Unusual silicate mineralization in fumarolic sublimates of the Tolbachik volcano, Kamchatka, Russia – Part 1: Neso-, cyclo-, ino- and phyllosilicates

This is the initial paper in a pair of articles devoted to silicate minerals from fumaroles of the Tolbachik volcano (Kamchatka, Russia). These papers contain the first systematic data on silicate mineralization of fumarolic genesis. In this article nesosilicates (forsterite, andradite and titanite), cyclosilicate (a Cu,Znrich analogue of roedderite), inosilicates (enstatite, clinoenstatite, diopside, aegirine, aegirine-augite, esseneite, “Cu,Mg-pyroxene”, wollastonite, potassic-fluoro-magnesio-arfvedsonite, potassic-fluoro-richterite and litidionite) and phyllosilicates (fluorophlogopite, yanzhuminite, “fluoreastonite” and the Sn analogue of dalyite) are characterized with a focus on chemistry, crystal-chemical features and occurrence. Unusual As5+-rich varieties of forsterite, andradite, titanite, pyroxenes, amphiboles and mica are described. General data on silicate-bearing active fumaroles and the diversity and distribution of silicates in fumarole deposits are reported. Evidence for the fumarolic origin of silicate mineralization is discussed.


Introduction
Active volcanic fumaroles can be considered natural laboratories which make it possible to study in situ the processes of mineral formation, geochemical behaviour and migration of many chemical elements.
Despite abundant data on minerals from volcanic fumaroles, silicate mineralization formed in these systems was characterized very scarcely. In particular, this might be due mainly to the proper identification of the origin of silicates from fumarolic deposits (including old, extinct fumaroles).
Even if silicates are in fumarolic vents, they are typically early minerals, which form crusts underlying "classic" sublimate incrustations that include sulfates, halides, oxides, etc. This fact hampers the reliable determination of genesis of silicates and requires an answer to the following question: are they formed with a participation of fumarolic gas or not? Another reason for the difficult identification of the origin of silicates in such mineral-forming systems is the absence of chemical criteria in literature data. Indicative impurities could be geochemical markers showing that a silicate mineral was deposited from the gas phase or, at least, crystallized in the system involving fumarolic gas.
It came as a surprise to find a rich and diverse (31 mineral species) silicate mineralization in sublimates of fumaroles related to the Tolbachik volcano. The majority of these minerals was unusual, remarkable in both chemical and crystal-chemical aspects. In the present paper and the companion article we describe these silicates and thus report the first systematic characteristics of silicate mineralization formed in fumaroles at the active volcano. We believe that diversity and chemical originality of silicates are caused by strongly oxidizing conditions of mineral formation and distinct "ore" geochemical specialization of the Tolbachik fumaroles (Pekov et al., 2018a). In the present paper we report data on neso-, cyclo-, ino-and phyllosilicates; the next paper will be devoted to the great variety of tecto-aluminosilicates. The discussion and conclusions concerning fumarolic silicate mineralization in general are given in the companion paper .

Location
The Tolbachik volcano belongs to the active Klyuchevskaya volcanic group, the greatest from the Kurilo-Kamchatsky volcanic belt. It is located at the central part of the Kamchatka peninsula. In addition to Tolbachik, this group includes two active volcanoes, Klyuchevskoy and Bezymiannyi, and several extinct volcanoes. All of them appeared dur-ing the Quaternary a few hundred thousand years ago. The Tolbachik volcanic massif consists of the extinct andesitic volcano Ostryi Tolbachik and active basaltic volcano Ploskiy Tolbachik (Fedotov and Markhinin, 1983).
The widest diversity of silicates was found in the Arsenatnaya fumarole situated at the summit of the second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption 1975-1976. This cone is a monogenetic volcano about 300 m high and approximately 0.1 km 3 in volume located 18 km SSW of Ploskiy Tolbachik. Its formation started on 9 August 1975 and was completed on 15 September 1975 (Fedotov and Markhinin, 1983). Arsenatnaya is an active fumarole discovered by the authors in July 2012. This fumarole is a linear system (about 15 m long and up to 4 m wide) of mineralized vents located in the interval from 0.3 to 4 m depth under the ground. The temperature inside the vents measured by the authors using a chromel-Alumel thermocouple in 2012-2018 varies from 360 to 490 • C and, in general, increases with depth. Arsenatnaya is the brightest example in the world of the strongly mineralized oxidizing-type fumarole. It was characterized in detail by Pekov et al. (2014Pekov et al. ( , 2018a. Some silicates were also found in other fumaroles located at the second scoria cone of the NB GTFE, including the famous Yadovitaya fumarole described by Vergasova and Filatov (2016).
Rich silicate mineralization also occurs in deposits of extinct fumaroles at Mountain 1004, a scoria cone located 2 km south of the second scoria cone of the NB GTFE. Mountain 1004 is a monogenetic volcano formed as a result of an ancient eruption of Tolbachik, about 2000 years ago (Naboko and Glavatskikh, 1992).
The mineralized pockets of the Arsenatnaya fumarole are located in an area mainly composed by blocks of scoria and volcanic bombs consisting of intermediate-type basalt.

