	core	Calcareous sulphidic baryte rock	BRT, CAL, PY, qz, sp, dol, (gal, ms, ccp)
505-14	core	Calcareous sulphidic baryte rock	CAL, QZ, BRT, PY SP, dol, gal, (ms, ccp, ap)
N81-43	outcrop	Biotite calcite schist (MB)	BT, QZ, CAL, MS, (pyh, ap, ccp)
708-4b	core	Quartz muscovite hyalophane schist	QZ, MS, HYAL, py, sp, cls, chl, rt, gn
708-10	core	Sulphidic quartz chert	QZ, PY, sp, hyal, dol, cls, gn, ccp, bt, ms, ap, (rt)
708-28	core	Calcareous sulphidic baryte rock	BRT, CAL, PY, qz, sp, dol, (gn, ms, ccp)

Sample locations indicated in Figure 1b. Core * indicates IGS BH samples, for which in brackets are thin section numbers [15]; the remaining core samples are from Dresser Minerals drill-holes [21]. All samples are from the Ben Eagach Schist except for 429-5, which is from the Ben Lawers Schist (BLS). (MB) = metabasite. † Mineral species are listed in order of decreasing modal abundance: UPPERCASE indicates >10%, lowercase 1%–10%, (in brackets) <1%. Mineral abbreviations are those recommended by [27]: Ap apatite; Brt baryte; Bt biotite (including phlogopite); Cal calcite; Cls celsian (barian feldspar with Ba > K + Na, includes compositionally similar cymrite); Ccp chalcopyrite; Chl chlorite; Dol dolomite; Gt garnet; Gn galena; Hb hornblende; Hyal hyalophane (an informal name for barian feldspar with K + Na > Ba); Mag magnetite; Ms muscovite; Pl plagioclase; Py pyrite; Pyh pyrrhotite; Qz quartz; Rt rutile; Sp sphalerite.

Table 1. Mineralogy of Foss deposit samples mentioned in the text.

Sample Name	Type	Rock Description	Mineralogy †
G100d	outcrop	Quartz celsian chert with sulphate pseudomorphs	CLS, QZ, ms, bt, py, rt, (sp)
G114a	outcrop	Quartz chert with pyrite laminae	QZ, PY, cls, hyal, ms, sp, pyh
429-5	core	Garnet hornblende mica schist (BLS)	QZ, PL, MS, GT, HB, DOL, bt, chl, cal
429-18	core	Baryte rock	BRT, py, cls, qz, bt, hyal, (sp, rt, cal)
BH9-28.6	core * (4330)	Quartz muscovite hyalophane schist	QZ, MS, HYAL, cls, py, sp, (rt, brt)
09-05 [BH9]	core	Muscovite dolomite schist (MB)	DOL, MS, QZ, PYH, (rt, ap)
N81-80	outcrop	Hyalophane chert	HYAL, QZ, bt, ms, py, rt, brt, sp, (ap)
424-8	core	Hyalophane biotite schist	HYAL, DOL, BT, QZ, PY, ms, pyh, rt, (sp)
BH3-27.8	core * (3963)	Banded baryte rock	BRT, QZ, PY, cal, mag, bt, dol
G171	outcrop	Baryte rock	BRT, qz, bt, cls, py, dol
702-11	core	Banded baryte–dolomite rock	BRT, DOL, py, qz, sp, bt, mag, cls, gn, (phy, ap)
207-5	core	Biotite calcite schist (MB)	BT, QZ, CAL, PL, chl, dol, py, rt, ms, ap, (ccp)
410-31	core	Sulphidic baryte rock	BRT, PY, QZ, sp, dol, ms, (gn, cal, ap, ccp)
703-9	core	Quartz chert and sulphide breccia	QZ, PY, CLS, SP, gn, ms, (ccp, ap)
503-23	core	Graphitic dolostone	DOL, sp, phy, qz, bt, ms, py, gn, (chl, rt, ccp, ap)
503-27	core	Graphitic garnet mica schist	QZ, MS, GT, pl, bt, chl, cal, ap, py, rt
705-22	core	Muscovite dolomite schist (metabasite)	DOL, MS, QZ, pyh, (rt, ap, ccp)
705-28	core	Calcareous sulphidic baryte rock	BRT, CAL, PY, qz, sp, dol, (gal, ms, ccp)
505-14	core	Calcareous sulphidic baryte rock	CAL, QZ, BRT, PY SP, dol, gal, (ms, ccp, ap)
N81-43	outcrop	Biotite calcite schist (MB)	BT, QZ, CAL, MS, (pyh, ap, ccp)
708-4b	core	Quartz muscovite hyalophane schist	QZ, MS, HYAL, py, sp, cls, chl, rt, gn
708-10	core	Sulphidic quartz chert	QZ, PY, sp, hyal, dol, cls, gn, ccp, bt, ms, ap, (rt)
708-28	core	Calcareous sulphidic baryte rock	BRT, CAL, PY, qz, sp, dol, (gn, ms, ccp)

Sample locations indicated in Figure 1b. Core * indicates IGS BH samples, for which in brackets are thin section numbers [15]; the remaining core samples are from Dresser Minerals drill-holes [21]. All samples are from the Ben Eagach Schist except for 429-5, which is from the Ben Lawers Schist (BLS). (MB) = metabasite. † Mineral species are listed in order of decreasing modal abundance: UPPERCASE indicates >10%, lowercase 1%–10%, (in brackets) <1%. Mineral abbreviations are those recommended by [27]: Ap apatite; Brt baryte; Bt biotite (including phlogopite); Cal calcite; Cls celsian (barian feldspar with Ba > K + Na, includes compositionally similar cymrite); Ccp chalcopyrite; Chl chlorite; Dol dolomite; Gt garnet; Gn galena; Hb hornblende; Hyal hyalophane (an informal name for barian feldspar with K + Na > Ba); Mag magnetite; Ms muscovite; Pl plagioclase; Py pyrite; Pyh pyrrhotite; Qz quartz; Rt rutile; Sp sphalerite.

Muscovite is a major component of the pelitic metasediments enclosing the Aberfeldy SEDEX mineralization and also of some hydrothermally altered metabasites adjoining the stratiform mineralization (Figure 2e; Table 1). Minor amounts of muscovite occur within the stratiform mineralization, although here, iron-poor tri-octahedral micas are of similar abundance. Muscovite is observed to coexist with most other silicate phases (except talc) and also with graphite, sulphides, and carbonates. In foliated schists proximal to mineralized beds, barian muscovite is commonly intergrown with cymrite which has been replaced, or partially replaced, by celsian subsequent to penetrative deformation (Figure 3b).

Fortey and Beddoe-Stephens [26] determined cell parameters for a Ba-muscovite containing ~8 wt% BaO and established that the structural type is 2M₁, the same as normal (Ba-free) muscovite. Coats et al. [14,15] reported the occurrence of ‘fuchsite’ (chromian muscovite) in the Aberfeldy deposits, although this mineral was not characterised by EMPA or structural analyses. Further research, presented here, shows that within baryte rock, disseminated flakes or lamellae of strongly pleochroic, brown-green-coloured micas (Figure 3f,g) are Ba-Cr muscovite, and some are actually barian biotite or altered biotite.

Biotite is widespread in metasediments within the Ben Eagach Schist but is generally not a major constituent (<10%) except in some calcareous mica schists and in biotitic metabasites (e.g., samples N81-43 and 207-5: Table 1, Figure 2b–d and Figure 3d). Biotite is typically a major component of the Ben Lawers Schist formation stratigraphically overlying the Ben Eagach Schist [18] (Figure 1a,b). Biotite (sensu stricto) seldom coexists with more than trace amounts of iron sulphides and is largely absent from the sulphidic graphitic schists that host the stratiform mineralization. Pale-brown-coloured, Fe-poor phlogopite occurs as a minor constituent of graphitic dolostone, of mineralized metasediments, and of some sulphidic quartz celsian and baryte rocks (Figure 3e) in which the abundance of iron sulphide is similar to or greater than that of the micas. Chlorite is often inter-laminated with biotite on a sub-millimetric scale in schistose rocks, and chlorite of retrograde metamorphic origin has locally replaced other ferromagnesian aluminosilicates.

