Chromium Members of the Pumpellyite Group: Shuiskite-(Cr), Ca 2 CrCr 2 [SiO 4 ][Si 2 O 6 (OH)](OH) 2 O, a New Mineral, and Shuiskite-(Mg), a New Species Name for Shuiskite

: A new pumpellyite-group mineral shuiskite-(Cr), ideally Ca 2 CrCr 2 [SiO 4 ][Si 2 O 6 (OH)] (OH) 2 O, was found at the Rudnaya mine, Glavnoe Saranovskoe deposit, Middle Urals, Russia. It occurs on the walls of 0.5 to 1 cm thick fractures in chromitite, ﬁlled with calcite, Cr-bearing clinochlore, and uvarovite. Shuiskite-(Cr) forms long prismatic to acicular crystals up to 0.1 × 0.5 × 7 mm elongated along [010] and slightly ﬂattened on [100]. The crystals are commonly combined into radial, sheaf-like aggregates. Most observed crystals are simple twins with a (001) composition plane. light. It optically biaxial D g / cm 3 IR spectrum chemical CaO MgO 3 3


Introduction
The pumpellyite-group members are low-grade metamorphic and hydrothermal minerals with the general formula Ca 2 XY 2 Si 3 O 14-n (OH) n , where X = Mg, Al, Mn 2+ , Mn 3+ , Fe 2+ , Fe 3+ , V 3+ and Cr 3+ , while Y = Al, Mn 3+ , Fe 3+ , V 3+ and Cr 3+ . In accordance with the IMA-accepted nomenclature [1], pumpellyite-group members are named based on the combination of the predominant cations at the Y (root name) and X (suffix-modifier) sites. Minerals with different cations predominant at the Y site have the different root names: Y Al-pumpellyite (the name first proposed by Palache and Vassar [2]), Y Fe 3+ -julgoldite [3], Y Mn 3+ -okhotskite [4], and Y V 3+ -poppiite [5]. Minerals with predominant Cr 3+ at the Y site have the root name (series name) shuiskite [6]. To date, the shuiskite series existed only formally and included one mineral species, shuiskite, with Mg as a dominant cation at the X site [6,7]; therefore, a suffix-modifier has not been used.
In 1985, a paper came out describing a variety of shuiskite with a high Cr and low H 2 O content; however, the distribution of Cr between the X and Y sites was not determined [8]. One of the authors of the present study (O.I.) was the senior author of the cited work and was able to find that specimen. We studied it in detail and showed that Cr is a dominant cation at both the X and Y sites, making it a new pumpellyite-group mineral shuiskite-(Cr), ideally Ca 2 CrCr 2 [SiO 4 ][Si 2 O 6 (OH)](OH) 2 O, as described in this paper. We proposed to name this new mineral species shuiskite-(Cr), and rename shuiskite to shuiskite-(Mg) as a mineral species with Mg prevailing at the X site and, thus, the ideal formula Ca 2 MgCr 2 [SiO 4 ][Si 2 O 6 (OH)](OH) 3 , in accordance with the IMA-accepted nomenclature [1].
Both the new mineral shuiskite-(Cr), its name, and the new name for shuiskite were approved by the IMA Commission on New Minerals, Nomenclature and Classification (IMA2019-117). Therefore, the name shuiskite was transferred from a species name to a root name, and the shuiskite series with the general formula Ca 2 XCr 2 [SiO 4 ][Si 2 O 6 (OH,O)](OH) 2 (OH,O), that includes shuiskite-(Mg) and shuiskite-(Cr) with X = Mg and Cr 3+ , respectively, appeared in the pumpellyite group.
The holotype specimen of shuiskite-(Cr) was deposited in the collection of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia with the registration number 5481/1. A part of the holotype was deposited in the collection of the Canadian Museum of Nature, Ottawa, Canada with the catalogue number CMNMC 87302.

Occurrence and General Appearance
The specimen that became a holotype of shuiskite-(Cr) was found in 1979 by V.A. Kuznetsov, a local geologist, at the Rudnaya underground chromite mine (level 280 m), located in the town of Sarany, Perm Krai, Middle Urals, Russia. This mine operates at the Glavnoe (Main) Saranovskoe deposit belonging to the Saranovskaya group of chromite deposits [8]. The Glavnoe Saranovskoe chromite deposit is also known as Saranovskoe (the Saranovskii mine, or, colloquially, Sarany, or Sarani), and should not be confused with the Biserskoe (the Yuzhno-Saranovskoe, or Southern Saranovskoe) deposit, where shuiskite-(Mg) was first found [6]. Both Biserskoe and Glavnoe Saranovskoe deposits belong to the Saranovskaya group of chromite deposits; Biserskoe is located 4 km south of Glavnoe Saranovskoe [9].

