Achyrophanite, (K,Na)₃(Fe³⁺,Ti,Al,Mg)₅O₂(AsO₄)₅, a New Mineral with the Novel Structure Type from Fumarolic Exhalations of the Tolbachik Volcano, Kamchatka, Russia

Abstract

The new mineral achyrophanite (K,Na)₃(Fe³⁺,Ti,Al,Mg)₅O₂(AsO₄)₅ was found in high-temperature sublimates of the Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is associated with aphthitalite-group sulfates, hematite, alluaudite-group arsenates (badalovite, calciojohillerite, johillerite, nickenichite, hatertite, and khrenovite), ozerovaite, pansnerite, arsenatrotitanite, yurmarinite, svabite, tilasite, katiarsite, yurgensonite, As-bearing sanidine, anhydrite, rutile, cassiterite, and pseudobrookite. Achyrophanite occurs as long-prismatic to acicular or, rarer, tabular crystals up to 0.02 × 0.2 × 1.5 mm, which form parallel, radiating, bush-like, or chaotic aggregates up to 3 mm across. It is transparent, straw-yellow to golden yellow, with strong vitreous luster. The mineral is brittle, with (001) perfect cleavage. D_calc is 3.814 g cm^–3. Achyrophanite is optically biaxial (+), α = 1.823(7), β = 1.840(7), γ = 1.895(7) (589 nm), 2V (meas.) = 60(10)°. Chemical composition (wt.%, electron microprobe) is: Na₂O 3.68, K₂O 9.32, CaO 0.38, MgO 1.37, MnO 0.08, CuO 0.82, ZnO 0.48, Al₂O₃ 2.09, Fe₂O₃ 20.42, SiO₂ 0.12, TiO₂ 7.35, P₂O₅ 0.14, V₂O₅ 0.33, As₂O₅ 51.88, SO₃ 1.04, and total 99.40. The empirical formula calculated based on 22 O apfu is Na_1.29K_2.15Ca_0.07Mg_0.34Mn_0.01Cu_0.11Zn_0.06Al_0.44Fe³⁺_2.77Ti_1.00Si_0.02P_0.02S_0.14V_0.04As_4.90O₂₂. Achyrophanite is orthorhombic, space group P222₁, a = 6.5824(2), b = 13.2488(4), c = 10.7613(3) Å, V = 938.48(5) ų and Z = 2. The strongest reflections of the PXRD pattern [d,Å(I)(hkl)] are 5.615(59)(101), 4.174(42)(022), 3.669(31)(130), 3.148(33)(103), 2.852(43)(141), 2.814(100)(042, 202), 2.689(29)(004), and 2.237(28)(152). The crystal structure of achyrophanite (solved from single-crystal XRD data, R = 4.47%) is unique. It is based on the octahedral-tetrahedral M-T-O pseudo-framework (M = Fe³⁺ with admixed Ti, Al, Mg, Na; T = As⁵⁺). Large-cation A sites (A = K, Na) are located in the channels of the pseudo-framework. The achyrophanite structure can be described as stuffed, with the defect heteropolyhedral pseudo-framework derivative of the orthorhombic Fe³⁺AsO₄ archetype. The mineral is named from the Greek άχυρον, straw, and φαίνομαι, to appear, in allusion to its typical straw-yellow color and long prismatic habit of crystals.

1. Introduction

Diverse and rich arsenate mineralization is one of the most interesting mineralogical and geochemical features of the high-temperature fumaroles of the oxidizing type born by the active Tolbachik volcano at Kamchatka (Russian Far East). Almost sixty H-free arsenates found here involve in their chemical composition many cations as species-defining constituents: Cu, Zn, Mg, Mn, Al, Fe³⁺, Ti, Zr, Sn, Ca, Na, and K; some fumarolic arsenates contain additional anions: (SO₄), (VO₄), (PO₄), O, F, or Cl. The uniqueness of this mineralization is brightly highlighted by the great number of arsenate minerals first discovered in exhalations of Tolbachik fumaroles, namely forty new mineral species ([1,2] and references therein).

In this paper, we describe the new mineral achyrophanite (Cyrillic: ахирoфанит) which has a unique crystal structure. The mineral is named from the Greek (in both ancient and modern Greek languages) άχυρον, straw, and φαίνομαι, to appear, in allusion to its straw-yellow color and long prismatic habit of crystals: typical aggregates of the mineral visually resemble straw bunches.

Both the new mineral and its name have been approved by the IMA Commission on New Minerals, Nomenclature and Classification, IMA2018–011. The type specimen is deposited in the systematic collection of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, with the catalogue number 96254.