3. Materials and Methods

Following transmitted light optical examination of micas in polished thin sections, wavelength-dispersive spectrometry (WDS) electron microprobe analyses were obtained using a Cambridge Instruments Microscan Mk. 5 (Cambridge, UK), equipped with rubidium acid phthalate and quartz diffracting crystals, at the University of Edinburgh. Operating conditions were a 20 kV accelerating potential, a beam current of 30 ± 0.05 nA, and a focused electron beam with a diameter of 1–2 μm. Counting times were routinely 40 s on peaks of both standards and samples and 20 s on backgrounds. Backgrounds were measured on standards and on the initial analysis of each phase in a sample; background values were re-determined when substantially different compositions of the same phase were analysed. Data reduction was performed by an on-line computer using the ZAF correction procedures of [28] and the absorption coefficients of [29]. Detection limits and analytical precision for typical analyses are shown in

Table 2. Lower limits of detection (det) and precision (2σ) of EMPA analyses of representative mica crystals.

Barian Muscovite (Sample 504-5)				Biotite (Sample 429-5)
	wt%	2σ	det		wt%	2σ	det
SiO2	42.76	0.27	0.06	SiO2	37.29	0.25	0.03
TiO2	0.59	0.06	0.04	TiO2	1.76	0.08	0.04
Al2O3	31.87	0.17	0.02	Al2O3	17.57	0.14	0.02
FeO	0.93	0.06	0.05	FeO	17.71	0.21	0.05
MnO	0.01	0.03	0.04	MnO	0.05	0.04	0.04
MgO	2.33	0.06	0.02	MgO	11.52	0.11	0.02
Na2O	0.34	0.02	0.02	Na2O	0.20	0.01	0.02
K2O	7.70	0.14	0.03	K2O	8.33	0.14	0.03
BaO	8.71	0.22	0.07
F	0.20	0.10	0.07
Total	95.44			Total	94.43

, calculated from the counting statistics by the method of [30]. The precision figures (expressed at 95% confidence level) obtained by this method are similar to those estimated by repeat analyses of points or small (10 μm²) areas, except where volatilization of light elements (F, Na and K) results in a significantly greater scatter in duplicate analyses.

Note that the EMPA oxide totals in Table 2 (and subsequent tables including the Supplementary Material) do not include H₂O. Totals considerably less than 100% do not necessarily indicate low-quality analyses; the analyses presented are merely missing components, principally H₂O (representing hydroxyl ions).

A partial overlap of TiKα and BaLα radiation can affect EMPA-WDS analyses of materials containing both elements. TiO₂ concentrations in micas analysed in this study are not sufficiently high to produce a significant (~0.1 wt%) BaO overestimate, but the TiO₂ content may be overestimated in minerals containing >10 wt% BaO. This was satisfactorily resolved by using collimating slits on the quartz spectrometer during the analysis of Ba-Cr micas and other particularly Ba-rich muscovites. Analysis of the Ba-V-Cr mica in sample 410-31 was more difficult because of overlaps between TiKα and VKα radiation, and between VKβ and CrKα radiation. A satisfactory resolution was achieved by determining Ti and V on the LiF crystal using collimating slits and by measuring CrKβ radiation on the quartz crystal together with Ba and other elements, again using collimating slits.

4. Results

4.1. Muscovite and Barian Muscovite

Approximately 170 EMPA analyses of muscovite and barian muscovite (in roughly equal proportions) were obtained from about sixty rock samples representing a wide range of parageneses. A representative selection of these analyses is given in

Table 3. EMPA analyses of representative dioctahedral micas in Aberfeldy stratiform mineralization and host rocks. Analyses are ranked from left to right in order of increasing wt% BaO.

Sample	429-5	503-27	09-05	705-22	N81-43	G100d	N81-80	705-22	G100d	G114a
Composition expressed as weight % oxides
SiO2	46.86	46.40	47.40	46.54	45.15	48.09	44.50	43.35	44.76	38.38
Al2O3	32.23	33.36	32.52	32.09	30.54	28.22	30.75	29.63	31.06	33.41
TiO2	0.39	0.44	0.53	0.62	0.82	1.56	1.50	1.40	1.44	0.69
V2O3
Cr2O3			0.09
MgO	1.78	1.42	2.35	3.07	2.35	3.97	2.16	3.11	3.22	1.64
MnO	0.02	0.02	0.02			0.12	0.02	0.03	0.10	0.02
FeO *	1.68	1.62	1.17	0.85	2.70	0.69	2.14	1.38	0.21	0.25
CaO							0.01
Na2O	0.77	0.63	0.64	0.72	0.54	0.15	0.30	0.25	0.30	0.69
K2O	9.54	9.83	9.21	9.22	8.73	8.89	8.64	7.87	7.75	6.16
BaO		0.23	1.13	3.19	4.47	5.70	6.46	7.97	9.10	10.86
Total	93.27	93.95	95.06	96.30	95.30	97.39	96.48	94.99	97.94	92.10
F		0.14	0.16	0.25		0.32	0.18	0.23	0.40	0.12
Atoms per formula unit on the basis of 22 positive charges
Si	3.180	3.135	3.165	3.125	3.120	3.245	3.080	3.070	3.070	2.845
Aliv †	0.820	0.865	0.835	0.875	0.880	0.755	0.920	0.930	0.930	1.155
Alvi	1.755	1.790	1.725	1.665	1.610	1.490	1.585	1.540	1.580	1.765
Al total	2.575	2.655	2.560	2.540	2.490	2.245	2.505	2.470	2.510	2.920
Ti	0.020	0.020	0.025	0.030	0.045	0.080	0.080	0.075	0.075	0.040
V
Cr
Mg	0.180	0.145	0.235	0.305	0.240	0.400	0.225	0.330	0.330	0.180
Mn
Fe *	0.095	0.090	0.065	0.050	0.155	0.040	0.125	0.080	0.010	0.015
Na	0.100	0.080	0.085	0.095	0.070	0.020	0.040	0.035	0.040	0.100
K	0.825	0.845	0.785	0.790	0.770	0.765	0.765	0.710	0.680	0.585
Ba		0.005	0.030	0.085	0.120	0.150	0.175	0.220	0.245	0.315
Total	6.975	6.975	6.950	7.020	7.010	6.945	6.995	6.990	6.960	7.000
∑I interlayer site	0.927	0.936	0.898	0.967	0.963	0.935	0.978	0.966	0.962	0.997
∑M octahedral site	3.806	3.836	3.785	3.715	3.660	3.502	3.597	3.567	3.580	3.766
Fe/(Fe + Mg)	0.346	0.390	0.218	0.134	0.392	0.089	0.357	0.199	0.035	0.079

* Total Fe expressed as Fe²⁺. † Al^iv cations calculated as 4-Si cations.

. The range in barium contents of analysed muscovites is illustrated in Figure 4b. After calculating molecular formulae and eliminating analyses showing evidence of contamination, inter-element variations in the dataset are evaluated by means of divariate and trivariate plots (Figure 4, Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9). The range in muscovite compositions and that between phase relationships are described before discussing (in Section 5) lattice substitutions that can account for these inter-element variations.

