Richardsite, Zn 2 CuGaS 4 , A New Gallium-Essential Member of the Stannite Group from the Gem Mines near Merelani, Tanzania

: The new mineral richardsite occurs as overgrowths of small (50–400 µ m) dark gray, disphenoidal crystals with no evident twinning, but epitaxically oriented on wurtzite–sphalerite crystals from the gem mines near Merelani, Lelatema Mountains, Simanjiro District, Manyara Region, Tanzania. Associated minerals also include graphite, diopside, and Ge,Ga-rich wurtzite. It is brittle, dark gray in color, and has a metallic luster. It appears dark bluish gray in reﬂected plane-polarized light, and is moderately bireﬂectant. It is distinctly anisotropic with violet to light-blue rotation tints with crossed polarizers. Reﬂectance percentages for R min and R max in air at the respective wavelengths are 23.5, 25.0 (471.1 nm); 27.4, 28.9 (548.3 nm); 28.1, 29.4 (586.6 and 722 cm − 1 . Richardsite is named in honor of Dr. R. Peter Richards in recognition of his extensive research and writing on topics related to understanding the genesis of the morphology of minerals. Its status as a new mineral and its name have been approved by the Commission of New Minerals, Nomenclature and Classiﬁcation of the International Mineralogical Association (No. 2019-136).


Introduction
In addition to tanzanite, the blue-purple gem variety of zoisite that is famous from the region, the gem mines near Merelani, Lelatema Mountains, Simanjiro District, Manyara Region, Tanzania, are host to several other unusually well-crystallized minerals, including tsavorite, the green gem variety of grossular, diopside, prehnite, fluorapatite, and even graphite [1][2][3][4][5]. The mines are also host to well-formed and uncommonly large crystals of pyrite, alabandite, and wurtzite as well as several rare sulfides, including clausthalite (PbSe), germanocolusite (Cu 13 VGe 3 S 16 ), and merelaniite (Mo 4 Pb 4 VSbS 15 ) [5][6][7]. A detailed study of the chemistry of intergrown sphalerite and wurtzite, which included samples the Merelani mines and from the Animas-Chocaya Mine complex, Quechisla district, Bolivia, was recently published [8]. The Merelani sphalerite and wurtzite are Mn-rich, and were found to contain several trace elements (e.g., Fe, Cu, Se, and Cd) with concentrations greater than 500 ppm and a discernable differentiation between the sphalerite and wurtzite. Noteworthy, 1450 ppm Ga in the wurtzite and 1750 ppm in the sphalerite phases were also reported [8], with estimated standard deviations of 30 and 80 ppm, respectively. In both the sphalerite and wurtzite phases, trace Ga and Cu concentrations were consistent with the coupled substitution Cu + + Ga 3+ ↔ 2Zn 2+ .
In the course of our ongoing project dealing with the characterization of the Merelani mineralization [2,[5][6][7], we recovered a specimen containing an exceptionally Ga-enriched stannite, with the Ga content indicating a new mineral species. This paper deals with the description of this mineral as new independent species, which was named richardsite. Richardsite is the first gallium-essential sulfide to be described from the Merelani area, joining a very short list of accepted Ga-defined species, of which only three others are sulfides: gallite CuGaS 2 , ishiharaite (Cu,Ga,Fe,In,Zn)S, and zincobriartite Cu 2 (Zn,Fe)(Ge,Ga)S 4 .
The new mineral and its name have been approved by the Commission of New Minerals, Nomenclature and Classification of the International Mineralogical Association (No. 2019-136). It is named in honor of Dr. R. Peter Richards (b. 1943), retired water-quality researcher at Heidelberg College (Tiffin, OH, USA) and consulting editor of the journal Rocks & Minerals, in recognition of his research and writing, spanning over four decades, on topics related to understanding the genesis of the morphology of minerals. Dr. Richards was a major contributor to the discovery and description of the new minerals carlsonite and huizingite-(Al), and the previously unknown 2H and 3R polytypes of sabieite, all from the Huron River shale fire in Huron County, Ohio, USA [9]. Holotype material is deposited in the collections of the Museo di Storia Naturale, Università degli Studi di Firenze, Via La Pira 4, I-50121, Firenze, Italy, catalogue number 3555/I, and the A. E. Seaman Mineral Museum, 1404 E. Sharon Ave., Houghton, Michigan 49931-1659, USA, catalogue number DM 31876.