2. Occurrence and Mineral Association

Achyrophanite was found in the Arsenatnaya fumarole situated at the second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption 1975–1976 (NB GTFE), Tolbachik volcano, Kamchatka Peninsula, Far-Eastern Region, Russia (55°41′N 160°14′E, 1200 m asl). This scoria cone is a monogenetic basalt volcano formed in 1975. Many fumaroles of the oxidizing type, including several richly mineralized ones, are still active here ([3,4,5] and references therein). Arsenatnaya is the largest fumarole at the second scoria cone of the NB GTFE. It is an outstanding mineralogical occurrence due to the greatest mineral diversity and originality: more than 200 mineral species have been reliably identified here, including 72 new minerals. The detailed data on the Arsenatnaya fumarole have been reported in [1,6].

The type material of achyrophanite was collected by us in July 2016 from the medium (about 2 m under the day surface) zone of Arsenatnaya. The temperature in this area measured using a chromel-alumel thermocouple during collection was ca. 430 °C. In July 2018, additional specimens were collected.

Achyrophanite is associated with aphthitalite-group sulfates (metathénardite, belomarinaite and aphthitalite), hematite, alluaudite-group arsenates (badalovite, calciojohillerite, johillerite, nickenichite, hatertite, and khrenovite forming a continuous solid-solution system), ozerovaite, pansnerite, arsenatrotitanite, yurmarinite, svabite, tilasite, katiarsite, yurgensonite, As-bearing sanidine, anhydrite, rutile (Sn-, Fe³⁺-, and Sb⁵⁺-enriched variety), cassiterite, and pseudobrookite.

3. Methods

The Raman spectrum of achyrophanite was obtained using an EnSpectr R532 spectrophotometer (Dept. of Mineralogy, Lomonosov Moscow State University) with a green laser (532 nm) at room temperature. The radiation power at the output of the laser source was about 16 mW. The spectrum was processed in the range from 100 to 4000 cm^–1 with the use of a holographic diffraction grating with 1800 mm⁻¹ and a resolution equal to 6 cm⁻¹. The diameter of the laser spot on the sample was about 10 μm with a 40× objective. The spectrum was acquired on a randomly oriented crystal. The mineral is stable under a laser beam.

Scanning electron microscopic (SEM) studies in secondary electron (SE) mode were carried out, and chemical composition was determined for all studied samples using a JEOL JSM-6480LV scanning electron microscope equipped with an INCA-Wave 500 wavelength-dispersive spectrometer (Laboratory of Analytical Techniques of High Spatial Resolution, Dept. of Petrology, Lomonosov Moscow State University), with an acceleration voltage of 20 kV, a beam current of 20 nA, and a 3 μm beam diameter. The standards used are as follows: Na, Al, Si (jadeite), K, P (KTiOPO₄), Ca (CaSiO₃), Mg (olivine), Mn, Ti (MnTiO₃), Cu (Cu), Zn, S (ZnS), Fe (FeS₂), V (V), and As (GaAs). Contents of other elements with atomic numbers > 6 are below detection limits.

Powder X-ray diffraction (XRD) data were collected using a Rigaku RAXIS Rapid II diffractometer (Rigaku Corporation, Tokyo, Japan) with a curved image plate detector and rotating anode with VariMAX microfocus optics, using CoKα radiation in Debye-Scherrer geometry at an accelerating voltage of 40 kV, current of 15 mA, and exposure time 15 min. The distance between the sample and the detector was 127.4 mm. The data were processed using osc2xrd software [7].

Single-crystal XRD studies were carried out using an Xcalibur S diffractometer equipped with a CCD detector (Oxford Diffraction, Oxford, UK) (MoKα radiation).

4. Results

4.1. General Appearance and Physical Properties

In the type material, achyrophanite occurs as long-prismatic (lath-, sword-, or spear-like) to acicular crystals elongated along [100] and usually flattened on [001], up to 0.02 × 0.2 × 1.5 mm, which form parallel, radiating, bush-like, or chaotic aggregates up to 3 mm across (Figure 1a–d and Figure 2). Rarely, they are combined in interrupted, open-work encrustations on the surface of basalt scoria altered by fumarolic gas. In the specimens collected in 2018, achyrophanite crystals (up to 0.5 mm) demonstrate another morphology: they are flattened prismatic to tabular (Figure 1e,f). The pinacoid {001} is the major form of achyrophanite crystals; lateral faces are typically represented by the pinacoid {010}; on some crystals the pinacoidal face {100} and/or non-indexed faces of the prismatic zone {hk0} are observed.

Achyrophanite is transparent, with strong vitreous luster and a white streak. The mineral is straw-yellow to golden yellow; the thinnest crystals are almost colorless. Achyrophanite is brittle; however, parallel intergrowths of flattened acicular crystals are flexible and elastic. Perfect cleavage on (001) was observed under the microscope. The fracture is uneven across a crystal elongation (observed under the microscope). Density could not be measured because crystals are thin and their aggregates are open-work. The density value calculated using the empirical formula and unit-cell volume found from single-crystal XRD data is 3.814 g cm^–3.