Silicate mineralization in Tolbachik fumaroles: general data
The Arsenatnaya fumarole is the major subject of the present study. It is remarkable in mineral diversity. The number of mineral species of exhalation origin and products of their supergene alteration identified is about 210, including 40 insufficiently studied mineral phases (Pekov et al., 2019). Among the sublimate minerals found in the Arsenatnaya fumarole, 26 valid species and five insufficiently studied mineral phases belong to silicates. They include representatives of the majority of subclasses known for natural silicates, in terms of topology of crystal structure, (invalid species names used for simplicity are given in quotation marks at their first usage in the paper): nesosilicates (forsterite, andradite and titanite), cyclosilicates (a Cu,Zn-rich analogue of roedderite), different inosilicates (enstatite, clinoenstatite, diopside, aegirine, aegirine-augite, esseneite, "Cu,Mg-pyroxene", wollastonite, potassic-fluoromagnesio-arfvedsonite, potassic-fluoro-richterite and litidionite), phyllosilicates (fluorophlogopite, yangzhumingite, "fluoreastonite" and the Sn analogue of dalyite) and tectoaluminosilicates (sanidine, anorthoclase, ferrisanidine, anorthite, barium feldspar of the celsian-anorthoclase series, leucite, nepheline, kalsilite, sodalite and hauyne). Evidence that these minerals do have a fumarolic origin is a key point. The assignment of these silicates to products of deposition from hot fumarole gas or interactions between gas and rocks which compose walls of fumarolic vents is based mainly on two signs. (1) These silicates occur typically as well-shaped crystals in the open space of vents and, which seems especially important, commonly overgrow undoubtedly sublimate minerals (sulfates, arsenates, oxides, halides, etc., including the minerals containing species-defining chalcophile metals) or form intergrowths, usually open-work aggregates, in which silicates and non-silicate minerals demonstrate signs of simultaneous crystallization. (2) The majority of silicate minerals in the Arsenatnaya fumarole, belonging to the most common structure types/archetypes (olivine, garnet, pyroxene, amphibole, mica, feldspar and feldspathoids), are characterized by chemical features unusual for the same minerals from other geological formations. The brightest feature is an enrichment of these silicates (with species-defining Si, Al, Mg, Fe 3+ , Ca, Na and/or K) by chalcophile and some other ore chemical elements, namely As (the major admixture), Cu, Zn, Sn, Mo and/or W. These ore elements can be in significant amounts (up to several percent by weight) in silicates from the Arsenatnaya fumarole (Table 1) and are species-defining constituents in sublimate minerals belonging to other chemical classes (arsenates, sulfates, oxides, molybdates, borates, etc.) which occur in close intergrowths with the silicates.
The general data on the studied silicates from the Arsenatnaya fumarole are given in Table 1. Silicate mineralization is mainly concentrated in intermediate and deep zones of Arsenatnaya ( Fig. 1). The main mineral associations corresponding to different zones are reported in Table 2.
In addition to the above-mentioned aggregates occurring in the open space of vents, some fumarolic silicates (commonly feldspars and fluorophlogopite, occasionally forsterite, kalsilite, sodalite, and hauyne) in the Arsenatnaya fumarole replace basalt. Such replacement rims are observed in near-surface parts of blocks of basalt scoria and volcanic bombs altered by fumarolic gas. The silicates forming these rims demonstrate the same chemical features as their crystals from the deposits originating in the open space of fumarolic vents.
In the Yadovitaya fumarole located at the same second scoria cone of the NB GTFE, only one silicate is known, namely potassic feldspar (sanidine).
At Mountain 1004 there are three paleofumarolic fields: the southern, southwestern and western fields, all bearing rich Cu and sporadic Pb mineralization including sublimate tenorite and anglesite and secondary, supergene atacamite, antlerite, chrysocolla, volborthite, mottramite, pyromorphite, etc. (Serafimova et al., 1994;Pekov et al., 2018b). We believe that the amount and diversity of oxy-salts and chlorides of chalcophile elements was greater here; however, many minerals disappeared with time because of their instability under atmospheric conditions. Nevertheless, the silicate mineralization that is stable under weathering is quite rich there. For these ancient fumarolic fields, diopside, enstatite, albite, orthoclase, leucite, hauyne, pargasite, phlogopite and sericite were mentioned and considered to belong to the post-eruptive association formed at temperatures below 600-800 • C (Naboko and Glavatskykh, 1992). Our studies of paleofumarolic incrustations from Mountain 1004 show the presence of the following silicates: diopside, enstatite, fluorophlogopite, indialite, sanidine, anorthoclase, leucite, and hauyne. The wide distribution of the primary and secondary copper oxy-salts tenorite and atacamite closely associated with these minerals and the presence of admixed Cu in these silicates (see below) may indirectly confirm that these silicate      Note. Dash means the absence of unit-cell data (for minerals forming tiny individuals or thin zones in zoned crystals of other silicates). Names of invalid or insufficiently studied minerals are given in quotation marks. 1 Indialite was found only at Mountain 1004. 2 Minerals identified using Raman spectroscopy, without X-ray diffraction data. Figure 1. Schematic section across the northern part of the Arsenatnaya fumarole. The scheme is drawn after Pekov et al. (2018a). The detailed description of each zone is given in the cited paper. The mineral diversity of zones containing silicates is listed in Table 3. crusts could form with participation of fumarolic gas. However, with the formation mechanism of these minerals not being well identified, we only assign it to the post-eruptive stages. Therefore, we prefer to discuss this mineralization separately from silicates from the Arsenatnaya fumarole, for which the crystallization in the oxidizing-type system with ore geochemical specialization seems doubtless.

