A New Mineral Ferrisanidine, K[Fe 3 + Si 3 O 8 ], the First Natural Feldspar with Species-Deﬁning Iron

: Ferrisanidine, K[Fe 3 + Si 3 O 8 ], the ﬁrst natural feldspar with species-deﬁning iron, is an analogue of sanidine bearing Fe 3 + instead of Al. It was found in exhalations of the active Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Fissure Tolbachik Eruption, Tolbachik volcano, Kamchatka Peninsula, Russia. The associated minerals are aegirine, cassiterite, hematite, sylvite, halite, johillerite, arsmirandite, axelite, aphthitalite. Ferrisanidine forms porous crusts composed by cavernous short prismatic crystals or irregular grains up to 10 µ m × 20 µ m. Ferrisanidine is transparent, colorless to white, the lustre is vitreous. D calc is 2.722 g · cm − 3 . The chemical composition of ferrisanidine (wt. %, electron microprobe) is: Na 2 O 0.25, K 2 O 15.15, Al 2 O 3 0.27, Fe 2 O 3 24.92, SiO 2 60.50, in total 101.09. The empirical formula calculated based on 8 O apfu is (K 0.97 Na 0.03 ) Σ 1.00 (Si 3.03 Fe 3 + 0.94 Al 0.02 ) Σ 3.99 O 8 . The crystal structure of ferrisanidine was studied using the Rietveld method, the ﬁnal R indices are: R p = 0.0053, R wp = 0.0075, R 1 = 0.0536. Parameters of the monoclinic unit cell are: a = 8.678(4), b = 13.144(8), c = 7.337(5) Å, β = 116.39(8) ◦ , V = 749.6(9) Å 3 . Space group is C 2 / m . The crystal structure of ferrisanidine is based on the sanidine-type “ferrisilicate” framework formed by disordered [SiO 4 ] and [Fe 3 + O 4 ] tetrahedra.


Introduction
High content of iron is extremely rare for feldspar-group minerals. In most cases, the content of Fe 2 O 3 in natural feldspars is not higher than 3 wt. % and all Fe-rich samples of minerals of this group belong to potassic feldspars [1,2]. The K-feldspar with distinct content of Fe 2 O 3 (2.5-3.0 wt. %) was first described by Alfred Lacroix in 1912 from granitic pegmatites at Madagascar. Its optical properties are close to those of sanidine [3]. Fe-enriched (6.3 wt. % Fe 2 O 3 ) potassic feldspar was described from manganese-rich skarns of the famous Långban deposit, Filipstad, Sweden [4]. "Sanidine" with 13.7 wt. % Fe 2 O 3 and the empirical formula (K 0.95 Na 0.03 )[Si 3.08 Fe 3+ 0.50 Al 0. 40 O 8 ] was reported by [5] from lamproites of Cancarix, Spain. Other researchers [6] published data on zoned crystals of sanidine with zones containing up to 18.4 wt. % Fe 2 O 3 from leucite-and sanidine-bearing lavas of Leucite Hills, Wyoming, USA. The empirical formula of the most Fe-rich zone in these crystals is (K 0.96 Na 0.04 ) Σ1.00 [(Si 3.02 Fe 3+ 0.70 Al 0.20 Mg 0.05 Ti 0.03 ) Σ4.00 O 8 ]. In both these cases, an Fe-dominant (Fe > Al in atom proportions) feldspar occurs as thin rims on sanidine crystals. However, no X-ray diffraction (XRD) data for natural samples of Fe-dominant feldspars were published and, therefore, nothing on crystallography of these minerals and the distribution of Si, Al and Fe in their crystal structures is known.
In this paper, we describe a natural feldspar chemically close to KFe 3+ Si 3 O 8 found in fumarole sublimates at the Tolbachik volcano, Kamchatka, Russia. This new mineral was named ferrisanidine as an analogue of sanidine with Fe 3+ instead of Al. Both the new mineral and its name have been approved by the IMA Commission on New Minerals, Nomenclature and Classification, IMA No. 2019-052. The type specimen is deposited in the collection of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, with catalogue number 96732.