Due to the foliated fabric and flake-like shape of muscovite crystals in most samples (e.g., Figure 3a), it was usually not possible to analyse cores and rims separately. In samples where this was possible, usually little difference was found between core and rim compositions. Within individual thin sections, muscovite grains are usually similar in composition, although large variations in Ba, Na, Ti, Al, Fe, and Mg contents are evident in the dataset as a whole. Within section (<1 cm) heterogeneity was observed in three situations: (1) between rock-matrix and encapsulated (armoured) inclusions of muscovite in quartz hyalophane schists with zoned hyalophane pseudomorphs of sulphate crystals (samples BH9-28.6 and N81-80: Table 1 and Figure 1b, Figure 2g and Figure 4a); (2) in muscovite associated with thin (1–2 mm), cross-cutting calcite–pyrite veinlets of retrograde metamorphic origin, for example in a biotitic metabasite, sample 207-5 (Table 1); and (3) in two samples of micaceous quartz celsian rock, G100d and 705-22 (Table 1 and Figure 1b), in which individual muscovite grains show zonation in Ba content whereby a central zone elongated parallel to the muscovite cleavage is relatively poor in Ba and is sandwiched by Ba-rich muscovite with no break in optical continuity. Comparable zoning in barian muscovite and phengite crystals was described by Grapes [31], who attributed the zoning to baryte dissolution in host metacherts during amphibolite-facies metamorphism, and by Chopin and Maluski [32] and Raith et al. [33], who attributed the zoning to localized retrogressive metasomatism.

The Na₂O content of Aberfeldy muscovite ranges from 0.1 wt% to 1.2 wt% (0.02–0.30 Na cations per formula unit), but calcium was found to be generally at or below the detection limit (<0.02 wt% CaO). On the Ba-K-Na ternary diagram (Figure 4a), the spread of barium-poor muscovite compositions parallel to the K-Na join indicates a solid solution with between ~7 and ~16 mol% of the paragonite end-member, Na₂A1₄(Si,Al)₈O₂₀(OH)₄. Although paragonite has not been identified as a separate species in samples from Foss, this extent of muscovite–paragonite solid solution is consistent with Na saturation at metamorphic temperatures of 400–560 °C (cf. [33,34,35]), which are consistent with peak metamorphic temperatures attained by the rocks in the study area [20]. Only two samples examined contain barian muscovite with a comparable paragonite content (N81-43 and G114a: Table 1 and Table 3, see also Figure 4a). The remaining barian muscovites (plus those analysed by Fortey and Beddoe-Stephens [26]) contain 1–7 mol% paragonite, but within this range, the sodium content generally increases with increasing barium content (Figure 4a). K/Na ratios in muscovite are generally higher than in the tri-octahedral micas and lower than in barium feldspar coexisting with muscovite (Figure 4 inset).

A notable feature of the majority of muscovite analyses is a deficiency in I-site occupancy. This is illustrated by a plot of Na + K cations against Ba cations (Figure 5a) in which many muscovites, including all those poor in barium, fall significantly below the line representing (Na + K + Ba) = 2. The presence of additional trace elements such as Rb, estimated from XRF data ([21] unpublished data) to constitute about 300 ppm of Ba-poor muscovite, is unlikely to account for this deficiency in interlayer site cations, which may be due to site vacancies or to the presence of (H₃O)⁺ ions associated with retrograde illitic alteration, as suggested by other researchers [2,36].

4.2. Chromium- and Vanadium-Bearing Barian Micas

Twenty-two analyses were obtained of chromium-rich, barian muscovite crystals occurring in eight samples from Foss examined in this study (

Table 4. EMPA analyses of selected Ba-Cr and Ba-V-Cr micas in Aberfeldy stratiform mineralization.

Sample	708-10	G114a	G100e a	G100e b	505-14	705-28	410-31 a	410-31 b
Composition expressed as weight % oxides
SiO2	46.39	40.53	42.42	33.94	37.29	36.66	36.40	34.77
Al2O3	27.95	26.30	27.21	18.42	16.41	16.59	13.67	13.77
TiO2	1.10	1.28	1.45	1.32	0.75	0.70	1.49	1.80
V2O3							10.82	10.07
Cr2O3	0.98	4.19	0.45	4.60	9.27	5.15	4.25	3.48
MgO	3.82	2.62	3.31	2.14	1.44	1.55	3.30	2.13
MnO	0.02	0.07	0.04	0.08	0.16	0.04	0.04	0.08
FeO *	0.51	0.38	0.35	0.25	3.09	2.30	7.52	10.25
ZnO							0.06	0.03
Na2O	0.11	0.25	0.24	0.15	0.09	0.10	0.09	0.05
K2O	8.73	5.82	7.86	4.78	6.42	6.10	6.64	5.83
BaO	6.27	11.48	8.34	15.70	11.02	11.13	9.86	12.33
Total	95.88	92.92	91.67	81.38	85.94	80.32	94.14	94.59
F	0.22	0.05	0.23		0.07		0.10
Atoms per formula unit on the basis of 22 positive charges
Si	3.205	3.030	3.125	3.095	3.180	3.295	2.930	2.870
Aliv †	0.795	0.970	0.875	0.905	0.820	0.705	1.070	1.130
Alvi	1.480	1.345	1.485	1.075	0.830	1.050	0.225	0.210
Al total	2.275	2.315	2.360	1.980	1.650	1.755	1.295	1.340
Ti	0.055	0.070	0.080	0.090	0.050	0.045	0.090	0.110
V							0.700	0.665
Cr	0.055	0.250	0.025	0.330	0.625	0.365	0.270	0.225
Mg	0.395	0.290	0.365	0.290	0.185	0.210	0.395	0.260
Mn					0.010			0.005
Fe *	0.030	0.025	0.020	0.020	0.220	0.175	0.505	0.710
Na	0.015	0.035	0.035	0.025	0.015	0.015	0.015	0.010
K	0.770	0.555	0.740	0.555	0.700	0.700	0.680	0.615
Ba	0.170	0.335	0.240	0.560	0.370	0.390	0.310	0.400
Total	6.970	6.905	6.990	6.945	7.005	6.950	7.190	7.210
∑I interlayer site	0.954	0.932	1.013	1.143	1.081	1.108	1.006	1.021
∑M octahedral site	3.494	3.331	3.462	2.885	2.744	2.898	1.715	1.734
Fe/(Fe + Mg)	0.070	0.075	0.056	0.062	0.546	0.454	0.561	0.730

* Total Fe expressed as Fe²⁺. † Al^iv cations calculated as 4-Si cations. ^a,b Analyses of different mica crystals within the same thin section of these samples.

). These samples are mostly of sulphidic quartzose cherts but include two sulphidic baryte rocks (410-31 and 705-28) (Figure 1b and Table 1). Although occasionally present in sufficient quantity to impart a distinctive green colour to the rocks (Figure 2f), Cr-rich muscovite is invariably a minor or trace constituent, and its presence does not indicate high bulk Cr contents. For example, a bulk rock analysis of sample G100e, which contains Cr-bearing muscovite (Table 1), shows that it contains only 75 ppm Cr ([21] unpublished XRF data). It is inferred that chromium is concentrated into the small amount of mica present because other suitable receptor phases, such as chromite, are scarce or absent. However, Cr may also be accommodated in rutile that is present in amounts <1% in some rocks (Table 1). Rutile in the Ba-Cr-muscovite-bearing sample G100e (Figure 1b and Table 1) contains up to 0.6 wt% Cr₂O₃ ([21] unpublished EMPA data).