Experimental procedures
The minerals described here were studied by scanning electron microscopy (SEM), electron-microprobe analysis (EMPA, WDS and EDS modes), powder and single-crystal X-ray diffraction, and Raman spectroscopy.
The SEM studies and quantitative EMPA were conducted using a digital scanning electron microscope Cam-Scan MV2300 (VEGA TS 5130MM) with an EDS INCA Energy 350 Tables 3 and S2. A full sphere of three-dimensional X-ray diffraction (XRD) data for studied single-crystal samples were collected using MoKα radiation (λ = 0.71073 Å) at room temperature on an Xcalibur S CCD diffractometer. Data reduction was performed using CrysAlisPro version 1.171.37.35 (Agilent Technologies, 2014).
Powder X-ray diffraction data were collected using a Rigaku R-AXIS Rapid II diffractometer (image plate), CoKα 1 /α 2 (λ = 1.79021 Å) radiation, 40 kV, 15 mA, a rotating anode with microfocus optics, Debye-Scherrer geometry, d = 127.4 mm and exposure 15 min. The data were integrated using the software package Osc2Tab (Britvin et al., 2017). Intensities of diffraction reflections and unit-cell parameters for minerals were calculated by means of the STOE WinXPOW v.2.08 program suite. The JANA program package (Petriček et al., 2014) was used for the refinement of crystal structures by the Rietveld method.
The Raman spectra were recorded using an EnSpectr R532 spectrometer with a green laser (532 nm) at room temperature. The power of the laser beam on the sample was about 7 mW. The spectrum was processed using the EnSpectr expert mode program in the range from 100 to 4000 cm −1 with the use of a holographic diffraction grat- The arrangement of described zones is shown in Fig. 2. Names of invalid minerals are given in quotation marks.
ing with 1800 lines cm −1 and a resolution equal to 5-8 cm −1 . The diameter of the focal spot on the sample was about 10 µm. Raman spectra were acquired on polycrystalline samples.