Background Information: Crystal Structure and Nomenclature of Potassic Feldspars
Feldspar-type crystal structures are well-studied. Crystal chemistry of feldspars is described in many publications including several handbooks [1,2, [14][15][16]. The feldspar structural archetype unites compounds with the general formula AT 4 O 8 . Until the present work, the species-defining constituents in valid feldspar-group minerals were as follows: tetrahedrally coordinated T = Si, Al, B, and As 5+ and extra-framework cations A = Na, K, Rb, NH 4 , Ca, Sr, and Ba.
The crystal structure of feldspars is based on a three-dimensional framework composed by corner-sharing TO 4 tetrahedra. The main structural unit is a chain of four-membered rings, called as "double crankshaft". These rings consist of tetrahedra T(1) and T(2) (in ordered species, T10, T1m, T20 and T2m). The double crankshafts repeat throughout space by c-translations and are linked with one another as shown in Figure 1.
Minerals 2019, 9, x FOR PEER REVIEW 2 of 18 Al in atom proportions) feldspar occurs as thin rims on sanidine crystals. However, no X-ray diffraction (XRD) data for natural samples of Fe-dominant feldspars were published and, therefore, nothing on crystallography of these minerals and the distribution of Si, Al and Fe in their crystal structures is known. Unlike natural Fe-rich feldspars, synthetic ferri-feldspars are well-studied. The first experimental data revealing the possibility of Fe 3+ presence in the feldspar-type crystal structure were obtained by [7], who synthesized so-called Fe-orthoclase. Later the investigation of the KAlSi3O8-KFe 3+ Si3O8 solid-solution series and transitions between monoclinic and triclinic forms of K[Fe 3+ Si3O8] were the focus of many studies [8][9][10][11][12][13].
In this paper, we describe a natural feldspar chemically close to KFe 3+ Si3O8 found in fumarole sublimates at the Tolbachik volcano, Kamchatka, Russia. This new mineral was named ferrisanidine as an analogue of sanidine with Fe 3+ instead of Al. Both the new mineral and its name have been approved by the IMA Commission on New Minerals, Nomenclature and Classification, IMA No. 2019-052. The type specimen is deposited in the collection of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, with catalogue number 96732.

Background Information: Crystal Structure and Nomenclature of Potassic Feldspars
Feldspar-type crystal structures are well-studied. Crystal chemistry of feldspars is described in many publications including several handbooks [1,2, [14][15][16]. The feldspar structural archetype unites compounds with the general formula AT4O8. Until the present work, the species-defining constituents in valid feldspar-group minerals were as follows: tetrahedrally coordinated T = Si, Al, B, and As 5+ and extra-framework cations A = Na, K, Rb, NH4, Ca, Sr, and Ba.
The crystal structure of feldspars is based on a three-dimensional framework composed by corner-sharing TO4 tetrahedra. The main structural unit is a chain of four-membered rings, called as "double crankshaft". These rings consist of tetrahedra T(1) and T(2) (in ordered species, T10, T1m, T20 and T2m). The double crankshafts repeat throughout space by c-translations and are linked with one another as shown in Figure 1.  The extra-framework A cations are located in the interstices, on the (010) mirror plane in monoclinic feldspars or near the plane in triclinic feldspars. All feldspars have topological monoclinic symmetry C2/m which can be lowered to C1 or I2/c in the fully T-ordered species.
Ordering at tetrahedral sites T causes an appearance of several polymorphs of potassic feldspar, ideally K[AlSi 3 O 8 ]. Sanidine (C2/m) is a disordered species; if the Al content at T(1) is < 0.50 atoms per formula unit (apfu), the term high sanidine is used, while the variety with Al content at T(1) between 0.50 and 0.75 apfu is named low sanidine. In orthoclase, a pseudomonoclinic (pseudo-symmetry C2/m) due to microtwinning [with microtwinned triclinic (C1) domains] polymorph, the Al content at T(1) is > 0.75 apfu. Microcline (triclinic, C1) possesses an ordered structure with Al content at T10 + T1m in the range of 0.75-1.00 apfu [1, 2,14,16,17] Occurrence The specimens with the new mineral were collected by us in July 2018 from the active Arsenatnaya fumarole situated at the summit of the Second scoria cone of the Northern Breakthrough of the Great Fissure Tolbachik Eruption of 1975-1976 (NB GFTE). This scoria cone is a monogenetic volcano about 300 m high and approximately 0.1 km 3 in volume that was formed in 1975 [18]. It is located 18 km SSW of the Ploskiy Tolbachik volcano in the central part of Kamchatka Peninsula, Far-Eastern Region, Russia. The description of the Arsenatnaya fumarole has been published by [19].