4.2. Optical Data

In plane-polarized transmitted light, achyrophanite is very weakly pleochroic, with the following absorption scheme: Z (very pale yellow) > Y = X (colorless). It is optically biaxial (+), α = 1.823(7), β = 1.840(7), γ = 1.895(7) (589 nm), 2V (meas.) = 60(10)°, and 2V (calc.) = 60°. Dispersion of optical axes is weak, r > v. Orientation is as follows: X = c, Y = a, Z = b. Extinction is straight, and elongation is negative.

4.3. Raman Spectroscopy

The Raman spectrum of achyrophanite (Figure 3) was interpreted according to [8]. The bands in the region between 1000 and 700 cm⁻¹ correspond to As⁵⁺–O stretching vibrations of AsO₄³⁻ anions. The presence of two strong bands with maxima at 865 and 777 cm⁻¹ and a distinct band at 921 cm⁻¹ is caused by significant distortion of AsO₄ tetrahedra marked as T(1)O₄, T(2)O₄, T(4)O₄, and, especially, T(5)O₄ (see structure data below). Bands in the region between 600 and 350 cm⁻¹ are assigned to bending vibrations of AsO₄ tetrahedra, Fe³⁺–O, Ti–O, Al–O, and Mg–O stretching vibrations. Bands with frequencies lower than 300 cm⁻¹ correspond to lattice modes. The absence of bands with frequencies higher than 1000 cm⁻¹ indicates the absence of groups with O–H, C–H, C–O, N–H, N–O, and B–O bonds in achyrophanite.

4.4. Chemical Data

Chemical composition of achyrophanite (type specimen) is given in

Table 1. Chemical composition (wt.%) of achyrophanite.

Constituent	Mean (for 8 Spot Analyses)	Range	Standard Deviation
Na2O	3.68	3.41–3.92	0.20
K2O	9.32	8.98–9.66	0.23
CaO	0.38	0.30–0.44	0.05
MgO	1.27	1.04–1.60	0.22
MnO	0.08	0.06–0.11	0.02
CuO	0.82	0.71–0.95	0.13
ZnO	0.48	0.37–0.58	0.08
Al2O3	2.09	1.53–2.81	0.47
Fe2O3 *	20.42	18.64–22.71	1.59
TiO2	7.35	6.60–7.96	0.55
SiO2	0.12	0.04–0.19	0.05
P2O5	0.14	0.04–0.20	0.05
V2O5	0.33	0.14–0.50	0.12
As2O5	51.88	50.61–54.25	1.17
SO3	1.04	0.21–1.91	0.70
Total	99.40

* Trivalent state of iron in achyrophanite is undoubtedly confirmed by structure data—interatomic distances and bond valence calculations (see Section 5). This also is in agreement with strongly oxidizing conditions of mineral formation in the Arsenatnaya fumarole: all iron minerals known here contain only Fe³⁺ [1,5].

.

The empirical formula calculated on the basis of 22 oxygen atoms per formula unit (apfu) is Na_1.29K_2.15Ca_0.07Mg_0.34Mn_0.01Cu_0.11Zn_0.06Al_0.44Fe³⁺_2.77Ti_1.00Si_0.02P_0.02S_0.14V_0.04As_4.90O₂₂ or, after the grouping of constituents; or, after the formal grouping of cations between T, M and A sites: ^A(K_2.15Na_1.29Ca_0.07)_Σ3.51^M(Fe³⁺_2.57Ti_1.00Al_0.44Mg_0.34Cu_0.11Zn_0.06Mn_0.01)_Σ4.53(As_4.90S_0.14V_0.04Si_0.02P_0.02)_Σ5.12O₂₂ (Z = 2). Some surplus of T and (M + A) constituents over the values required by the idealized formula, 5 and 8 apfu, respectively, is caused by the existence of additional low-occupied positions T’ and M’ in the structure of achyrophanite. The surplus of (K + Na + Ca) over the idealized value for A cations = 3 apfu is caused by the location of Na not only at the A sites but also at the M(4) and M’ sites (see below).

The simplified formula of achyrophanite is (K,Na)₃(Fe³⁺,Ti,Al,Mg)₅O₂(AsO₄)₅. The idealized formula with whole-number coefficients (K₂Na)(Fe³⁺₄Ti)O₂(AsO₄)₅ requires Na₂O 2.82, K₂O 8.57, Fe₂O₃ 29.06, TiO₂ 7.27, As₂O₅ 52.28, total 100 wt%.

The Gladstone–Dale compatibility index [9] for achyrophanite is −0.029 (excellent).

4.5. X-Ray Crystallography and Crystal Structure

Powder XRD data of achyrophanite are given in

Table 2. Powder X-ray diffraction data (d in Å) of achyrophanite.