Nesosilicates
The overwhelming number of nesosilicates in the Arsenatnaya fumarole occur in Zone VIb (Table 2), excluding minor finds of titanite in Zone IV.

Forsterite
Forsterite is a typical mineral of the deepest zones of the Arsenatnaya fumarole. It associates with hematite, hauyne, anhydrite and members of the svabite-fluorapatite series. The morphology of its colourless or pale pinkish transparent crystals (up to 0.1 mm in size) is diverse. There are perfect prismatic crystals with well-developed faces of two prisms (Fig. 2a), lens-shaped crystals (Fig. 2b), twins and trillings (110) (Fig. 2c). The bright red forsterite crystals (< 50 µm in size) typically form open-work aggregates with hematite. Such vivid colour is due to the inclusions of fine-powder hematite (Fig. 3). In Zone IVb the major silicate mineral associated with forsterite is hauyne. Both form white crusts replacing basalt scoria and open-work clusters in cavities. Lamellar aggregations of forsterite "cemented" by hauyne (Fig. 2d) were also observed.
The most typical forsterite from Arsenatnaya is chemically close to end-member Mg 2 SiO 4 . Some samples of "ordinary" forsterite from this locality contain the following impurities (up to, wt %): MnO 2.4, Fe 2 O 3 2.3, As 2 O 5 2.4, P 2 O 5 2.4 and CuO 0.4 (see also analysis 1, Table 3) (iron is considered trivalent due to strongly oxidizing conditions of mineral formation in the Tolbachik fumaroles; Pekov et al., 2018a).
One of the studied samples of forsterite is outstanding in both chemical and crystal chemical aspects. There are zoned crystals containing up to 16.0 wt % As 2 O 5 and up to 12.9 wt % P 2 O 5 . The empirical formulae (calculated on the basis of four O atoms per formula unit, apfu) corresponding to composition of the P-and As-richest zones are (Mg 1.82 Mn 0.01 ) 1.83 [(Si 0.58 P 0.26 As 0.14 ) 0.98 O 4 ] and (Mg 1.88 Mn 0.01 ) 1.89 [(Si 0.69 As 0.20 P 0.09 ) 0.98 O 4 ] respectively (Table 3, analyses 2 and 3). A special paper is devoted to the crystal chemistry and genetic features of this variety of forsterite -the P-and As-richest natural olivine (Shchipalkina et al., 2019).

Andradite
Minerals of the garnet supergroup are mainly represented in Arsenatnaya by members of the berzeliite (Ca 2 Na)Mg 2 (AsO 4 ) 3 -schäferite (Ca 2 Na)Mg 2 (VO 4 ) 3 solid-solution series, widespread in the Zones VIa and VIb of the fumarole (Pekov et al., 2018). The silicate garnet andradite is found in the same association (Table 2) but is not so abundant. Andradite forms crystal clusters or crusts overgrowing hematite and usually covered by anhydrite (Fig. 4a, b). The zonation of its brown or dark brownish-red crystals is due to variations in Fe : Al ratio and admixed Sn content (Fig. 4b). The content of grossu-

Titanite
Titanite is rare in the Arsenatnaya fumarole. It occurs as tiny crystals, grains or crystal clusters up to 70 µm across in arsenate silicate crusts replacing basalt scoria (Fig. 5) (Table 3, analyses 6 and 7).

Cyclosilicate, the Cu,Zn-rich analogue of roedderite
A member of the osumilite group with the simplified formula (Na, K, ) 3 (Mg, Zn, Cu) 5−x [Si 12 O 30 ] (Table 1) was found in Zone IV of Arsenatnaya. It forms hexagonal prismatic crystals combined in parallel intergrowths up to 50 µm in size in association with tridymite, litidionite, cassiterite and Asbearing sanidine (Fig. 6). The assignment of this mineral to the osumilite structure type on the basis of chemical composition and crystal morphology was confirmed by the Raman spectrum (Fig. 6). Chemical data make it possible to assume that the mineral is a Zn-and Cu-rich variety of roedderite, ideally KNaMg 5 [Si 12 O 30 ] (Alietti et al., 1994), or its hypothetic Mg−(Cu/Zn)-ordered analogue.
0.04 0.14 0.20 0.02 0.15 0.04 0.12 ] to ortho-or clinopyroxene was determined by single-crystal X-ray diffraction. The unit-cell parameters for both minerals are given in Table 1.
Enstatite forms well-shaped prismatic (Fig. 7a) transparent colourless to pale yellowish crystals associated with fluorophlogopite, hematite and members of the fluorapatitesvabite series. Its crystals also overgrow diopside crystals (Fig. 8a). Chemically, the mineral is typically close to the end-member. In some samples the following impurities were detected (up to, wt %): CaO and MnO 1.0, CuO 0.4, Fe 2 O 3 1.9, and Al 2 O 3 1.3. Arsenic was not detected in enstatite.
Clinoenstatite occurs as transparent colourless or greenish columnar to acicular crystals up to 0.1 mm long (Fig. 7b).  -