Occurrence
Richardsite occurs on the faces of a cluster of dark orange-brown wurtzite-sphalerite crystals (to~2.5 cm across) on a single-known specimen (4.2 cm × 2.6 cm × 1.5 cm) (Figure 1) from the Merelani gem mines. The specimen was obtained in November 2019 through the secondary mineral market, and its precise origin from among the numerous mine workings is unknown. In addition to the primary wurtzite-sphalerite, associated minerals include a second generation of epitaxic sphalerite on the earlier wurtzite-sphalerite, grains of Ge,Ga-rich wurtzite, minor hexagonal graphite crystals, and minor transparent, pale green crystals of diopside. The order of crystallization appears to be (wurtzite-sphalerite)/sphalerite/(richardsite + Ge,Ga-rich wurtzite)/(diopside + graphite).
Numerous studies and reviews are available in the literature on the geology of the Merelani gem deposits and models of formation of the gem crystals, particularly for zoisite (tanzanite) and grossular (tsavorite) (see, for example, [1,4,[10][11][12][13] and references therein). However, despite the significance of the large sulfide crystals [6] and associated sulfide deposits at the Merelani gem mines, we are not aware of any studies to date of their geological extent, significance, or formation.

Analytical Methods
Reflectance values were measured in air using an MPM-200 Zeiss microphotometer (Zeiss, Jena, Germany) equipped with an MSP-20 system processor on a Zeiss Axioplan ore microscope. The filament temperature was approximately 3350 K. An interference filter was adjusted, in turn, to select four wavelengths for measurement (471.1, 548.3, 586.6, and 652.3 nm). Readings were taken for the

Analytical Methods
Reflectance values were measured in air using an MPM-200 Zeiss microphotometer (Zeiss, Jena, Germany) equipped with an MSP-20 system processor on a Zeiss Axioplan ore microscope. The filament temperature was approximately 3350 K. An interference filter was adjusted, in turn, to select four wavelengths for measurement (471.1, 548.3, 586.6, and 652.3 nm). Readings were taken for the specimen Minerals 2020, 10, 467 4 of 10 and the standard (SiC) maintained under the same focus conditions. The diameter of the circular measuring area was 0.05 mm.
Unpolarized micro-Raman spectra were obtained in nearly back-scattered geometry with a Jobin-Yvon Horiba LabRAM HR800 instrument (HORIBA Jobin Yvon, Edison, NJ, USA) equipped with a motorized x-y stage, an Olympus BX41 microscope (Olympus, Tokyo, Japan) with a 100× objective, polarized incident HeNe laser radiation (632.8 nm), and a neutral density filter (D0.3). Spectra were collected through multiple acquisitions with single counting times of 10 s, and repeated on natural and broken surfaces (not polished) of several crystal grains. No damage from the laser was observed on the samples under these conditions. Quantitative chemical analyses were carried out using a JEOL 8200 microprobe (JEOL, Akishima, Japan), WDS mode, 20 kV, 20 nA, 1 µm beam size, with counting times of 20 s for peak and 10 s for background). For the WDS analyses, the following lines (standards in parentheses) were used: SKα (sphalerite), FeKα (pyrite), CuKα (synthetic Cu 2 S), ZnKα (sphalerite), GaKα (synthetic Ga 2 S 3 ), GeKα (synthetic Ge 2 S 3 ), MnKα (synthetic MnS), and SnLβ (synthetic SnS).
Single-crystal X-ray studies were carried out using a Bruker D8 Venture Photon 100 CMOS (Bruker, Billerica, MA, USA) equipped with graphite-monochromatized MoKα radiation (λ = 0.71073 Å) operating at 60 kV. The detector-to-crystal distance was 50 mm. Data were collected using ω and ϕ scan modes, in 0.5 • slices, with an exposure time of 45 s per frame. Single-crystal X-ray diffraction intensity data were integrated and corrected using the software package APEX3 (Bruker AXS Inc., Madison, WI, USA, [14]). A total of 955 unique reflections was collected.
X-ray powder diffraction data were collected with a Bruker D8 Venture Photon 100 CMOS using copper radiation (CuKα, λ = 1.54138 Å). The observed diffraction rings were converted to a conventional powder diffraction pattern using APEX3 [14].

Appearance and Physical Properties
Richardsite occurs as overgrowths of small crystals that appear to be epitaxically oriented on the crystal faces of a cluster of wurtzite-sphalerite crystals that is approximately 2.5 cm in maximum dimension ( Figure 1). Second-generation sphalerite crystals are crystallographically oriented on the faces of the primary wurtzite-sphalerite. The richardsite appears to selectively occur more richly on some faces of the wurtzite-sphalerite than others, and does not to occur at all on the faces of the second-generation sphalerite. Richardsite crystals exhibit subhedral morphology with pseudo-tetrahedral dispenoidal habit and stepped surfaces. No twinning has been observed. The typical size of richardsite crystals is about 50 to 150 µm, while the maximum size observed is about 400 µm. The physical properties of richardsite are summarized in Table 1.