Within individual thin sections, crystals of Cr-rich mica vary in their depth of colour from very pale to vivid emerald green (Figure 3f,g), corresponding to variations in Cr content. A striking feature is their high barium content, up to 15.7 wt% BaO, which generally increases with increasing Cr content, reaching a maximum of 9.3 wt% Cr₂O₃ (Figure 4b and Table 4). However, ten of the analyses are suspect because they have oxide totals of <90% and have markedly lower Al contents and somewhat lower Si, Na, and F contents than the remaining Ba-Cr muscovite analyses. Ba-Cr micas in the baryte rock samples 505-14 and 705-28 have the highest measured Cr contents and are rich in Fe compared with most other Ba-Cr micas which have lower Fe/(Fe + Mg) ratios. This might suggest an affinity with the tri-octahedral micas, but Figure 5d shows that the Ba-Cr micas lie within the trend of phengitic substitution (Mg + Fe for Al) in Aberfeldy barian muscovite.

A vanadium-rich, barian green mica was found in one sample of sulphidic baryte rock, 410-31. This mineral occurs as irregular grains and flakes enclosed by baryte and pyrite crystals and varies in body colour from a murky olive-brown to translucent pale green, the latter variety being strongly pleochroic from almost colourless to viridian green and mid-brown. Three EMPA analyses of this mica (two are included in Table 4) show that it contains 3.4%–4.3% Cr₂O₃ and 9.3%–10.8% V₂O₃ (assuming that Cr and V are entirely trivalent), in addition to considerable amounts of FeO and MgO (total, 11%–14%) and 9.9%–12.3% BaO. Despite these features, and the similarity in SiO₂ and Al₂O₃ content to barian biotites, the M-site occupancy (~1.7 apfu: Table 4) is <2.5 apfu, indicating that this mica is a member of the di-octahedral group [1,4]. However, it is possible that V³⁺ and Cr³⁺ have largely substituted for Al³⁺ in the octahedral (M) site of the muscovite lattice, coupled with an increase in tetrahedral Al and decrease in Si (Figure 5d). This is discussed further in Section 5. One of the analysed crystals contains micron-scale, thin laminae of opaque mineral inferred to be exsolution laminae: quantitative analysis proved difficult but qualitative spectra showed that the opaque mineral comprises a V-Cr-Fe oxide.

4.3. Biotite, Phlogopite, and Barian Biotite

Ninety analyses were obtained of tri-octahedral micas in over forty samples. This includes twenty analyses of barian biotite (defined as ≥0.10 Ba cations per formula unit) and an equal proportion of biotite and phlogopite analyses (

Table 5. EMPA analyses of representative trioctahedral micas in Aberfeldy stratiform mineralization and host rocks. Analyses are ranked from left to right in order of increasing wt% BaO.

Sample	429-5	503-27	503-23	N81-43	G100d	708-10	207-5	BH3-27.8	424-8	702-11
Composition expressed as weight % oxides
SiO2	37.15	34.68	38.03	37.01	42.01	40.44	36.47	37.56	34.87	35.99
Al2O3	17.49	18.25	16.46	16.77	14.56	14.16	16.46	12.56	14.56	12.89
TiO2	1.84	1.46	1.10	2.64	0.54	0.82	2.36	0.15	3.36	1.75
Cr2O3						0.34
MgO	11.95	9.22	16.67	11.81	24.75	24.61	14.66	16.81	13.24	14.37
MnO	0.03	0.26	0.11	0.09	0.29		0.07	0.12	0.05	0.02
FeO *	17.28	21.34	11.11	17.56	1.29	1.54	13.39	15.11	15.56	16.05
CaO
Na2O	0.18	0.13	0.10	0.17	0.19	0.08	0.17	0.12	0.16	0.14
K2O	8.21	7.67	8.78	8.75	8.91	8.80	8.40	7.51	7.70	6.99
BaO		0.06	0.07	1.33	2.81	2.99	3.08	4.55	4.54	5.46
Total	94.13	93.07	92.43	96.13	95.35	93.78	95.06	94.49	94.04	93.66
F		0.24	0.68	0.09	2.71	3.08	0.63	1.62	0.90	0.97
Atoms per formula unit on the basis of 22 positive charges
Si	2.805	2.710	2.850	2.785	2.965	2.915	2.735	2.910	2.735	2.850
Aliv †	1.195	1.290	1.150	1.215	1.035	1.085	1.265	1.090	1.265	1.150
Alvi	0.360	0.390	0.305	0.270	0.175	0.120	0.190	0.055	0.080	0.050
Al total	1.555	1.680	1.455	1.485	1.210	1.205	1.455	1.145	1.345	1.200
Ti	0.105	0.085	0.060	0.150	0.030	0.045	0.135	0.010	0.200	0.105
Cr						0.020
Mg	1.345	1.075	1.860	1.325	2.605	2.645	1.640	1.940	1.550	1.695
Mn			0.005		0.015			0.010
Fe *	1.090	1.395	0.695	1.105	0.075	0.095	0.840	0.980	1.020	1.060
Na	0.025	0.020	0.015	0.025	0.025	0.010	0.025	0.020	0.025	0.020
K	0.790	0.765	0.840	0.840	0.805	0.810	0.805	0.740	0.770	0.705
Ba				0.040	0.080	0.085	0.090	0.140	0.140	0.170
Total	7.715	7.730	7.780	7.755	7.810	7.830	7.725	7.895	7.785	7.805
∑I interlayer site	0.817	0.787	0.856	0.904	0.907	0.906	0.918	0.898	0.935	0.897
∑M octahedral site	3.264	3.357	3.237	3.123	3.082	3.043	2.992	3.046	2.935	2.964
Fe/(Fe + Mg)	0.448	0.565	0.272	0.455	0.028	0.034	0.339	0.335	0.397	0.385

* Total Fe expressed as Fe²⁺. † Al^iv cations calculated as 4-Si cations.

). Tri-octahedral micas were analysed in twenty-five samples from which analyses of coexisting muscovite (or barian muscovite) were also obtained. In a number of cases, phlogopite with a low iron content (0.5–1.5 wt% FeO) was misidentified as muscovite in optical studies before EMPA analyses were obtained. In all analyses, the total iron content is expressed as wt% FeO, although some trivalent iron is probably present in biotite, and particularly in the green-pleochroic barian biotites associated with baryte.

In all but three of the sixteen samples in which phlogopites were analysed, this mineral was found to be enriched in barium (1.7–3.4 wt% BaO). Barian biotites form a compositionally distinct group in which Fe/(Fe + Mg) ratios generally range from 0.3 to 0.5 and BaO contents range from 3.1 to 5.5 wt% (Table 5 and Figure 6a). To this extent, the iron and barium contents of barium-enriched tri-octahedral micas are inter-related. Phlogopites also contain less aluminium (in both octahedral and tetrahedral sites) than Ba-poor biotites, but barian biotites have a broad range in Al content (Figure 6d and Figure 7a). The significance of these relationships to substitution mechanisms in these micas are discussed below (Section 5.2).

Unlike the di-octahedral micas, the tri-octahedral micas do not show any association between barium and titanium contents, but TiO₂ contents (up to 3.4 wt%) generally increase with increasing Fe/(Fe + Mg) ratio (Figure 6a) and Al^iv content (Figure 6d). Relatively high Ti contents are characteristic of biotites occurring in metabasic rocks such as N81-43 and 424-8, whereas barian biotites found in baryte rock are relatively poor in titanium (samples BH3-27.8 and G171; Figure 6a). This indicates that the partitioning of Ti into tri-octahedral micas is partly controlled by bulk rock composition: baryte rock contains negligible Ti whereas metabasites contain several percent TiO₂ [15,21]. This also applies to manganese and chromium: biotite MnO contents are generally <0.2 wt%, but 0.5–0.7 wt% MnO occurs in phlogopite in sample 429-18, which also contains Mn-calcite, and 0.1–0.3 wt% Cr₂O₃ was detected in phlogopite coexisting with Ba-Cr muscovite in sample 708-10 (Table 1, Table 4 and Table 5).