Methods
The chemical composition of ferrisanidine was studied using electron microprobe. The analyses were carried out with a JEOL JXA-8230 instrument (WDS and EDS modes) at the Laboratory of Analytical Techniques of High Spatial Resolution, Dept. of Petrology, Moscow State University, Moscow, Russia. Standard operating conditions included an accelerating voltage of 20 kV and beam current of 10 nA, beam was rastered on an area of 4 µm × 4 µm. The data reduction was carried out by means of an INCA Energy 300 software package. The following standards were used for quantitative analysis: potassic feldspar (K), albite (Na), anorthite (Al and Si) and Fe (Fe). Contents of other elements with atomic numbers higher than carbon are below detection limits.
The Raman spectrum of ferrisanidine was recorded using an EnSpectr R532 spectrometer (Enhanced Spectrometry, Inc., Torrance, CA, USA) with a green laser (532 nm) at room temperature. The output power of the laser beam was about 7 mW. The spectrum was processed using the EnSpectr expert mode program (version PRO, Enhanced Spectrometry, Inc., Torrance, CA, USA) in the range from 100 to 4000 cm −1 with the use of a holographic diffraction grating with 1800 lines/cm and a resolution equals about 6 cm −1 . The diameter of the focal spot on the sample was about 10 µm. The Raman spectrum was acquired on a polycrystalline sample.
Single-crystal XRD studies of ferrisanidine could not be performed because of small size and imperfection (splitting and sponginess) of its crystals. The powder XRD data was obtain on the crushed small piece of ferrisanidine-aegirine aggregate (0.1-0.2 mm) using a Rigaku R-AXIS Rapid II diffractometer (Rigaku Corporation, Tokyo, Japan) equipped with a cylindrical image plate detector and rotating anode with the microfocus optics (Debye-Scherrer geometry; d = 127.4 mm; CoKα radiation). The raw powder XRD data were collected using the osc2xrd program suite designed by [22]. Calculated intensities were obtained by means of the STOE WinXPOW program (Version 2.08, STOE & Cie GmbH, Darmstadt, Germany) suite based on the atomic coordinates and unit-cell parameters.

Morphology, Physical Properties and Optical Data
Ferrisanidine forms porous, sometimes broken crusts up to 0.1 mm across and up to 20 µm thick composed by crude, cavernous short prismatic crystals (Figure 2a,b) or irregular in shape grains up to 10 µm × 20 µm. Another morphological variety of ferrisanidine ( Figure 3) is represented by clusters of prismatic slightly split porous crystals overgrowing fine-grained cassiterite crusts. Ferrisanidine also occurs as thin single-crystal crusts epitaxially overgrowing crystals of sanidine ( Figure 4).

Morphology, Physical Properties and Optical Data
Ferrisanidine forms porous, sometimes broken crusts up to 0.1 mm across and up to 20 μm thick composed by crude, cavernous short prismatic crystals (Figure 2a,b) or irregular in shape grains up to 10 μm × 20 μm. Another morphological variety of ferrisanidine ( Figure 3) is represented by clusters of prismatic slightly split porous crystals overgrowing fine-grained cassiterite crusts. Ferrisanidine also occurs as thin single-crystal crusts epitaxially overgrowing crystals of sanidine ( Figure 4).