Iobs	dobs	Icalc *	dcalc **	h k l
5	13.21	13	13.249	010
2	8.36	8	8.353	011
9	6.62	8, 1	6.624, 6.582	020, 100
11	5.900	11	5.894	110
59	5.615	6, 88	5.641, 5.615	021, 101
9	5.376	10	5.381	002
4	4.980	2	4.985	012
3	4.683	1	4.669	120
3	4.417	1	4.416	030
1	4.278	1	4.283	121
42	4.174	41	4.177	022
2	4.077	1	4.086	031
3	3.972	2	3.974	112
31	3.669	39	3.667	130
6	3.527	5	3.527	122
4	3.469	3	3.471	131
14	3.413	13	3.414	032
6	3.315	1, 6	3.312, 3.291	040, 200
33	3.148	5, 37	3.154, 3.150	023, 103
8	3.064	1, 17	3.064, 3.062	113, 211
5	3.032	6	3.030	132
11	2.951	2, 17	2.959, 2.947	140, 220
43	2.852	45	2.853	141
100	2.814	61, 100	2.821, 2.808	042, 202
29	2.689	43	2.690	004
5	2.642	2, 4, 2	2.650, 2.639, 2.637	050, 230, 014
7	2.587	9	2.585	222
5	2.565	1, 9	2.564, 2.563	133, 231
1	2.490	1	2.490	104
2	2.443	1	2.447	114
1	2.398	1	2.396	151
1	2.371	1	2.369	232
2	2.296	2	2.298	034
2	2.284	3, 1	2.282, 2.282	143, 241
28	2.237	37	2.236	152
2	2.210	1	2.208	060
9	2.169	11, 1	2.169, 2.165	134, 310
2	2.144	1	2.142	242
6	2.127	8	2.126	233
4	2.087	3, 1, 1	2.088, 2.083, 2.083	044, 204, 320
4	2.041	3	2.046	105
5	2.029	9	2.027	251
5	2.009	4	2.008	312
1	1.929	2	1.927	252
2	1.884	2	1.884	234
3	1.873	6	1.872	303
5	1.847	12	1.846	332
2	1.834	3	1.834	260
10	1.815	1, 17	1.819, 1.815	170, 154
4	1.789	6, 3	1.789, 1.785	253, 215
17	1.764	31	1.763	244
6	1.739	8, 5	1.740, 1.736	145, 262
2	1.707	3	1.707	064
5	1.692	16, 2	1.690, 1.687	350, 314
2	1.667	2	1.668	235
4	1.658	1, 5	1.662, 1.656	036, 080
2	1.647	1, 10	1.647, 1.646	324, 400
2	1.631	4	1.630	343
2	1.614	3, 3, 1	1.615, 1.612, 1.611	411, 352, 136
7	1.585	6, 7	1.588, 1.583	181, 082
10	1.576	10, 17, 6	1.577, 1.575, 1.574	046, 206, 402
2	1.539	2, 1	1.539, 1.536	182, 305
1	1.530	3	1.526	431
2	1.515	3	1.515	264
2	1.509	2	1.507	174
3	1.495	6	1.497	107
3	1.481	1, 1, 4	1.483, 1.482, 1.479	236, 432, 280
4	1.467	7	1.466	183
3	1.449	12	1.449	156
4	1.438	6	1.437	190
8	1.432	27, 1	1.431, 1.428	354, 363
2	1.419	5, 4	1.421, 1.417	442, 433
2	1.405	5	1.404	404
2	1.393	4	1.394	345
3	1.388	3, 1, 3	1.388, 1.386, 1.385	192, 451, 372
2	1.374	5	1.373	424

* For the calculated patterns, only reflections with intensities ≥ 1 are given; ** for the unit-cell parameters obtained from single-crystal data. The strongest reflections are marked in bold type.

. The orthorhombic unit-cell parameters calculated from the powder data are as follows: a = 6.579(2), b = 13.255(3), c = 10.763(4) Å, and V = 938.6(8) ų.

Single-crystal XRD studies of achyrophanite, including the obtaining of the dataset used for the structure determination, were carried out at room temperature. A full sphere of three-dimensional data was collected. Data reduction was performed using CrysAlisPro Version 1.171.39.46 [10]. The data were corrected for Lorentz factor and polarization effects. The crystal structure was solved by direct methods and refined using the SHELX software package (version SHELXL-97) [11] to R = 0.0447 for 2088 independent reflections with I > 2σ(I). The crystal data and the experimental details are presented in

Table 3. Crystal data, data collection information, and structure refinement details for achyrophanite.