O O
Note: Dash means "below detection limit". BoFC means a basis of formula calculation, i.e. number of oxygen atoms per formula unit (apfu), except for 1 for titanite sum of all cations = 3 apfu, 2 for amphiboles B + C + T = Na + Ca + Si + P + As + V + Al + Fe + Ti + Sn + Zn + Cu + Mn + Mg = 15 apfu in accordance with the general formula of amphiboles AB 2 C 5 [T 8 O 22 ]W 2 (Hawthorne et al., 2012), and 3 for mica O + F = 12. Names of invalid minerals are given in quotation marks.
The main crystal forms are {011}, {010} and {110}. The major associated minerals are svabite, sodalite and sanidine. The main impurities in clinoenstatite from the Arsenatnaya fumarole are (up to, wt %): 1.6 MnO, 0.3 CuO, 0.7 Fe 2 O 3 and 0.2 Al 2 O 3 . The comparison of unit-cell parameters of our mineral and low and high clinoenstatite (Smyth, 1974) shows that our pyroxene is low clinoenstatite.

Clinopyroxenes of the diopside-esseneite-aegirine solid-solution system
Members of the diopside-esseneite-aegirine solid-solution system are widespread in hematite-clinopyroxene-anhydrite incrustations in Zones Vb, VIa and VIb. The main pyroxene of this system is diopside. Typically it forms practically monomineralic, or with anhydrite, incrustations con-  sisting of well-shaped short-prismatic crystals (up to 0.2 mm in size) and open-work aggregates overgrowing basalt scoria or hematite crusts (Fig. 8b). The colour of diopside is variable: bright yellow, orange, brownish green, green, light brown or reddish brown, to brown red or brick red. Aegirine in the Arsenatnaya fumarole occurs as elongated light yellow to bright sulfur-yellow prismatic crystals up to 0.3 mm (Fig. 7c, d) associated with hematite, fluorophlogopite, sanidine, sodalite, cassiterite, Na-rich sylvite and various arsenates. Esseneite forms tiny (10-20 µm in size) inclusions in anorthoclase and sanidine or areas in diopside crystals in Zones IVa and IVb. The chemical composition of these clinopyroxenes varies widely. The Al : Fe 3+ and Ca : Na ratios in members of the discussed solid-solution system are shown in Fig. 9. Diopside is characterized by both Mg : Fe 3+ ratio and content of Al 2 O 3 varying significantly (from 0.00 to 0.49 Al apfu). It forms a solid solution with esseneite, ideally CaFe 3+ [AlSiO 6 ], with a distinct positive correlation between Al 3+ and Fe 3+ contents. The absence of the hedenbergite component CaFe 2+ [Si 2 O 6 ] is due to the highly oxidizing conditions of mineral formation. The Si : Al ratio in pyroxenes of the diopside-esseneite-aegirine system varies as displayed in Fig. 9c. The linear dependence demonstrates that Al preferably occupies the tetrahedral sites in the structure. The deviation to the left from the ideal line is connected with As 5+ or Fe 3+ impurities and to the right with abundance of Al for tetrahedral sites.

Cu,Mg-pyroxene
In addition to these pyroxenes common for the Arsenatnaya fumarole, a specific Cu-rich pyroxene was found in several samples ( ]. This pyroxene forms thin greenish-brown or brown crust (up to 7 µm thick) on crystals of light brown diopside (Fig. 8c). The insufficient quality of the powder XRD pattern of this Cu-rich pyroxene (due  to scarcity of pure material) hampers the determination of its crystallographic characteristics. By analogy with data on the synthetic pyroxene CuMg[Si 2 O 6 ] possessing the enstatitetype structure (Tachi et al., 1997), we suggest that this mineral could be orthorhombic. Unit-cell dimensions calculated from powder XRD data based on this assumption are given in Table 1.