Figure 6. (a) Ti cations vs. molecular ratio of Fe/(Fe + Mg) in Aberfeldy micas. Comparatively Ti-poor barian biotite in samples BH3-27.8 and G171 are within baryte rock (Table 1). (b) Weight percent fluorine vs. molecular ratio of Fe/(Fe + Mg) in Aberfeldy micas. (c) Ti cations vs. octahedral cation site deficiency in tri-octahedral micas analysed, showing a weak positive association. M-site deficiency calculated as 2–(Fe + Mg + Mn + Ti + Cr) cations. (d) Ti cations vs. tetrahedral Al cations in analysed tri-octahedral micas, showing weak positive association. (e) Octahedral cation plot after Gessmann et al. [37] ([1] Figure 178). (f) Aberfeldy tri-octahedral micas compared with mantle biotite compositions on a Gao and Green [38] coupled substitution diagram ([1] Figure 179). In (e,f), Aberfeldy mica compositions are compared with published analyses of tri-octahedral micas from worldwide localities [1,9,39].

Figure 6. (a) Ti cations vs. molecular ratio of Fe/(Fe + Mg) in Aberfeldy micas. Comparatively Ti-poor barian biotite in samples BH3-27.8 and G171 are within baryte rock (Table 1). (b) Weight percent fluorine vs. molecular ratio of Fe/(Fe + Mg) in Aberfeldy micas. (c) Ti cations vs. octahedral cation site deficiency in tri-octahedral micas analysed, showing a weak positive association. M-site deficiency calculated as 2–(Fe + Mg + Mn + Ti + Cr) cations. (d) Ti cations vs. tetrahedral Al cations in analysed tri-octahedral micas, showing weak positive association. (e) Octahedral cation plot after Gessmann et al. [37] ([1] Figure 178). (f) Aberfeldy tri-octahedral micas compared with mantle biotite compositions on a Gao and Green [38] coupled substitution diagram ([1] Figure 179). In (e,f), Aberfeldy mica compositions are compared with published analyses of tri-octahedral micas from worldwide localities [1,9,39].

The tri-octahedral micas from Aberfeldy have generally low sodium contents compared with muscovite, which displays a wider range in Na (Figure 4a). The tri-octahedral micas have a slightly greater deficiency in the interlayer (I) site occupancy than the di-octahedral micas (Figure 5a), but the tri-octahedral micas generally have relatively higher fluorine contents (Figure 6b). Fluorine substitutes for (OH)⁻ in the interlayer site, and as commonly observed by previous authors, F contents of the tri-octahedral micas are inversely proportional to the Fe/(Fe + Mg) ratio (up to ~3.6 wt% F in phlogopite in sample 708-5B; Table 5). An explanation for this phenomenon, termed ‘Fe-F’ avoidance (cf. Munoz [40]), has been provided by nuclear magnetic resonance studies (e.g., [41]), which indicate that (OH)⁻ groups are in direct coordination with Fe²⁺ in phlogopites, whereas fluorine ions probably form homogeneous domains and are coordinated with Mg²⁺ ions.

4.4. Altered (Interlayer-Deficient) Biotite

In several samples examined with the electron microprobe, dark micas yielded unusually low oxide totals (84–94 wt%), although these crystals are optically indistinguishable from normal biotite. The low oxide totals are principally due to lower-than-expected abundances of total alkalis (expressed as wt% alkaline oxides). This may be due to depletion associated with retrograde metamorphic alteration, since a partial to complete replacement of biotite by chlorite is commonly observed in samples from the study area. All stages are observed from the onset of alteration marked by interlayer site cation totals falling below 1.8 to the complete loss of K, Na, and Ba from the mica structure. The loss of alkaline elements (probably replaced by [H₃O]⁺ ions) is generally accompanied by increases in iron content and Fe/(Fe + Mg) ratio, coupled with decreases in Al and Ti content. Dempster [42] observed similar chemical changes associated with the formation of hydrobiotites in paragonite-bearing schists of the Appin Group in the central Grampian Highlands. This alteration is distinct from that which formed secondary chlorite and probably occurred late in metamorphism as retrograde reactions associated with fluid infiltration.

The dark green coloured mica flakes, described as ‘fuchsite’ by Coats et al. [15], which occur in several samples of baryte rock from IGS BH1 and BH2 (Figure 1b), appear to be altered barian biotites. A similar pattern of alteration (loss of K, Na, Al, and Si) in Ba-Cr micas was noted in the current study.

5. Discussion

5.1. Di-Octahedral Mica Compositions and Substitution Mechanisms

Fortey and Beddoe-Stephens [26] reported up to 8.3 wt% BaO in barian muscovite, of which eleven analyses from Aberfeldy deposit samples were presented. However, in muscovite coexisting with barium feldspar, the BaO content was consistently found to be about 6 wt%. The authors suggested that this could represent an effective saturation level in muscovite in equilibrium with barium feldspar under the metamorphic conditions attained, established by subsequent research as 530 ± 30 °C and 9 ± 1 kbar [20]. In the muscovites analysed in the current study, BaO contents range from nearly zero to 10.6 wt% but show a bimodal distribution with peaks corresponding to 0–0.05 and 0.15–0.22 Ba cations per formula unit (Figure 4b). The hypothesis suggested by Fortey and Beddoe-Stephens [26] is clearly not substantiated, since in at least twelve samples examined, barian muscovite containing >0.20 Ba cations (>7 wt% BaO) coexists with celsian or hyalophane. In addition, Ba-Cr muscovite containing 0.34 Ba cations (11.5 wt% BaO) coexists with celsian in two samples studied (G114a and 703-9, Table 5 and Figure 1b).

Inspection of the molecular formula (Table 3 and Figure 5 and Figure 6) indicates that the substitution of divalent Ba ions for K + Na ions in the interlayer cation site is charge-balanced, as initially proposed by Wendlandt [43], by a coupled substitution of Al³⁺ for Si⁴⁺ in the tetrahedral site:

Ba²⁺ Al³⁺_iv = K⁺ + Si⁴⁺

(1)

As shown by Mitchell [44], the coupled substitution Ba + Al^iv = K + Si (phlogopite-kinoshitalite) produces a 1:1 slope, whereas the substitution mechanism Ba + vacancy = 2K would produce a 2:1 slope. Figure 5b shows that Aberfeldy di-octahedral micas show a 1:1 slope, indicating that the incorporation of barium is predominantly by means of Wendlandt’s [43] mechanism (reaction 1).

Fortey and Beddoe-Stephens [26] consider that this substitution is independent of the common phengitic (‘Tschermakitic’) substitution:

(Mg,Fe)²⁺ Si⁴⁺ = Al³⁺_iv Al ³⁺_vi

(2)

In Figure 5c, the dashed line represents substitution (1), i.e., Ba + Al^iv = K + Si^iv, starting from stoichiometric muscovite in which Ba = 0.0 and K + Si^iv = 1.0. The Aberfeldy muscovite compositional field roughly parallels this line intersecting the y-axis at Al^iv units of between 0.9 and 0.6, representing the same substitution offset by a phengitic substitution of between 0.1 and 0.4 formula units. Barian muscovites have higher octahedral site contents (Mg, Fe, and other cations in the M site excluding Al) than Ba-poor muscovites that have similar total Si contents (Figure 5d) and total Al contents (Figure 7a1).

The operation of both substitution mechanisms in muscovite from the Aberfeldy deposits accounts for the absence of simple relationships between Ba, Si, Al^iv, and (Mg,Fe) contents. The range of phengitic substitution is comparable to muscovites from other areas, which attained similar metamorphic grades [1]. Lower total Al and higher (Mg,Fe) and Si contents indicate a greater degree of phengitic substitution in some barian muscovites (in samples BH7-81.6, G100a, and G100d), whereas a considerably less phengitic substitution is apparent in another group of aluminous barian muscovites (samples BH9-28.6 and G114a: Table 1) in which substitution scheme (1) predominates. The spread of points on either side of the ideal substitution lines (Figure 5b and Figure 7a) may be related to other substitutions and to the incomplete interlayer site occupancy which, as noted above, is characteristic of barium-poor muscovites from Aberfeldy.