Structural Formula	(K0.47Na0.47?0.06)2(K0.50?0.50)4(?0.75K0.25)4 (?0.94Ca0.03Na0.03)4(Fe0.48Ti0.30Al0.22)2 (Fe0.70Ti0.30)4(Fe0.36Mg0.28Al0.20Ti0.16)2(Fe0.74Mg0.05Na0.17Al0.04)2(?0.75Cu0.11Na0.08Zn0.06)2 (AsO4)8[(As0.68S0.32)O4]2[(?0.75As0.25)O0.5 (?0.75O0.25)2]2O3
Formula weight	2171.74
Temperature, K	293(2)
Radiation and wavelength, Å	MoKα; 0.71073
Crystal system, space group, Z	Orthorhombic, P2221, 1
Unit-cell dimensions, Å/°	a = 6.5824(2) b = 13.2488(4) c = 10.7613(3)
V, Å3	938.48(5)
Absorption coefficient μ, mm−1	12.140
F000	1022
Crystal size, mm	0.02 × 0.17 × 0.77
Diffractometer	Xcalibur S CCD
θ range for data collection, °/Collection mode	3.09–28.27/full sphere
Index ranges	−8 ≤ h ≤ 8, −17 ≤ k ≤ 17, −14 ≤ l ≤ 14
Reflections collected	15,862
Independent reflections	2335 (Rint = 0.0626)
Independent reflections with I > 2σ(I)	2088
Data reduction	CrysAlisPro, Agilent Technologies (Oxfordshire, UK), v. 1.171.37.34 [10]
Absorption correction	gaussian
Structure solution	direct methods
Refinement method	full-matrix least-squares on F2
Number of refined parameters	190
Final R indices [I > 2σ(I)]	R1 = 0.0447, wR2 = 0.1065
R indices (all data)	R1 = 0.0507, wR2 = 0.1102
GoF	1.089
Largest diff. peak and hole, e/Å3	1.76 and −1.59

, atom coordinates and equivalent displacement parameters in

Table 4. Coordinates and equivalent displacement parameters (U_eq, in Ų) of atoms, site occupancy factors (s.o.f.) and site multiplicities (Q) for achyrophanite.

Site	x	y	z	Ueq	s.o.f.	Q
A(1)	0.6728(7)	0.0	0.5	0.0545(15)	K0.47Na0.47	2
A(2)	0.198(2)	0.3889(8)	0.828(2)	0.035(4)	K0.25	4
A(3)	0.2239(7)	0.3724(5)	0.9404(7)	0.0472(17)	K0.50	4
A’	0.730(7)	0.184(4)	0.500(6)	0.033(10) *	Ca0.03Na0.03	4
M(1)	0.0	0.13054(17)	0.75	0.0113(4)	Fe3+0.48Ti0.30Al0.22	2
M(2)	0.24503(16)	0.25063(9)	0.50445(16)	0.0082(2)	Fe3+0.70Ti0.30	4
M(3)	0.5	0.39561(17)	0.25	0.0077(5)	Fe3+0.36Mg0.28Al0.20Ti0.16	2
M(4)	0.0	0.11671(15)	0.25	0.0087(4)	Fe3+0.74Na0.17Mg0.05Al0.04	2
M’	0.5	0.6055(6)	0.25	0.025(2)	Cu0.11Na0.08Zn0.06	2
T(1)	0.5	0.18623(9)	0.75	0.0084(2)	As1.00	2
T(2)	0.18208(15)	0.0	0.50	0.0070(2)	As1.00	2
T(3)	0.31817(15)	0.5	0.50	0.0088(2)	As1.00	2
T(4)	0.0	0.32747(10)	0.25	0.0087(3)	As0.68S0.32 **	2
T(5)	0.5	0.18211(9)	0.25	0.0084(2)	As1.00	2
T’	0.0	0.3259(4)	0.75	0.0174(10)	As0.25	2
O(1)	0.2967(10)	0.1146(5)	0.7845(6)	0.0125(14)	O1.00	4
O(2)	0.4763(10)	0.4944(6)	0.3774(5)	0.0223(14)	O1.00	4
O(3)	0.6994(11)	0.1136(5)	0.2905(5)	0.0121(14)	O1.00	4
O(4)	0.1940(8)	0.3968(5)	0.2086(6)	0.0205(16)	O1.00	4
O(5)	0.0245(8)	0.0004(5)	0.3750(4)	0.0123(10)	O1.00	4
O(6)	0.3291(8)	0.1020(4)	0.4949(7)	0.0142(11)	O1.00	4
O(7)	0.4531(10)	0.2698(4)	0.3642(6)	0.0115(15)	O1.00	4
O(8)	0.1699(8)	0.3972(4)	0.5045(8)	0.0188(12)	O1.00	4
O(9)	0.4580(10)	0.2679(4)	0.6306(5)	0.0109(15)	O1.00	4
O(10)	0.0404(9)	0.2279(4)	0.6294(5)	0.0116(14)	O1.00	4
O(11)	0.0396(13)	0.2421(4)	0.3606(5)	0.029(2)	O1.00	4
O(12)	0.175(8)	0.405(3)	0.799(8)	0.035(4)	O0.25	4

* U_iso; ** this site is As dominant with admixed tetrahedrally coordinated S, P, Si, and V, which are jointly marked here as S.

, selected interatomic distances in

Table 5. Selected interatomic distances (Å) in the structure of achyrophanite.