Analytical Methods
Chemical data for shuiskite-(Cr) were obtained using a Tescan VEGA-II XMU scanning electron microscope equipped with an EDS INCA Energy 450 and a WDS INCA-Wave 700 (Institute of Experimental Mineralogy, Chernogolovka, Russia) with an acceleration voltage of 20 kV, a beam current of 10 nA, and a beam diameter of 5 μm. The following standards were used: wollastonite (Ca), Mg (MgO), Al (Al2O3), Cr (Cr), Ti (Ti), Si (SiO2). H2O content was not determined directly because of the paucity of the available material. CO2 content was not measured because bands that could be assigned to C-O vibrations are absent in the infrared (IR) spectrum of shuiskite-(Cr).
IR absorption spectra of shuiskite-(Cr) and shuiskite-(Mg) were obtained from powdered samples mixed with dried KBr, pelletized, and analysed using an ALPHA FTIR spectrometer (Bruker Optics) at a resolution of 4 cm -1 . Sixteen scans were collected. The IR spectrum of an analogous pellet of pure KBr was used as a reference.
Powder X-ray diffraction data were collected with a Rigaku R-AXIS Rapid II single-crystal diffractometer (St. Petersburg State University, St. Petersburg, Russia) equipped with a cylindrical image plate detector using Debye-Scherrer geometry (d = 127.4 mm, CoKα radiation). The data were integrated using the software package Osc2Tab [11].
Single-crystal X-ray studies were carried out using an Oxford Xcalibur S diffractometer (Moscow State University, Moscow, Russia) equipped with a CCD detector (MoKα radiation). The structure was solved by direct methods and refined on the basis of 2793 independent reflections with I > 2σ(I) to R1 = 0.0469 using the SHELXL-2018/3 program package [12]. The structure was refined using the dataset containing the specific twin information, i.e., the overlapping as well as the non-overlapping reflections (so-called HKLF 5 format), as a two-component twin with a domain ratio of 87:13.

Physical Properties and Optical Data
Shuiskite-(Cr) is transparent in thin crystals and translucent in thicker ones. It changes colour depending on the light source like alexandrite, the chromian variety of chrysoberyl, and some other Cr-bearing minerals: in aggregates, shuiskite-(Cr) is greenish-black under daylight or purplish-black under incandescent light; in separate crystals, it is green to light-green and purple or greyish-purple, respectively. The streak is grey-green. The lustre is vitreous. The mineral is non-fluorescent under ultraviolet rays. The Mohs hardness is 6. Cleavage is {001} distinct. The fracture is uneven. The density, calculated using the empirical formula and unit-cell volume refined from the single-crystal XRD data, is 3.432 g/cm 3 . Shuiskite-(Cr) occurs together with pink Cr-bearing clinochlore and bright green uvarovite on the walls of 0.5 to 1 cm thick fractures in chromitite, filled with colourless calcite.

Analytical Methods
Chemical data for shuiskite-(Cr) were obtained using a Tescan VEGA-II XMU scanning electron microscope equipped with an EDS INCA Energy 450 and a WDS INCA-Wave 700 (Institute of Experimental Mineralogy, Chernogolovka, Russia) with an acceleration voltage of 20 kV, a beam current of 10 nA, and a beam diameter of 5 µm. The following standards were used: wollastonite (Ca), Mg (MgO), Al (Al 2 O 3 ), Cr (Cr), Ti (Ti), Si (SiO 2 ). H 2 O content was not determined directly because of the paucity of the available material. CO 2 content was not measured because bands that could be assigned to C-O vibrations are absent in the infrared (IR) spectrum of shuiskite-(Cr).
IR absorption spectra of shuiskite-(Cr) and shuiskite-(Mg) were obtained from powdered samples mixed with dried KBr, pelletized, and analysed using an ALPHA FTIR spectrometer (Bruker Optics) at a resolution of 4 cm -1 . Sixteen scans were collected. The IR spectrum of an analogous pellet of pure KBr was used as a reference.
Powder X-ray diffraction data were collected with a Rigaku R-AXIS Rapid II single-crystal diffractometer (St. Petersburg State University, St. Petersburg, Russia) equipped with a cylindrical image plate detector using Debye-Scherrer geometry (d = 127.4 mm, CoKα radiation). The data were integrated using the software package Osc2Tab [11].
Single-crystal X-ray studies were carried out using an Oxford Xcalibur S diffractometer (Moscow State University, Moscow, Russia) equipped with a CCD detector (MoKα radiation). The structure was solved by direct methods and refined on the basis of 2793 independent reflections with I > 2σ(I) to R1 = 0.0469 using the SHELXL-2018/3 program package [12]. The structure was refined using the dataset containing the specific twin information, i.e., the overlapping as well as the non-overlapping reflections (so-called HKLF 5 format), as a two-component twin with a domain ratio of 87:13.