The relative proportions of Mg and Fe also vary considerably, and lower Fe/(Fe + Mg) ratios are characteristic of most barian muscovites (Figure 6a and Figure 7b). The variation in this ratio in muscovite is analogous to that in biotite, and the explanation for this and also Mg-Fe partitioning between coexisting micas is discussed below. Barian muscovites also generally have higher TiO₂ contents (up to 1.8 wt%) than barium-poor muscovites (Figure 6a).

Figure 7. (a1) Total Al cations vs. total of M-site cations excluding Al in analysed micas, excluding those with illitic alteration. Dashed lines indicate ideal Al^iv substitution trends in di-octahedral micas (lower) and tri-octahedral micas (upper). Three analyses of Ba-V-Cr mica in sample 410-31 plot between the trend lines. (a2) Comparison of Aberfeldy mica compositional fields with published analyses of barian mica from worldwide localities [1,9,10,11,13,33,39,45,46,47,48,49,50]. (b1) Total Al cations vs. molecular ratio of Fe/(Fe + Mg) in analysed micas, excluding those depleted in alkali elements. Legend for symbols as in a1. Tie lines connect co-existing biotite and muscovite compositions in samples in which both were analysed (Table 1, Figure 1b). Sample N81-80 shows an unusually wide range of Fe/(Fe + Mg) ratios in muscovite. The inset ternary diagram shows compositions of Ba-V-Cr mica in baryte rock sample 410-31 compared with Hemlo mica compositions (Fleet [1] Figure 35); tie lines connect di-octahedral and tri-octahedral micas in the same samples. (b2) Comparison of Aberfeldy mica compositional fields with published analyses of barian micas from worldwide localities (legend as in (a2)).

Figure 7. (a1) Total Al cations vs. total of M-site cations excluding Al in analysed micas, excluding those with illitic alteration. Dashed lines indicate ideal Al^iv substitution trends in di-octahedral micas (lower) and tri-octahedral micas (upper). Three analyses of Ba-V-Cr mica in sample 410-31 plot between the trend lines. (a2) Comparison of Aberfeldy mica compositional fields with published analyses of barian mica from worldwide localities [1,9,10,11,13,33,39,45,46,47,48,49,50]. (b1) Total Al cations vs. molecular ratio of Fe/(Fe + Mg) in analysed micas, excluding those depleted in alkali elements. Legend for symbols as in a1. Tie lines connect co-existing biotite and muscovite compositions in samples in which both were analysed (Table 1, Figure 1b). Sample N81-80 shows an unusually wide range of Fe/(Fe + Mg) ratios in muscovite. The inset ternary diagram shows compositions of Ba-V-Cr mica in baryte rock sample 410-31 compared with Hemlo mica compositions (Fleet [1] Figure 35); tie lines connect di-octahedral and tri-octahedral micas in the same samples. (b2) Comparison of Aberfeldy mica compositional fields with published analyses of barian micas from worldwide localities (legend as in (a2)).

5.2. Tri-Octahedral Mica Compositions and Substitution Mechanisms

The incorporation of barium into tri-octahedral micas in the Aberfeldy rocks appears to be by a mechanism similar to that described above (reactions 1 and 2) for muscovite. Barian biotites and phlogopites generally contain less Al (in both M and T sites) and more Fe + Mg than Ba-poor biotite (Figure 7a,b) and, therefore, differ from most phlogopites described from igneous rocks, in which higher Ba contents are correlated with increases in Al content. Nepheline-hosted phlogopites containing up to 20 wt% BaO and 14 wt% TiO₂ were described by Mansker et al. [51], and Ba-rich, Al-poor phlogopite is quite common in nephelinites and other undersaturated basic eruptive rocks and carbonatites [1,52]. Majka et al. [6] report biotite with up to 13.4 wt% BaO and 8.4 wt% TiO₂ in calcareous, ultra-high-pressure metasediments. In Aberfeldy biotites, TiO₂ contents increase with increasing Fe/(Fe + Mg) ratio and Al^iv content (Figure 6a,d), and similar relationships are recorded in previous studies of Ti- and Ba-bearing tri-octahedral micas (e.g., [33,53,54]).

Variation in the Al content of Ba-poor tri-octahedral micas from Aberfeldy may be largely accounted for by a limited solid solution between the phlogopite and siderophyllite end-members (Figure 7b) by means of the substitution reaction:

6Mg^vi Si^iv = 5Fe^vi Al^vi Al^iv

(3)

An excess of Al in most phlogopites and barium-poor biotites, which plot to the right of the phlogopite-siderophyllite line in Figure 7b, could be explained by a greater component of the eastonitic (Tschermakitic) substitution reaction (2). Barium incorporation by substitution reaction (1) in effect counterbalances the siderophyllite substitution in barian biotites, but not to the extent that the sum of Al + Si cations falls below four per formula unit.

Titanium solubility and substitution mechanisms in phlogopites have been investigated experimentally (e.g., [53]) and in natural occurrences (e.g., [55] and subsequent authors). Several researchers (summarised by [1]) suggest that titanium is incorporated by two substitution reactions, respectively:

2(Mg,Fe)^vi = Ti^vi □^vi

(4)

(Mg,Fe)^vi 2Si^iv = Ti^vi 2Al^iv

(5)

Crude positive correlations between Ti content and apparent cation deficiency in the octahedral (M) site (Figure 6c), and with Al^iv contents (Figure 6d), suggest that both reactions may partly explain Ti substitution in tri-octahedral micas in the Aberfeldy samples. However, reaction [4] is largely discredited by studies demonstrating a lack of cation vacancies in tri-octahedral micas (see Henderson [7]).

Substitution mechanisms for the incorporation of Ti and Ba in phlogopite have been widely investigated [4,7,8,43,51,56,57]. Righter et al. [54] and others have suggested that coupled variations in the chemistry of mantle-derived Ba-Ti phlogopites can be accounted for by a combination of reactions (1), (3), and (5), such as the following:

K 3(Mg,Fe)^vi 3Si^iv = Ba 2Ti^vi 3Al^iv

(6)

or

Ba 3Ti 4Al^iv = 2K 4(Mg,Fe) 4Si

(7)

Figure 6e shows that compositions of some Aberfeldy biotites may involve Tschermakitic substitution, whereas barian biotite and phlogopite do not and instead show ‘muscovite substitution’ (di-octahedral vacancy) as indicated by reaction (8) [3,37]:

2Al^vi + □ ^vi = 3(Mg,Fe)

(8)

Yindongzi-Daxigou Ba-biotite compositions ([9]) may be partially accounted for by Tschermakitic substitution, whereas Ti-poor (<0.5 wt% TiO₂) Ba-phlogopites in the Franklin Marble ([39]) show no evidence of this mechanism and plot on the trend of muscovite substitution.

5.3. Compositional Variations in Micas Coexisting in Rock Samples

The similar Fe content of micas in grain contact with sulphides and micas elsewhere in the same thin section indicate the general attainment of equilibrium, with respect to Fe-partitioning, on a centimetre scale in the Aberfeldy rocks. As noted above, wide ranges in Fe:Mg ratios are found in both the di-octahedral and tri-octahedral micas examined in this study, but coexisting micas within individual samples have similar Fe/(Fe + Mg) ratios (Figure 7b).