T(1)—O(1)	1.682(6) × 2	T(4)—O(4)	1.635(3) × 2
—O(9)	1.702(6) × 2	—O(11)	1.663(3) × 2
T(2)—O(6)	1.663(5) × 2	T(5)—O(3)	1.655(7) × 2
—O(5)	1.698(5) × 2	—O(7)	1.719(6) × 2
T(3)—O(8)	1.677(5) × 2	M’—O(12)	1.649(11) × 2
—O(2)	1.682(6) × 2	—O(10)	1.855(4) × 2
M(1)—O(10)	1.849(6) × 2	M(3)—O(2)	1.901(7) × 2
—O(1)	1.999(7) × 2	—O(4)	2.063(6) × 2
—O(5)	2.201(6) × 2	—O(7)	2.093(6) × 2
M(2)—O(10)	1.927(6)	M(4)—O(3)	2.026(7) × 2
—O(9)	1.965(6)	—O(5)	2.052(6) × 2
—O(8)	2.004(6)	—O(11)	2.060(7) × 2
—O(6)	2.048(6)
—O(7)	2.054(6)	A(1)—O(6)	2.636(7) × 2
—O(11)	2.058(7)	—O(5)	2.677(7) × 2
		—O(3)	2.716(6) × 2
M’—O(2)	2.018(9) × 2	—O(1)	2.780(6) × 2
—O(9)	2.131(9) × 2
—O(12)	2.21(7) × 2	A(2)—O(10)	2.689(13)
		—O(2)	2.695(12)
A’—O(3)	2.45(6)	—O(9)	2.810(15)
—O(1)	2.50(6)	—O(12)	2.82(3)
—O(9)	2.53(5)	—O(4)	2.867(11)
—O(10)	2.54(5)	—O(8)	3.02(3)
—O(7)	2.60(5)
—O(11)	2.64(5)	A(3)—O(9)	2.624(8)
—O(6)	2.86(5)	—O(8)	2.680(7)
		—O(10)	2.695(8)
		—O(2)	2.733(9)
		—O(4)	2.911(10)

, and bond valence calculations in

Table 6. Bond valence calculations for achyrophanite.

	T(1)	T(2)	T(3)	T(4)	T(5)	T’ 0.25	M(1)	M(2)	M(3)	M(4)	M’	A(1)	A(2)	A(3)	Σ
O(1)	1.26 x2↓						0.52 ×2↓					0.11 ×2↓			1.89
O(2)			1.26 ×2↓						0.64 ×2↓		0.11 ×2↓		0.05	0.10	2.16
O(3)					1.35 ×2↓					0.48 ×2↓		0.14 ×2↓			1.97
O(4)				1.28 ×2↓					0.41 ×2↓				0.03	0.06	1.78
O(5)		1.20 ×2↓					0.31 ×2↓			0.46 ×2↓		0.15 ×2↓			2.12
O(6)		1.32 ×2↓						0.48				0.17 ×2↓			1.97
O(7)					1.14 ×2↓			0.47	0.43 ×2↓						2.04
O(8)			1.28 ×2↓					0.54					0.02	0.11	1.95
O(9)	1.19 ×2↓							0.60			0.08 ×2↓		0.04	0.13	2.04
O(10)						0.20 ×2↓	0.78 ×2↓	0.66					0.06	0.11	1.81
O(11)				1.19 ×2↓				0.47		0.44 ×2↓					2.10
O(12)						0.34 ×2↓					0.08 ×2↓		0.04		0.46
Σ	4.90	5.04	5.08	4.94 *	4.98	1.08	3.22	3.22	2.76	2.76	0.60	1.14	0.24	0.51

Bond-valence parameters are taken from [12]. Site occupancies were taken into account: T(4) = As_0.68S_0.32, T’ = As_0.25, M(1) = Fe³⁺_0.48Ti_0.30Al_0.22, M(2) = Fe³⁺_0.70Ti_0.30, M(3) = Fe³⁺_0.36Mg_0.28Al_0.20Ti_0.16, M(4) = Fe³⁺_0.74Mg_0.05Na_0.17Al_0.04, M’ = Cu_0.11Na_0.08Zn_0.06, A(1) = K_0.47Na_0.47, A(2) = K_0.25, A(3) = K_0.50, O(12) = O_0.25. A’ = Ca_0.03Na_0.03 is not included in bond-valence calculations due to very low occupancy factor of the site. * The sum was obtained for T(4) = As_0.68S_0.32 but we should note that the value will be increased taking into account other impurities: V, Si, P. For pure As in this site the bond valence sum could be 5.50.

.