Physical Properties and Optical Data
Shuiskite-(Cr) is transparent in thin crystals and translucent in thicker ones. It changes colour depending on the light source like alexandrite, the chromian variety of chrysoberyl, and some other Cr-bearing minerals: in aggregates, shuiskite-(Cr) is greenish-black under daylight or purplish-black under incandescent light; in separate crystals, it is green to light-green and purple or greyish-purple, Minerals 2020, 10, 390 4 of 11 respectively. The streak is grey-green. The lustre is vitreous. The mineral is non-fluorescent under ultraviolet rays. The Mohs hardness is 6. Cleavage is {001} distinct. The fracture is uneven. The density, calculated using the empirical formula and unit-cell volume refined from the single-crystal XRD data, is 3.432 g/cm 3 .

Chemical Data
Chemical data for shuiskite-(Cr) are given in Table 1 Shuiskite-(Cr) does not react with a diluted aqueous HCl solution at room temperature.
The bands at 827-836 cm -1 , and possibly the weak band at 681 cm -1 , may be tentatively assigned to M···O-H modes (where M is a metal cation at the X or Y site), but these bands may also correspond to mixed vibrations involving M···O-H angles and silicate groups. The weak bands in the ranges 1110-1160 and 1930-1950 cm -1 correspond to overtones or combination modes. The wavenumbers of the weak bands at 2207 and 2180 cm -1 are too high for a first overtone or a combination mode. The presence of these bands in the IR spectra can be explained by the presence of silanol groups Si-OH, confirming structural data that show a significant protonation of O (10).
The bands in the range of O-H-stretching vibrations (in the range from 2900 to 3520 cm -1 ) in the IR spectrum of shuiskite-(Cr) are shifted towards lower frequencies as compared to shuiskite-(Mg), which corresponds to stronger hydrogen bonds formed by the OH groups in the former mineral; 2.
The low-frequency shifts in the bands of shuiskite-(Cr) relative to those of shuiskite-(Mg) in the range 360-490 cm -1 (mixed modes involving (Cr,Mg)-O stretching vibrations) are due to the fact that Cr 3+ cation is heavier than Mg 2+.

Single-Crystal X-Ray Diffraction Data and Description of The Crystal Structure
The single-crystal X-ray diffraction data were indexed in the C2/m space group with the following unit-cell parameters: a = 19.2436(6), b = 5.9999(2), c = 8.8316(3) Å, β = 97.833(3) • , and V = 1010.17(6) Å 3 . The details on the data collection and structure refinement are given in Table 3. The Coordinates and equivalent displacement parameters of the atoms are given in Table 4, selected interatomic distances in Table 5, and bond valence calculations in Table 6. The crystallographic information file (CIF) for shuiskite-(Cr) is available as Supplementary Material (see below).      (Figure 3). The X site is occupied by 0.52Cr + 0.48Mg, with the average <X-O> distance of 2.026 Å, while the Y site is occupied by 0.70Cr + 0.30Al, with the average <Y-O> distance of 1.970 Å.  Table 6). The mixed occupancy at the O(10) site means that disilicate groups [Si2O6(OH,O)] are present. Weak bands at 2207 and 2180 cm -1 in the IR spectra of shuiskite-(Cr) confirm the presence of silanol groups Si-OH (Figure 2b). The remaining ten oxygen sites are occupied by O 2− anions. Both BVS and IR data indicate the absence of H2O 0 in shuiskite-(Cr). The distribution of OH groups in shuiskite-(Cr) is similar to that found in other Cr-bearing pumpellyite-group minerals [7,[15][16][17].  Table 6). The mixed occupancy at the O(10) site means that disilicate groups [Si 2 O 6 (OH,O)] are present. Weak bands at 2207 and 2180 cm -1 in the IR spectra of shuiskite-(Cr) confirm the presence of silanol groups Si-OH (Figure 2b). The remaining ten oxygen sites are occupied by O 2− anions. Both BVS and IR data indicate the absence of H 2 O 0 in shuiskite-(Cr). The distribution of OH groups in shuiskite-(Cr) is similar to that found in other Cr-bearing pumpellyite-group minerals [7,[15][16][17]. The

Discussion
Our data show that Cr 3+ can be the predominant cation at both the Y and X sites in pumpellyite-group minerals, resulting in the formation of the second Y Cr-dominant member of the pumpellyite group-shuiskite-(Cr).
All the works on Cr-enriched pumpellyite-group minerals show that Cr is distributed between both the X and Y sites; however, the distribution is uneven. At a relatively low Cr content (up to 16-17 wt.% Cr 2 O 3 ), Cr prefers the X site rather than the Y site resulting in the formation of a Cr-rich variety of pumpellyite-(Mg), Ca 2 (Mg,Cr)(Al,Cr) 2 Table 7. As proposed by Yoshiasa and Matsumoto [18], the substitution of Mg for Cr 3+ at the X site in pumpellyite-group minerals follows the mechanism Mg 2+ + OH − → Cr 3+ + O 2− with corresponding anion substitutions at the O(11) site. Table 7. Comparative data for shuiskite-(Cr) and shuiskite-(Mg).

Mineral
Shuiskite Funding: This research received no external funding.