An exception to this generalisation is sample N81-80 (Table 1) in which muscovite crystals have variable Fe and Mg contents, whereas phlogopites are essentially uniform in composition. N81-80 is a fine-grained quartz-hyalophane chert containing pseudomorphs of sulphate crystals (Figure 2g), and these textural and mineral compositional features indicate that small-scale (millimetric) disequilibrium was maintained in this rock during amphibolite-facies regional metamorphism. This can be explained by muscovite–consuming reactions during burial or regional metamorphism, involving the reduction of baryte and the formation of barium feldspar that incorporated liberated Ba²⁺. The reduced sulphur liberated by these reactions was incorporated into pyrite, as indicated by its exceptionally heavy sulphur isotope composition [24].

In the remaining samples examined, which do not contain other ferromagnesian silicates, coexisting muscovite and biotite (or phlogopite) have Fe-Mg partition coefficients close to unity (Figure 8a). However, in rocks containing garnet, chlorite, and/or hornblende, biotite generally has higher Fe/(Fe + Mg) ratios than coexisting muscovite, and partition coefficients range from 1.3 to 1.5. This evidence for equilibration between coexisting ferromagnesian silicates permits the application of Fe-Mg exchange geothermometers and barometers [20]. The low iron content of both muscovite and biotite occurring in sulphide-bearing graphitic dolostones and in many mineralized lithologies (in which other ferromagnesian silicates are usually absent) is attributed to low effective Fe/(Fe + Mg) ratios due to the partitioning of iron into sulphides during metamorphic mineral neoformation or recrystallization [1,58]). The corresponding increase in effective bulk Fe/(Fe + Mg) ratios, together with low oxygen fugacities, accounts for the scarcity of garnet, chlorite, and biotite in the sulphidic graphitic schists hosting the stratiform mineralization.

Ferry [59] found a systematic relationship between biotite composition and the identity of iron sulphides in graphitic schists from south-central Maine, such that all rocks with pyrite + pyrrhotite contained phlogopite with Fe/(Fe + Mg) ratios of 0.02–0.06, whereas if only pyrrhotite was present, then this ratio varied from 0.16 to 0.47 in the tri-octahedral micas. Ferry [59] suggested that the unique composition of phlogopite coexisting with pyrite + pyrrhotite may reflect initial crystallization in an invariant assemblage and demonstrated that biotite does not participate in desulphidation reactions until pyrite is eliminated from the assemblage. By analogy, the presence of biotite rather than phlogopite in graphitic schists of the stratigraphically lower (unmineralized) part of the Ben Eagach Schist at Foss may be related to the absence of pyrite (other than as a retrograde replacement of pyrrhotite) in these rocks.

Figure 8. (a) Molecular ratios of Fe/(Fe + Mg) in biotite vs. ratios in coexisting muscovite in analysed samples containing both micas. Micas in sample N81-80 have compositions far from the 1:1 line indicating disequilibrium. (b) Octahedral Al cations in biotite vs. octahedral Al cations in muscovite in analysed samples containing both micas. Magenta line fitted through the data points by classical linear regression after excluding the outlier sample 207-5 (see text). The dashed red lines represent loci of constant K_D values. In both (a,b), the symbols represent different rock samples; where more than one analysis was obtained, the plotted points are average values for the rock thin section.

Figure 8. (a) Molecular ratios of Fe/(Fe + Mg) in biotite vs. ratios in coexisting muscovite in analysed samples containing both micas. Micas in sample N81-80 have compositions far from the 1:1 line indicating disequilibrium. (b) Octahedral Al cations in biotite vs. octahedral Al cations in muscovite in analysed samples containing both micas. Magenta line fitted through the data points by classical linear regression after excluding the outlier sample 207-5 (see text). The dashed red lines represent loci of constant K_D values. In both (a,b), the symbols represent different rock samples; where more than one analysis was obtained, the plotted points are average values for the rock thin section.

Many authors have observed a general increase in the Al contents of both muscovite and biotite with increasing metamorphic grade. For example, Wenke [60] found that the partition coefficient, K_D = Al^vi_musc/Al^vi_biot, decreased from 14 to 3 in passing from the biotite zone into the staurolite zone in the central Alps. The similar range of K_D found in coexisting micas in the Aberfeldy rocks (Figure 8b) cannot be related to variation in the metamorphic grade, considering that all the samples in this study are from a spatially small rock body (Figure 1b) within an orogenic belt several hundred kilometres across. This systematic relationship is a function of the similar extents of Tschermakitic substitution in coexisting micas with similar Ba contents and Fe/(Fe + Mg) ratios; and therefore, it indirectly reflects a bulk compositional control. The scatter on either side of the trend line in Figure 8b may be due to localised disequilibrium: for example, some of the Ba-rich muscovite in sample 207-5 is spatially associated with late metamorphic veinlets, mentioned in Section 4.1.

5.4. Other SEDEX Ba- (±Cr ±V) Mica Occurrences and Comparisons with Aberfeldy Barian Micas

Chromian muscovite is comparatively common worldwide [1], but Ba-Cr varieties are rare. Barium-free chromian phengite containing up to 16.25% Cr₂O₃ was found by Mevel and Kienast [61] in a high-pressure metamorphosed gabbro. Matthews [62] presents an analysis of ‘fuchsite’ containing 1.07% BaO and 3.09% Cr₂O₃ from zoned ultramafic pods on Skye, and a hydromuscovite containing 1.32% BaO and 0.46% Cr₂O₃ was reported by Neiva [63]. Dymek et al. [10] describe green micas from pyritic metasediments of Archaean age in southern West Greenland, which range from nearly pure muscovite to types with up to ~8% BaO and ~17% Cr₂O₃. A compositional discontinuity (not evident in Aberfeldy micas) was observed by Dymek et al. [10] between micas with 0.20–0.45 and 0.80–0.95 cations Cr per formula unit (based on a total of 22 positive charges). These enrichments are coupled with high Mg, Fe, and Ti contents (up to 2%–3% each), and the authors describe coupled substitution mechanisms similar to those inferred in this study.

EMPA data for Ba-Cr-micas from the Ghattihosahalli SEDEX baryte deposit in India are provided by Devaraju et al. [64] and Raith et al. [33]. The mineralization is of Mesoarchean age and hosted in lower-amphibolite facies metasediments. The Ba-Cr-micas occur in various lithologies, namely impure baryte rock with coexisting barian feldspar, barian siliceous metasediments lacking barian feldspar, and low-barian kyanite-muscovite schists. The dataset provided by Raith et al. [33] is for micas from this spectrum of lithologies, comprising 42 muscovite analyses with up to 16.82 wt% Cr₂O₃ and 14.02 wt% BaO (in different crystals), plus 3 phlogopite analyses with up to 8.97 wt% Cr₂O₃ and 8.06 wt% BaO. The authors show that compositions of the di-octahedral micas were controlled by both bulk rock chemistry and fluid-mediated metamorphic processes. Most micas have a celadonite component of around ~20 mol%. Na-equilibration occurred between coexisting barian feldspar and micas during peak-metamorphic conditions (~550 °C and 4–5 kbar). These characteristics are very similar to those of barian feldspars and micas in the Aberfeldy SEDEX mineralization, although Cr is comparatively uncommon in Aberfeldy micas probably reflecting a low bulk rock Cr content ([15,21]).

Globally, occurrences of vanadian micas are associated with lode gold deposits and ‘red bed’ sediment-hosted U-V±Cu deposits, in both of which V has been remobilized from black shales and/or mafic igneous rocks. Kazachenko et al. [65] described vanadian muscovite containing up to 2.73% V₂O₃ (with no barium) in carbonaceous metasediments in the Primorye region of Russia. Vanadium-rich barian micas (V-rich muscovite and roscoelite) containing 6–18 wt% V₂O₃ and 7% BaO were reported by Heinrich and Levinson [66], and Snetsinger [12] reported a Ba-V muscovite also from a metamorphosed black shale. Canet et al. [48] describe V-rich biotite associated with other V- and Cr-bearing minerals including garnet, amphiboles, titanite, and allanite-(Ce), disseminated within contact metamorphosed V-rich aluminous and carbonaceous shales in the Poblet area of southwestern Catalonia, Spain. The Ba-V-Cr mica described here in a metamorphosed baryte rock from the Aberfeldy SEDEX deposits (sample 410-31) appears to be the first global occurrence to be reported in this particular geological setting.