5. Discussion

5.1. Structure Description

The crystal structure of achyrophanite (Figure 4a–c) is unique. It is based on the heteropolyhedral (octahedral-tetrahedral) M-T-O pseudo-framework. Cations M centre five crystallographically non-equivalent octahedrally coordinated sites: four of them are fully occupied and Fe³⁺-dominant [M(1–4)] with admixed Ti, Al, and Mg [and Na in M(4)], whereas the fifth site (M’) is 25% occupied and assumed to contain Cu²⁺ with subordinate Na and Zn. The cation distribution (Table 4) is based on the refined numbers of electrons, interatomic distances (Table 5) and bond valence calculations (Table 6) and takes into account electron-microprobe data (Table 1). The M(1)O₆ and M(4)O₆ octahedra share edges to form dimers as well as M(3)O₆ and M’O₆ octahedra. The neighboring dimers are connected with each other via the M(2)-centered octahedra [each M(2)O₆ octahedron shares four oxygen vertices with the octahedra of the dimers] to form an octahedral pseudo-framework (Figure 5a).

Interatomic distances (Table 5) and bond valence calculations (Table 6) undoubtedly confirm the trivalent state of iron in achyrophanite.

The structure of achyrophanite contains six crystallographically non-equivalent tetrahedrally coordinated sites T. Four of them [T(1,2,3,5)] are fully occupied by As⁵⁺. The T(4) site is filled by As with admixed lighter components; according to electron-microprobe data, there are S (prevailing), V, P, and Si (Table 1); it is refined as As_0.68S_0.32, assuming that “S” means all these admixtures together. The T’ site is predominantly vacant: ☐_0.75As_0.25. TO₄ tetrahedra share with MO₆ octahedra all vertices [T(2,3)] or two vertices and one edge [T(1,4,5) and T’] and, thus, “reinforce” the linkages in the octahedral pseudo-framework forming the heteropolyhedral pseudo-framework (Figure 5b). This pseudo-framework could also be described as a building unit formed by the (001) layers of octahedral dimers linked by common edges to the T(1,4,5)- and T’-centered tetrahedra (Figure 5c). Adjacent layers are linked with each other via the chains of alternating M(2)O₆ octahedra and T(2,3)O₄ tetrahedra (Figure 5d) sharing vertices with each other and with polyhedra of the layers.

Large-cation sites are located in the channels of the pseudo-framework (Figure 4a–c). Almost fully occupied site A(1) [K_0.47Na_0.47☐_0.06] and low-occupied site A’ [☐_0.94Ca_0.03Na_0.03] are located at the distance of 2.47 Å from one another and, thus, cannot be filled simultaneously. We do not also exclude that a minor amount of Ca could also be located in the A(1) site. The A(2) [☐_0.75K_0.25] and A(3) [K_0.50☐_0.50] sites are located at a distance of 1.24 Å from one another and also cannot be filled simultaneously. These sites are also close to T’- and M’-centered polyhedra (both 25% filled), and, thus, the fragment with T’O₄ tetrahedron and M’O₆ octahedron and the fragment with A(2)/A(3) sites cannot be occupied jointly.

5.2. Comparative Crystal Chemistry

The above-described heteropolyhedral M-T-O pseudo-framework can be considered as a base of the crystal structure of achyrophanite. The simplified structural formula of this pseudo-framework can be written as ^M(1−5)M₁₀^M’(☐_0.75M_0.25O_1.5)₂{^T(1−5)(AsO₄)₁₀^T’[☐_0.75(AsO₄)_0.25]₂} (Z = 1). Topologically the same pseudo-framework forms the crystal structure of the synthetic orthorhombic (space group Imam) modification of Fe³⁺AsO₄ (Figure 4d–f) with the following unit-cell dimensions: a = 13.468(2), b = 6.5255(11), c = 10.7678(18) Å, V = 946.3(3) ų and Z = 12 [13,14]. The unit-cell parameters of this compound and achyrophanite are very close. As it is shown in Figure 4, the structure of achyrophanite is in fact a stuffed derivative of the structure of this modification of Fe³⁺AsO₄. The porous character of the latter was emphasized by Bazán et al. [14], who reported it as “the new textural porous orthorhombic Fe(AsO₄) phase”. Indeed, the pseudo-framework contains two systems of broad channels. In the synthetic Fe³⁺AsO₄, these channels are empty (Figure 4d,f), whereas in achyrophanite, they are stuffed by A cations (Figure 4a–c). This is accompanied by the transition from a non-defect, full-cationic heteropolyhedral pseudo-framework ^MFe³⁺₁₂(^TAsO₄)₁₂ (Z = 1) of the orthorhombic Fe³⁺AsO₄ to the pseudo-framework (M_10.5☐_1.5)_Σ12O₃[(AsO₄)_10.5☐_1.5]_Σ12 (Z = 1) with vacancy defects in both T and M positions in achyrophanite.

Besides the orthorhombic modification of Fe³⁺AsO₄ [13,14], a similar octahedral-tetrahedral framework was found in α-Cr³⁺PO₄ [15,16,17], α-Cr³⁺AsO₄ [17], and Rh³⁺PO₄ [18].

Thus, achyrophanite demonstrates a novel structure type: stuffed, with the defect heteropolyhedral pseudo-framework derivative of the orthorhombic Fe³⁺AsO₄ archetype.