Wollastonite
A mineral with the Ca : Si ratio very close to 1 : 1 was detected in the core of one andradite crystal (Fig. 4a). The small size of this inclusion (about 10 µm) made it impossible to determine the polymorph of CaSiO 3 . However, the conditions of mineral formation in the Arsenatnaya fumarole allow us to suggest that this silicate is probably common wollastonite, which is stable in the pressure range 0-30 kbar and at temperatures less than 1130 • C (Swamy and Dubrovinsky, 1997). This mineral from the Arsenatnaya fumarole contains (wt %): 0.7 Fe 2 O 3 , 0.2 MgO, 0.3 MnO and 0.4 As 2 O 5 (Table 3,

Litidionite
Litidionite occurs as bright blue coarse prismatic crystals up to 0.02 mm long growing on sylvite, as balls up to 60 µm in diameter on aggregates of Na-bearing sylvite and as lamellar aggregates overgrowing tridymite ( Fig. 10c, d) (Fig. 11). The colour of the mineral varies from vivid orange and light brown to pale yellow or white; thin flakes are colourless.
The main difference between this fluorophlogopite and typical phlogopite-fluorophlogopite series from other geological formations is the absence of hydroxyl groups in samples from Arsenatnaya (Table 3, analyses 24 and 25). This is confirmed by both IR and Raman spectroscopy data. The Figure 9. Ratios of Fe 3+ to Al (total, i.e. octahedrally and tetrahedrally coordinated Al) (a), Ca to Na (b) and Si to Al (total, i.e. both octahedrally and tetrahedrally coordinated Al) (c) in pyroxenes of the diopside (Di)-aegirine (Aeg)-esseneite (Ess) solid-solution system from the Arsenatnaya fumarole: -aegirine, -aegirine-augite, • -enstatite and clinoenstatite, -esseneite, -diopside. The type of diagram (b) was proposed by Morimoto et al. (1988). content of O 2 substituting F − in the mineral from Arsenatnaya reaches 0.30 apfu.

Yangzhumingite
This mica, visually identical to fluorophlogopite, corresponds to the idealized formula KMg 2.5 [Si 4 O 10 ]F 2 . It was considered to be yangzhumingite (Miyawaki et al., 2011) and identified only in a few samples.
The chemical composition of cyclo-, ino-and phyllosilicates from paleofumaroles of Mountain 1004 is given in Table S2. In contrast to data reported by Naboko and Glavatskykh (1992), enstatite and fluorophlogopite studied by the authors contain admixed copper (up to 0.3 and 5.9 wt % CuO respectively). All studied samples of mica are fluorophlogopite with a F content close to 2.0 apfu.
Indialite deserves special attention. Unlike other abovementioned silicates, it was found at Tolbachik only within the western paleofumarole field of Mountain 1004. This hightemperature hexagonal dimorph of cordierite was identified using powder XRD. The characteristic reflections of the powder XRD pattern of indialite from the Mountain 1004 are (d in ångströms, I ) 8.47 (100), 4.89 (22), 4.09 (35), 3.38 (37), 3.14 (44), 3.03 (40), 2.64 (13) and 1.69 (14). The main characteristic of the XRD pattern of cordierite dimorphous with indialite is the presence of three well-resolved peaks in the range d = 3.05-3.01 Å, whereas indialite shows a single reflection at d ≈ 3.03 Å (Miyashiro, 1957). The presence of this single peak in the X-ray diffraction pattern allows us to identify the mineral from Mountain 1004 as indialite. It occurs as well-shaped short prismatic hexagonal crystals up to 50 µm in size associated with corundum and spinel (Fig. 13) Data availability. All data used are given in the tables, figures and text and in the Supplement to this paper.
Author contributions. NVS and IVP wrote the paper. NVS and NVZ carried out the crystal structure analysis. NVS obtained Raman spectroscopic data. NVS and SNB obtained and processed the X-ray diffraction data. NNK and DAV processed electron microprobe. IVP and EGS collected and prepared samples.