Morphology, Physical Properties and Optical Data
Ferrisanidine forms porous, sometimes broken crusts up to 0.1 mm across and up to 20 μm thick composed by crude, cavernous short prismatic crystals (Figure 2a,b) or irregular in shape grains up to 10 μm × 20 μm. Another morphological variety of ferrisanidine ( Figure 3) is represented by clusters of prismatic slightly split porous crystals overgrowing fine-grained cassiterite crusts. Ferrisanidine also occurs as thin single-crystal crusts epitaxially overgrowing crystals of sanidine ( Figure 4).    Ferrisanidine is transparent, colorless to white, with white streak. The lustre is vitreous. The mineral is brittle, perfect cleavage typical for feldspars was observed under the scanning electron microscope. Hardness and density could not be measured because crystals are tiny and aggregates are porous. The density value calculated from the empirical formula is 2.722 g·cm −3 .
Optical characteristics of ferrisanidine could not be correctly measured because of small size and porous character of crystals (Figures 2a,b and 5). Here, we give the data reported for the synthetic analogue of ferrisanidine by [8]. The superior, very low value of the Gladstone-Dale compatibility index [23] for ferrisanidine calculated using these refractive indices [1 − (Kp/Kc) = 0.002] indicates that these optical data correspond to the mineral well. The synthetic analogue of ferrisanidine is biaxial (-), α = 1.584(1), β = 1.595(1), γ = 1.605(1) (589 nm), 2V (meas.) = 85°, 2V(calc.) = 86.5°. Dispersion of optical axes was not observed. Orientation: Y = b, Z^c = 16(4)° [8]. Ferrisanidine is colorless and nonpleochroic under the microscope.  Ferrisanidine is transparent, colorless to white, with white streak. The lustre is vitreous. The mineral is brittle, perfect cleavage typical for feldspars was observed under the scanning electron microscope. Hardness and density could not be measured because crystals are tiny and aggregates are porous. The density value calculated from the empirical formula is 2.722 g·cm −3 .
Optical characteristics of ferrisanidine could not be correctly measured because of small size and porous character of crystals (Figure 2a,b and Figure 5). Here, we give the data reported for the synthetic analogue of ferrisanidine by [8]. The superior, very low value of the Gladstone-Dale compatibility index [23] for ferrisanidine calculated using these refractive indices [1 − (K p /K c ) = 0.002] indicates that these optical data correspond to the mineral well. The synthetic analogue of ferrisanidine is biaxial  Ferrisanidine is transparent, colorless to white, with white streak. The lustre is vitreous. The mineral is brittle, perfect cleavage typical for feldspars was observed under the scanning electron microscope. Hardness and density could not be measured because crystals are tiny and aggregates are porous. The density value calculated from the empirical formula is 2.722 g·cm −3 .

Chemical Data
Ferrisanidine possesses a rather stable chemical composition (Table 1).

Raman Spectroscopy
The Raman spectra of ferrisanidine, sanidine, orthoclase and microcline have some common features but significantly differ from each other in number of bands and their positions (wavenumbers) [24,25] (Figure 6).

Raman Spectroscopy
The Raman spectra of ferrisanidine, sanidine, orthoclase and microcline have some common features but significantly differ from each other in number of bands and their positions (wavenumbers) [24,25] (Figure 6). Figure 6. The Raman spectra of ferrisanidine from the Arsenatnaya fumarole (a) and sanidine from the extinct fumaroles of the Mountain 1004 (Tolbachik volcano) described in [21] (b); the Raman spectra of orthoclase and microcline given by [25] (c).
According to calculations, results of group analyses and vibrational studies undertaken by [24][25][26], there are several types of bands in the Raman spectra of feldspar-group minerals: Group I (450-520 cm −1 )-strongest bands which belong to the ring-breathing modes of four-membered tetrahedral Figure 6. The Raman spectra of ferrisanidine from the Arsenatnaya fumarole (a) and sanidine from the extinct fumaroles of the Mountain 1004 (Tolbachik volcano) described in [21] (b); the Raman spectra of orthoclase and microcline given by [25] (c).
According to calculations, results of group analyses and vibrational studies undertaken by [24][25][26], there are several types of bands in the Raman spectra of feldspar-group minerals: Group I (450-520 cm −1 )-strongest bands which belong to the ring-breathing modes of four-membered tetrahedral rings; Group II (<300 cm −1 )-bands corresponding to rotation-translation modes of the four-membered ring; Group III (300-450 cm −1 )-cage-shear modes; Group IV (700-900 cm −1 )-deformation modes of the tetrahedra; Group V (900-1200 cm −1 )-vibrational stretching modes of the tetrahedra. Wavenumbers of distinct bands in the Raman spectrum of ferrisanidine ( Figure 6a) are (cm −1 , s-strong band, sh-shoulder): 138s, 162sh, 303s, 386, 446s, 489s, 524sh, 597, 628, 968s, 1090. The shift of some bands in the Raman spectrum of ferrisanidine in the low-frequency range and high intensity of vibrational stretching bands of the tetrahedra united in four-membered rings (Group V), in comparison with the spectrum of sanidine (Figure 6b), are due to the presence of Fe 3+ instead of Al in tetrahedrally coordinated sites of framework. The absence of strong bands related to the various modes of the Fe 3+ O 4 tetrahedron (for example, at 730 cm −1 ) in the shown spectra of ferrisanidine may be due to the orientation of crystals in aggregate of the studied sample.