Optical Properties
In reflected plane-polarized light, richardsite appears dark bluish gray in color and is moderately bireflectant. Between crossed polarizers, it is distinctly anisotropic with violet to light-blue rotation tints. Richardsite shows neither pleochroism nor internal reflections, and no optical indications of growth zonation are evident. Reflectance data of richardsite at four wavelengths are summarized in Table 2.

Raman Spectroscopy
The Raman spectrum of richardsite is shown in Figure 2. The most distinct Raman bands occur at 276, 309, 350, and 366 cm −1 , with the peak at 309 cm −1 being the narrowest and most intense. Broader and less intense bands occur at 172, 676, and 722 cm −1 . The second-most intense peak in most spectra taken is that at 366 cm −1 , however, the relative intensities of the 366 and 350 cm −1 peaks tend to vary in spectra taken across the crystal grain and can reach the intensity of the 309 cm −1 peak in some spectra. Overall, the Raman spectrum of richardsite is similar to that of renierite, (Cu 1+ ,Zn) 11 Fe 4 (Ge 4+ ,As 5+ ) 2 S 16 (RRUFF ID: 050428 514 nm [15]). The peak at 350 cm −1 may be due to the presence of a Ge,Ga-rich Cu-Zn sulfide (also containing Fe, Al, Sn, Mn, and Sn) that is sometimes intermixed with richardsite and has a very intense Raman response at this frequency shift.

Optical Properties
In reflected plane-polarized light, richardsite appears dark bluish gray in color and is moderately bireflectant. Between crossed polarizers, it is distinctly anisotropic with violet to lightblue rotation tints. Richardsite shows neither pleochroism nor internal reflections, and no optical indications of growth zonation are evident. Reflectance data of richardsite at four wavelengths are summarized in Table 2.

Raman Spectroscopy
The Raman spectrum of richardsite is shown in Figure 2. The most distinct Raman bands occur at 276, 309, 350, and 366 cm −1 , with the peak at 309 cm −1 being the narrowest and most intense. Broader and less intense bands occur at 172, 676, and 722 cm −1 . The second-most intense peak in most spectra taken is that at 366 cm −1 , however, the relative intensities of the 366 and 350 cm −1 peaks tend to vary in spectra taken across the crystal grain and can reach the intensity of the 309 cm −1 peak in some spectra. Overall, the Raman spectrum of richardsite is similar to that of renierite, (Cu 1+ ,Zn)11Fe4(Ge 4+ ,As 5+ )2S16 (RRUFF ID: 050428 514 nm [15]). The peak at 350 cm −1 may be due to the presence of a Ge,Ga-rich Cu-Zn sulfide (also containing Fe, Al, Sn, Mn, and Sn) that is sometimes intermixed with richardsite and has a very intense Raman response at this frequency shift.
Based on factor group analysis, richardsite, as a stannite-group mineral, may be expected to have 14 Raman-active modes [16,17]. The two A1-symmetry modes, which involve vibrations of the S atoms, are expected to be the most intense. Definitive symmetry assignments of the Raman peaks would require more detailed experimental studies, such as polarized Raman spectroscopy, checking for resonance effects, and infrared spectroscopy, which are beyond the scope of this paper.

Chemical Composition and X-ray Crystallography
A preliminary chemical analysis using energy-dispersive X-ray spectrometry performed on several crystal fragments, including the one used for the structural study, did not indicate the presence of elements (Z > 9) other than Cu, Zn, Ga, S, and minor amounts of Mn, Sn, Fe, and Ge. Subsequent electron microprobe analyses (n = 4) revealed the fragment used for the structural study Based on factor group analysis, richardsite, as a stannite-group mineral, may be expected to have 14 Raman-active modes [16,17]. The two A 1 -symmetry modes, which involve vibrations of the S atoms, are expected to be the most intense. Definitive symmetry assignments of the Raman peaks would require more detailed experimental studies, such as polarized Raman spectroscopy, checking for resonance effects, and infrared spectroscopy, which are beyond the scope of this paper.