The general topology of the heteropolyhedral M-T-O pseudo-framework in achyrophanite corresponds to the space group Imma (in standard setting) as well as one in the synthetic Fe³⁺AsO₄—[13,14]. However, our attempt to refine the structure of achyrophanite in this space group was not successful. The symmetry lowering to space group P222₁ found for achyrophanite is caused by significantly different occupancies of several cationic sites in the M-T-O pseudo-framework. Perhaps it is related to the presence of large-cation sites in the mineral in contrast with Fe³⁺AsO₄.

Achyrophanite is microtwinned: twinning by merohedry Class I [19] with the twin domains ratio 53/47 was found in the studied crystal. Probably the symmetric relationship between the structure of achyrophanite and the archetype structure of the orthorhombic Fe³⁺AsO₄ is a cause of this merohedral twinning.

In unit-cell dimensions, achyrophanite and the orthorhombic Fe³⁺AsO₄ are close to katiarsite, KTiO(AsO₄), a mineral recently discovered in the same Arsenatnaya fumarole [20]. Katiarsite is an analogue of the well-studied synthetic compound named KTA, which belongs to the well-known KTP [KTiO(PO₄)] structure type. The crystal structure of KTA is based on undulating chains of corner-linked TiO₆ octahedra. These chains are cross-linked by isolated AsO₄ tetrahedra to form a three-dimensional mixed framework containing K⁺ cations in channels [21,22,23]. However, the structures of achyrophanite, Fe³⁺AsO₄, and KTP-type compounds, including katiarsite, are quite different, although the undulating octahedral chains of the same configuration could be identified in the octahedral pseudo-framework of achyrophanite. Single octahedral chains in KTA lie in the bc plane and are parallel to [011] and [01-1] (Figure 6a); in achyrophanite such chains lying in the ac plane are interconnected (Figure 6b). These structural units are presumably responsible for the similarity of the a and c unit-cell parameters of achyrophanite with the b and c, respectively, of KTA. In achyrophanite there are two such layer fragments formed by octahedral chains and AsO₄ tetrahedra per unit cell (Figure 6d). In KTA there are also two separate (100) “layers” containing the octahedral chains per unit cell (Figure 6c). These peculiarities determine the analogy of the b unit-cell parameter of achyrophanite (13.249 Å) with the a parameter of KTA (13.174 Å). The arrangement of AsO₄ tetrahedra is also close in both achyrophanite and KTA (Figure 7). It is worthy to note that orthorhombic Fe³⁺AsO₄ was obtained from the synthetic (NH₄)[Fe³⁺(AsO₄)F] as the result of its heating in air at 525 °C. (NH₄)[Fe³⁺(AsO₄)F] is related to KTA in stoichiometry, space group, unit-cell dimensions, and the structure of the heteropolyhedral Fe-As-O-F framework [13]. Their comparison highlights the relationship between all the above-discussed compounds.

Table 7. Unit-cell dimensions of achyrophanite and structurally related compounds.

Mineral/ Compound	Achyrophanite	Synthetic Fe3+AsO4	Katiarsite KTiO(AsO4)	Synthetic KTiO(AsO4) (KTA)	Synthetic (NH4)Fe3+(AsO4)F
Sp. gr. Unit cell a (Å) b (Å) c (Å) V (Å3)	P2221 6.5824(2) 13.2488(4) 10.7613(3) 938.48(5)	Imam 13.468(2) 6.525(1) 10.768(2) 946.3(3)	Pna21 13.174(4) 6.5635(10) 10.805(2) 934.3(3)	Pna21 13.1185(5) 6.5656(2) 10.7804(3) 928.5	Pna21 13.270(2) 6.629(1) 10.866(1) 955.8(2)
Reference	This work	[13,14]	[20]	[22]	[13,14]

presents unit-cell dimensions of achyrophanite and these structurally related compounds.

5.3. Genetic Notes

Achyrophanite is a fumarolic mineral crystallized at temperatures not lower than 430–450 °C. It was deposited directly from the gas phase as a volcanic sublimate or, more probably, formed as a result of the interaction between fumarolic gas and basalt scoria. The latter could be a source of Mg, Al, and Ti, which have low volatilities in such post-volcanic systems at temperatures up to 400–500 °C [24,25].

Evolution of gas composition in the fumarole mineral-forming system results in alteration of achyrophanite. We observed partial or complete pseudomorphs of arsenatrotitanite, ideally NaTiO(AsO₄), sometimes together with bradaczekite NaCu₄(AsO₄)₃, after crystals of achyrophanite (Figure 8). They are associated with sylvite, halite, tenorite, hematite, lehmannite, badalovite, johillerite, rutile, and pseudobrookite. Gas evolution is probably accompanied by Na activity increase.

The discovery of achyrophanite seems interesting firstly from the crystal chemical viewpoint: this mineral demonstrates an original structure type and unusual cation substitution schemes that are probably caused by its origin—in a volcanic fumarole system with non-stationary physical and chemical parameters.