Crystal Structure
The crystal structure of ferrisanidine was refined on a powder sample using the Rietveld method. Data treatment and the Rietveld structure analysis were carried out with the JANA software (version 2006, Institute of Physics, Praha, Czech Republic) [27]. A total of 13701 observed intensity envelope points were used in the refinement. The profiles of individual reflections were modeled using a Pseudo-Voigt function. The refinement of the crystal structure of ferrisanidine was complicated by the presence of admixed aegirine in the studied powder sample (Figure 7). Minerals 2019, 9, x FOR PEER REVIEW 7 of 18 rings; Group II (<300 cm −1 )-bands corresponding to rotation-translation modes of the fourmembered ring; Group III (300-450 cm −1 )-cage-shear modes; Group IV (700-900 cm −1 )-deformation modes of the tetrahedra; Group V (900-1200 cm −1 )-vibrational stretching modes of the tetrahedra. Wavenumbers of distinct bands in the Raman spectrum of ferrisanidine ( Figure 6a) are (cm −1 , sstrong band, sh-shoulder): 138s, 162sh, 303s, 386, 446s, 489s, 524sh, 597, 628, 968s, 1090. The shift of some bands in the Raman spectrum of ferrisanidine in the low-frequency range and high intensity of vibrational stretching bands of the tetrahedra united in four-membered rings (Group V), in comparison with the spectrum of sanidine (Figure 6b), are due to the presence of Fe 3+ instead of Al in tetrahedrally coordinated sites of framework. The absence of strong bands related to the various modes of the Fe 3+ O4 tetrahedron (for example, at 730 cm −1 ) in the shown spectra of ferrisanidine may be due to the orientation of crystals in aggregate of the studied sample.

Crystal Structure
The crystal structure of ferrisanidine was refined on a powder sample using the Rietveld method. Data treatment and the Rietveld structure analysis were carried out with the JANA software (version 2006, Institute of Physics, Praha, Czech Republic) [27]. A total of 13701 observed intensity envelope points were used in the refinement. The profiles of individual reflections were modeled using a Pseudo-Voigt function. The refinement of the сrystal structure of ferrisanidine was complicated by the presence of admixed aegirine in the studied powder sample (Figure 7).  Tables 5 and 6) and aegirine [28] (Phase #1) were used. Black graph is the experimental pattern of the mixture consisting of ferrisanidine and aegirine. The difference between the observed and calculated patterns is shown as a curve in the bottom part of the figure.
The most indicative reflections for both phases belong to the 2θ range 7.8-60° (CoKα radiation). The refined fraction of each phase, ferrisanidine and aegirine, in the mixture is 50%. As the initial model for the structure refinement of ferrisanidine we used the dataset on low AlFe-sanidine obtained by [12]. Unit-cell parameters of aegirine were refined using the structure model published by [28].
The comparative table including powder XRD patterns of ferrisanidine, its synthetic analogue and sanidine is given ( Table 2).  Tables 5 and 6) and aegirine [28] (Phase #1) were used. Black graph is the experimental pattern of the mixture consisting of ferrisanidine and aegirine. The difference between the observed and calculated patterns is shown as a curve in the bottom part of the figure.
The most indicative reflections for both phases belong to the 2θ range 7.8-60 • (CoKα radiation). The refined fraction of each phase, ferrisanidine and aegirine, in the mixture is 50%. As the initial model for the structure refinement of ferrisanidine we used the dataset on low AlFe-sanidine obtained by [12]. Unit-cell parameters of aegirine were refined using the structure model published by [28].
The comparative table including powder XRD patterns of ferrisanidine, its synthetic analogue and sanidine is given ( Table 2).  Data collection information and crystal structure refinement details for ferrisanidine are given in Table 3. Table 3. Crystal data and Rietveld refinement details for ferrisanidine.