Vanadian muscovite is the characteristic mineral of the ‘green leader’ marker horizon in the Kalgoorlie goldfield, Western Australia [1]. Vanadian micas from host rocks of the Hemlo gold deposit, Ontario [13], range continuously in composition from Ba-V-muscovite to roscoelite, KV₂[AlSi₃O₁₀](OH)₂, with up to 17.6 wt% V₂O₃. Vanadian phlogopite is also occasionally present. In the total Al cations vs. sum of M cations graph (Figure 7a), Hemlo Ba-V di-octahedral micas plot along the same trend to high transition metal and low Al contents as Ba-Cr muscovite from other localities including Aberfeldy. Vanadian phlogopite from Hemlo and vanadian biotite from the Poblet SEDEX deposit [48] plot close to the biotite-phlogopite ideal line though deficient in Al or transitional metal content (celadonite substitution). Conversely, on this diagram, Aberfeldy sample 410-31 Ba-V-Cr mica plots between the trends of di-octahedral and tri-octahedral micas, though closer to the former.

In Hemlo samples, coexisting di-octahedral and tri-octahedral vanadian micas show similar V enrichments, as shown in the ternary diagram inset in Figure 7b. On this diagram, the compositions of the Ba-V-Cr mica in Aberfeldy baryte rock sample 410-31 plot alongside Hemlo V-phlogopite. This is due to the low Al^vi content of the Ba-V-Cr mica in sample 410-31. This mica plots on the tri-octahedral trend in the total Al cations vs. Fe/(Fe + Mg) diagram (Figure 7b), supporting the inference that it is a member of the tri-octahedral mica group, despite its exceptionally high Fe content (Table 4 and Figure 5d). Ba-biotite in the Yindongzi-Daxigou Pb-Zn-Ag-Fe deposits [9] displays a higher transition metal content relative to the total Al content (Figure 7a) and higher Fe/(Fe + Mg) ratios (Figure 7b) than all Aberfeldy micas. Yindongzi-Daxigou Ba-muscovite [9] has similarly high Fe/(Fe + Mg) ratios indicating equilibrium formation alongside the Ba-biotite.

Aberfeldy muscovite and barian muscovite are compositionally similar to other occurrences worldwide (Figure 9a–d), although Aberfeldy muscovite and biotite show a somewhat greater range in I-site deficiency (Figure 9a) and phengitic substitution (Figure 9c,d). Due to greater phengitic substitution (discussed in Section 5.1), Aberfeldy muscovite generally has lower Al^iv and higher Si contents than Isua green micas [10], Berisal Complex Ba-muscovite [45,46], Ghattihosahalli [33,64], and Benallt deposit micas [49]. However, muscovite in the Yindongzi-Daxigou, Kipishi, and Hemlo ore deposits [9,11,47] show phengitic substitution comparable in extent to that of Aberfeldy Ba-muscovite. Outliers on these graphs are three analyses of Ba-V phlogopite from Hemlo (Figure 9c) and, once again, the unusual Fe-rich Ba-V-Cr mica in Aberfeldy sample 410-31 (Figure 9d).

Figure 9. Comparisons of Aberfeldy mica compositions (as per Figure 5) with published analyses of Ba- and Ba-Cr±V-micas from other localities worldwide [9,10,11,13,39,45,46,47,49,64]. Four analyses of Berisal ganterite [45] lie outside the compositional fields of other micas. (a) Ba cations vs. sum of Na + K cations. The dashed red line indicates ideal interlayer site occupancy (Na + K + Ba = 1). (b) Sum of Ba + Al^iv cations vs. sum of K + Si cations in di-octahedral micas. (c) Ba cations vs. tetrahedral Al cations in di-octahedral micas. Three outlier analyses of Hemlo Ba-V micas [13] are vanadian phlogopite. (d) Si cations vs. sum of Fe + Mg cations in di-octahedral micas. Ba-V-Cr mica in Aberfeldy baryte sample 410-31 has an exceptionally high Fe content (see text), whereas Benallt Ba-muscovite has a notably low Fe + Mg content [49]. Dashed lines indicate the coupled substitution scheme (1) (see Section 5.1).

Figure 9. Comparisons of Aberfeldy mica compositions (as per Figure 5) with published analyses of Ba- and Ba-Cr±V-micas from other localities worldwide [9,10,11,13,39,45,46,47,49,64]. Four analyses of Berisal ganterite [45] lie outside the compositional fields of other micas. (a) Ba cations vs. sum of Na + K cations. The dashed red line indicates ideal interlayer site occupancy (Na + K + Ba = 1). (b) Sum of Ba + Al^iv cations vs. sum of K + Si cations in di-octahedral micas. (c) Ba cations vs. tetrahedral Al cations in di-octahedral micas. Three outlier analyses of Hemlo Ba-V micas [13] are vanadian phlogopite. (d) Si cations vs. sum of Fe + Mg cations in di-octahedral micas. Ba-V-Cr mica in Aberfeldy baryte sample 410-31 has an exceptionally high Fe content (see text), whereas Benallt Ba-muscovite has a notably low Fe + Mg content [49]. Dashed lines indicate the coupled substitution scheme (1) (see Section 5.1).

6. Conclusions

Aberfeldy barian micas are compositionally similar to barian micas in other metamorphosed SEDEX baryte deposits and barium-rich metasediments worldwide, and they exhibit evidence of similar cationic substitution mechanisms to micas in the other occurrences. Ba²⁺ incorporation in both di-octahedral and tri-octahedral micas is mainly by the coupled substitution of Ba + Al^iv for K + Si. In muscovite, the paragonite component is distinctly higher in Ba-poor muscovite of the metasedimentary host rocks (up to ~18% of the I-site) than in barian muscovite associated with stratiform mineralization (typically <6% of the I-site). The extent of phengitic (celadonite, Tschermakitic) substitution is typical of muscovite in amphibolite-facies metasediments, with higher Fe + Mg in barian muscovite and lower Fe + Mg in non-barian muscovite. Tri-octahedral micas show greater deficiency in I-site occupancy than barian muscovite. The Ti content of Aberfeldy non-barian biotite is typical of Ti-biotite, whereas barian biotite (and phlogopite) is relatively depleted in Ti. Muscovite substitution (di-octahedral vacancy) dominates in barian biotite and phlogopite, rather than Tschermakitic substitution, which is more evident in Ba-poor biotite in host rock contexts.

Chromium-bearing, barian muscovite occurs in minor amounts mainly in baryte and barium-feldspar rocks. Ba-Cr muscovite compositions are generally similar to Cr-poor barian muscovite but extend the range to higher barium contents, reaching 15.70 wt% BaO in one analysis.

An unusual Ba-V-Cr mica occurs in one sample of baryte rock (410-31, Foss East). V-rich barian micas are previously known [13] from the Hemlo lode gold deposit, Ontario, although the Aberfeldy example has a substantially higher barium content, up to 12.33 wt% BaO compared to a maximum of 4.35 wt% BaO in published analyses of Hemlo muscovite [13]. The Ba-V-Cr mica described here has an exceptionally high Fe content and Fe/(Fe + Mg) ratio, and low total Al content, and it is, accordingly, difficult to classify in terms of di-octahedral or tri-octahedral mica. Further analysis of this mica (beyond the scope of the current study) is warranted to determine its crystal structure and the contents and oxidation state of iron and other transition metals.