Chemical Composition and X-ray Crystallography
A preliminary chemical analysis using energy-dispersive X-ray spectrometry performed on several crystal fragments, including the one used for the structural study, did not indicate the presence of Minerals 2020, 10, 467 6 of 10 elements (Z > 9) other than Cu, Zn, Ga, S, and minor amounts of Mn, Sn, Fe, and Ge. Subsequent electron microprobe analyses (n = 4) revealed the fragment used for the structural study to be homogeneous within analytical error. Microprobe data are presented in Table 3. Detection limits are <0.01 wt.% for the major elements (Ga, Zn, Cu, S), and <0.02 wt.% for the minor elements (Mn, Sn, Fe, Ge). Single-crystal X-ray diffraction indicates that richardsite is tetragonal, with a = 5.3626(2) Å, c = 10.5873(5) Å, V = 304.46(2) Å 3 , and Z = 2. It belongs to space group I42m (#121) and point group 42m. Least squares refinement of X-ray powder diffraction data (Table 4) give the tetragonal unit cell-parameter values as a = 5.3622(3) Å, c = 10.5844(10) Å, and V = 304.33(3) Å 3 . Table 4. Observed and calculated 1 X-ray powder diffraction data (d-spacings in Å) for richardsite. The strongest four estimated relative intensities I are given in bold type. The observed tetragonal unit-cell together with the obtained chemical formula suggests that richardsite is a new member of the stannite group. However, two closely related models have been proposed by Hall et al. [18] for the structure of these quaternary chalcogenides, which are topologically equivalent, but differ in the distributions of metals among the positions at (0,0,0), (0, 1 2 , 1 4 ), and (0, 1 2 , 3 4 ) [19].
In order to determine the distribution of metal atoms in richardsite without symmetry constraints, the structure was refined in both space groups, and better agreement was obtained in I42m. The crystal structure was refined using the program SHELXL-97 [21] up to R1 = 0.0284 for 655 reflections with F o > 4σ(F o ) and 14 parameters. The refined mean electron number at the metal sites, using scattering curves for neutral atoms taken from the International Tables for Crystallography [22], was 30 (Wyckoff position 4d), 31 (2a), and 29 (2b); thus, given also the observed mean bond distances and the chemical data, Zn, Ga, and Cu were assigned, respectively, to the three tetrahedral sites. Of course, due to the iso-electronic nature of its constituent elements (Cu = 29, Zn = 30, Ga = 31) together with the ambiguity in their valence states, the metal partitioning in richardsite is, however, not straightforward. According to Brese and O'Keeffe [23], the ideal distance (in Å) in a regular tetrahedron decreases following the sequence: 2.370/Cu + , 2.346/Zn 2+ , 2.288/Ga 3+ , 2.116/Cu 2+ , and this distribution is in keeping with the site-assignment proposed here for richardsite (Table 6). Furthermore, the chemical data clearly point to a new mineral species, regardless of the site distribution. Final atomic coordinates and equivalent isotropic displacement parameters are given in Table 5, and selected metal-sulfur (Me-S) bond distances are shown in Table 6. The Crystallographic Information File (CIF) is available as Supplementary Material. Table 5. Atoms, Wyckoff positions, atom coordinates, and isotropic displacement parameters (U iso in Å 2 ) for richardsite.

Atom
Wyckoff  The structure of richardsite consists of a cubic close packing (ccp) array of sulfur atoms tetrahedrally bonded with metal atoms occupying one half of the ccp tetrahedral voids (Figure 3). The ordering of the metal atoms leads to a sphalerite(sph)-derivative tetragonal unit-cell, with a ≈ a sph and c ≈ 2a sph . The packing of the S atoms slightly deviates from the ideal, however, primarily due to the presence of Ga.
Minerals 2020, 10, 467 8 of 10 Figure 3. The crystal structure of richardsite. Cu, Zn, Ga, and S atoms are given as light blue, dark blue, orange, and yellow circles, respectively. The unit cell of the structure is outlined in black, and its orientation is indicated at the top left.