Site
x/a y/b z/c * U eq Q Site Occupancy Selected interatomic distances in the ferrisanidine structure compared with data for synthetic low AlFe-sanidine [12] are presented in Table 5.

Discussion
The primary goal of the crystal structure study of our new mineral was to identify the polymorph of K[Fe 3+ Si 3 O 8 ].
The XRD study clearly demonstrates that our mineral is much closer to the most disordered potassic feldspar, sanidine rather to microcline. The comparison of calculated powder XRD patterns of ferrisanidine and microcline [21] is shown in Figure 8. The characteristic reflections that allow distinguishing these two minerals belong to the 2θ range 15-40 • . In the powder XRD pattern of sanidine three the most important reflections inherent to microcline are absent, namely (d, Å-I, %; intensities I are given for CoKα) 1-11 (5.926-5), 1-30 (3.705-46) and 1-31 (2.954-41).   Analysis of both powder XRD and Raman spectroscopic data shows that our new mineral from Tolbachik could not be considered as Fe-dominant analogue of orthoclase. The reasons are as follows: (1) the refined numbers of electrons in the T sites and T-O distances in tetrahedra show that Si and Fe 3+ are disordered with Fe 3+ at the T1 site <0.75, that corresponds to sanidine but not to orthoclase; (2) the Raman spectrum of orthoclase includes a well-resolved triplet in the region of 450-520 cm −1 , split bands in the 280-285 cm −1 region and resolved bands in the 745-820 cm −1 region (Figure 6), whereas the spectrum of ferrisanidine is closer to that of sanidine, which does not demonstrate such splitting of bands (Figure 6a). The comparison of sanidine and ferrisanidine is given in Table 6.  The species diversity in the feldspar group is due mainly to the substitutions in the extra-framework cation sites and different schemes of Al/Si-ordering at tetrahedrally coordinated sites of the framework. Until this work, only two valid minerals of the feldspar group with species-defining tetrahedral components except of Si and Al were known, namely, the borosilicate reedmergnerite Na[B 3+ Si 3 O 8 ] [32] and the alumino-arsenate filatovite K[(Al,Zn) 2 (As,Si) 2 O 8 ] [33]. They are both very rare and known from few specific geological formations. Reedmergnerite occurs in dolomitic oil shales of the Green River formation in Utah, USA [32,34] and in peralkaline pegmatites of two alkaline complexes, Dara-i-Pioz in Tajikistan [4] and Lovozero at Kola peninsula, Russia [35]. Filatovite and intermediate members of the sanidine-filatovite solid-solution series are known only from sublimates of fumaroles at the Second scoria cone of the NB GFTE including the Arsenatnaya fumarole [20,33] Different substitutions at framework sites are known in synthetic feldspars: Ga 3+ (r i = 0.47 Å), B 3+ (r i = 0.11 Å) and Fe 3+ (r i = 0.49 Å) substitute Al (r i = 0.39 Å) whereas Ge 4+ (r i = 0.39 Å), P 5+ (r i = 0.17 Å) and As 5+ (r i = 0.34 Å) substitute Si (r i = 0.26 Å) (the ionic radii are taken from [36]). These compounds were the focus of many studies. A lot of works were devoted to the various aspects of these phases: ways of synthesis, phase transitions, crystal chemical characteristics and physical properties [4,13,37,38]. However, the geochemistry of Ga and Ge do not facilitate an appearance of such feldspars in nature. Natural feldspars with species-defining P are also unknown; this component was reported in minerals of this group only as impurity: up to 5.0 wt. % P 2 O 5 [21,[39][40][41][42][43][44].