Discussion
Minerals of the stannite group are quaternary chalcogenides, typically with the general formula T12T2T3X4, where T1, T2, and T3 correspond to tetrahedrally coordinated cations, and X corresponds to monatomic anions [24]. Among the mineral species, including richardsite, accepted by the Commission of New Minerals, Nomenclature and Classification of the International Mineralogical Association, T1 = Ag, Cu, Zn; T2 = Ag, Cu, Cd, Fe, Hg, Zn; T3 = As, Ga, Ge, In, Sb, Sn; and X = S, Se. Group members are generally tetragonal but can also be orthorhombic, and their structures can be considered derivatives of the sphalerite (or chalcopyrite) structure type [20,25], with the types of cations and their ordering in the tetrahedral sites affecting the resulting overall symmetry of the structures.
A wide variety of ternary (I-III-VI2) and quaternary (I2-II-IV-VI4) chalcogenides (I = Cu, Ag; II = Zn, Cd, Mn; III = Al, Ga, In; IV = Ge, Sn; VI = S, Se, Te) have been the subject of recent interest for their potential applications in photovoltaic devices, thermoelectric devices, and solar energy conversion materials [30]. The difficultly of distinguishing between the kesterite and stannite structures, particularly with the high potential for (Cu + Zn) disorder, has been noted for the (Cu,Zn)containing quaternary phases (see [30] and references therein). Quaternary chalcogenides containing Ga do not appear to have been synthesized until more recently, as in a study of wurtzite and stannite phases of Cu2ZnAS4−x and CuZn2AS4 (A = Al, Ga, In) nanocrystals [31]. These nanocrystals were synthesized using the colloidal hot-injection method as disordered-wurtzite phases. Upon annealing for 2-2.5 h in an N2 atmosphere at temperatures of 400-450 °C for Cu2ZnAS4−x and 500 °C for CuZn2AS4, the nanocrystals transformed to ordered stannite phases. Single-crystal X-ray studies and structure refinements have not been carried out on these synthetic materials, however.
First-principles calculations for both Cu2ZnAS4−x [31] and CuZn2AS4 [31,32] materials indicate that they are direct band gap materials with high absorption coefficients for visible light and, as such, they show initial promise as radiation-absorbing materials for solar cells. First-principles calculations [32] also show the CuZn2AS4 materials to be p-type semiconductors, and that the stannite-type structure is energetically more stable than the kesterite-and wurtzite-type structures.

Discussion
Minerals of the stannite group are quaternary chalcogenides, typically with the general formula T1 2 T2T3X 4 , where T1, T2, and T3 correspond to tetrahedrally coordinated cations, and X corresponds to monatomic anions [24]. Among the mineral species, including richardsite, accepted by the Commission of New Minerals, Nomenclature and Classification of the International Mineralogical Association, T1 = Ag, Cu, Zn; T2 = Ag, Cu, Cd, Fe, Hg, Zn; T3 = As, Ga, Ge, In, Sb, Sn; and X = S, Se. Group members are generally tetragonal but can also be orthorhombic, and their structures can be considered derivatives of the sphalerite (or chalcopyrite) structure type [20,25], with the types of cations and their ordering in the tetrahedral sites affecting the resulting overall symmetry of the structures.
A wide variety of ternary (I-III-VI 2 ) and quaternary (I 2 -II-IV-VI 4 ) chalcogenides (I = Cu, Ag; II = Zn, Cd, Mn; III = Al, Ga, In; IV = Ge, Sn; VI = S, Se, Te) have been the subject of recent interest for their potential applications in photovoltaic devices, thermoelectric devices, and solar energy conversion materials [30]. The difficultly of distinguishing between the kesterite and stannite structures, particularly with the high potential for (Cu + Zn) disorder, has been noted for the (Cu,Zn)-containing quaternary phases (see [30] and references therein). Quaternary chalcogenides containing Ga do not appear to have been synthesized until more recently, as in a study of wurtzite and stannite phases of Cu 2 ZnAS 4−x and CuZn 2 AS 4 (A = Al, Ga, In) nanocrystals [31]. These nanocrystals were synthesized using the colloidal hot-injection method as disordered-wurtzite phases. Upon annealing for 2-2.5 h in an N 2 atmosphere at temperatures of 400-450 • C for Cu 2 ZnAS 4−x and 500 • C for CuZn 2 AS 4 , the nanocrystals transformed to ordered stannite phases. Single-crystal X-ray studies and structure refinements have not been carried out on these synthetic materials, however.
First-principles calculations for both Cu 2 ZnAS 4−x [31] and CuZn 2 AS 4 [31,32] materials indicate that they are direct band gap materials with high absorption coefficients for visible light and, as such, they show initial promise as radiation-absorbing materials for solar cells. First-principles calculations [32] also show the CuZn 2 AS 4 materials to be p-type semiconductors, and that the stannite-type structure is energetically more stable than the kesterite-and wurtzite-type structures.
Author Contributions: J.A.J. recognized the potential uniqueness of the specimen and performed the initial SEM-EDS and Raman studies. L.B. performed the X-ray diffraction experiments and analysis, electron microprobe analyses, and reflectivity measurements. Both authors wrote the manuscript and have read and agreed to the published version of the manuscript. All authors have read and agreed to the published version of the manuscript.
Funding: Analytical work performed at Michigan Technological University's Applied Chemical and Morphological Analysis Laboratory was supported in part by the Edith Dunn and E. W. Heinrich Mineralogical Research Trust. The research was also funded by MIUR-PRIN2017, project "TEOREM deciphering geological processes using Terrestrial and Extraterrestrial ORE Minerals", prot. 2017AK8C32 (PI: Luca Bindi).