The crystal data of natural and synthetic Fe-rich sanidine-like feldspars are shown in Table 7. The unit-cell parameters of natural Fe-rich sanidine from Leucite Hills (Table 7) were obtained by selected-area electron diffraction (SAED) method [6]. The increase of Fe 3+ content at tetrahedral sites affects the corresponding increase of all unit-cell parameters in the series K[AlSi 3 O 8 ]-K[Fe 3+ Si 3 O 8 ], which is clearly revealed on the powder XRD patterns in the increase of d values in this series (Table 2). This is the result of a substitution of Al for Fe 3+ . Ferrisanidine is the only mineral in the feldspar group with a tetrahedrally coordinated component larger than Al 3+ . The Si-O-Al connections in ferrisanidine are replaced by the Fe 3+ -O-Si ones and in addition the Fe 3+ -O-Fe 3+ connections can appear, because of the Si/Fe 3+ disorder at tetrahedral sites. It is known that feldspar-type crystal structures have a quite rigid framework. It constrains the free rotation of tetrahedra necessary for relaxation the strains due to the incorporation of different components (extra-framework and framework) [45]. One of the main indicators of the strain in silicates, e.g., feldspar-type crystal structures, is the average T-O-T angle, which is close to 140 • for relaxed Si-O-Si [45]. This value slightly differs from the ideal one in different potassic feldspars, both natural and synthetic, with various tetrahedrally coordinated components. In sanidine the average T-O-T angle is 141.  [47] and in ferrisanidine-137.5 • . Both these feldspars have the largest unit-cell volumes: 801.7 and 749.6 Å 3 , respectively. The strain in ferrisanidine and its synthetic analogues is caused by the more considerable mismatch of Fe 3+ and Si size, than Al and Si, occupying the same sites. This is also the trigger for the highest ordering rate in K[Fe 3+ Si 3 O 8 ] than in K[AlSi 3 O 8 ]. As it was reported by [9], the phase transition from monoclinic K[Fe 3+ Si 3 O 8 ] to triclinic polymorph happens at a temperature about 704 ± 6 • C, which is 200 • C higher than for phase transition in K[AlSi 3 O 8 ]. Moreover, the ordering kinetics in K[Fe 3+ Si 3 O 8 ] is higher, than in synthetic K[GaSi 3 O 8 ] [13]. The "conservation" of disordered K[Fe 3+ Si 3 O 8 ] in nature is rather surprising and proposed to be a quenched phase. We believe that ferrisanidine, similar to other silicates found in Tolbachik fumaroles, was deposited directly from the fumarolic gas as a volcanic sublimate at temperature range 500-700 • C [20].
The conditions of formation of silicates in Arsenatnaya and other oxidizing-type Tolbachik fumaroles (atmospheric pressure, temperature range 500-700 • C and transportation of chemical components by hot gas) are non-typical for geological settings ( [19,20]. As our data show, sanidine, including its As 5+ -bearing varieties, intermediate members of the sanidine-filatovite series, is a common mineral in fumarolic deposits of the Tolbachik volcano. However, the sanidine-filatovite series feldspars are usually Fe-poor: <0.6 wt. % Fe 2 O 3 . Only in several specimens up to 3.4 wt. % Fe 2 O 3 was detected [20]. Thus, no continuous solid-solution series between ferrisanidine and sanidine was found in the Tolbachik fumaroles. Moreover, the epitaxy of ferrisanidine on sanidine crystals ( Figure 4) is an evident sign of their sequential crystallization with a break. The appearance of ferrisanidine K[Fe 3+ Si 3 O 8 ], containing <0.3 wt. % Na 2 O, in close association with aegirine NaFe 3+ [Si 2 O 6 ] could be an indirect evidence of the instability of ferrious feldspars with higher content of sodium. The phase with chemical composition NaFe 3+ Si 3 O 8 is known only as a glass ( [48,49]. The cause for preferable crystallization of potassic ferri-feldspar rather than sodic ferri-feldspar NaFe 3+ Si 3 O 8 probably has a crystal chemical nature. The framework composed of Fe 3+ O 4 and SiO 4 tetrahedra is characterized by the significant elongation of T-O bonds and distortion of angles. This affects the A-O distances and, in general, the oxygen surrounding of extra-framework A cation. The Na + cation is smaller than K + and such irregular and distorted surrounding as it is not compatible with the feldspar structure with [Fe 3+ Si 3 O 8 ] − framework. Author Contributions: N.V.S. and I.V.P. wrote the paper. N.V.S. carried out the crystal structure analysis. N.V.S. and S.N.B. obtained and processed X-ray diffraction data. N.N.K. and M.F.V. obtained and processed electron microprobe and Raman spectroscopic data. I.V.P and E.G.S. collected and prepared samples.
Funding: This work was supported by the Russian Science Foundation, grant No. 19-17-00050.