Tsygankoite, Mn8Tl8Hg2(Sb21Pb2Tl)Σ24S48, a New Sulfosalt from the Vorontsovskoe Gold Deposit, Northern Urals, Russia

Tsygankoite, ideally Mn8Tl8Hg2(Sb21Pb2Tl)Σ24S48, is a new sulfosalt discovered at the Vorontsovskoe gold deposit, Northern Urals, Russia. It occurs as lath-like elongated crystals up to 0.2 mm embedded in calcite–dolomite–clinochlore matrix. The associated minerals also include aktashite, alabandite, arsenopyrite, barite, cinnabar, fluorapatite, orpiment, pyrite, realgar, routhierite, sphalerite, tilasite, and titanite. The new mineral is non-fluorescent, black, and opaque with a metallic lustre and black streak. It is brittle with an uneven fracture and no obvious parting and cleavage. Its Vickers hardness (VHN10) is 144 kg/mm2 (range 131–167 kg/mm2) and its calculated density is 5.450 g cm. In reflected light, tsygankoite is white; between crossed polars it is dark grey to black. It is strongly anisotropic: rotation tints vary from light grey to dark grey to black. Pleochroism and internal reflections are not observed. The chemical composition of tsygankoite (wt %, electron-microprobe data) is: Mn 6.29, Hg 5.42, Tl 26.05, Pb 5.84, As 3.39, Sb 30.89, S 21.87, total 99.75. The empirical formula, calculated on the basis of 90 atoms pfu, is: Mn8.06Tl8.00Hg1.90(Sb17.87As3.19Pb1.99Tl0.97)Σ24.02S48.03. Tsygankoite is monoclinic, space group C2/m, a = 21.362(4) A, b = 3.8579(10) A, c = 27.135(4) A, β = 106.944(14)°, V = 2139.19(17) A3 and Z = 1. The five strongest diffraction peaks from X-ray powder pattern (listed as (d,A(I)(hkl)) are: 3.587(100)(112), 3.353(70)(−114), 3.204(88)(405), 2.841(72)(−513), and 2.786(99)(−514). The crystal structure of tsygankoite was refined from single-crystal X-ray diffraction data to R = 0.0607 and consists of an alternation of two thick layer-like arrays, one based on PbS-archetype and the second on SnS-archetype. Tsygankoite has been approved by the IMA-CNMNC under the number 2017-088. It is named for Mikhail V. Tsyganko, a mineral collector from Severouralsk, Northern Urals, Russia, who collected the samples where the new mineral was discovered.


Introduction
The actively exploited Vorontsovskoe gold deposit in Northern Urals, Russia, similarly to the famous Lengenbach deposit in Switzerland or Allchar in Macedonia, was found to be the remarkable source of a particularly interesting Tl-Hg mineralization. Besides several very rare minerals-such as bernardite, chabournéite, christite, dalnegroite, hutchinsonite, imhofite, parapierrotite, picotpaulite, sicherite, vrbaite, etc.-this deposit also yielded three new Tl-bearing minerals already approved by IMA-CNMNC (vorontsovite, ferrovorontsovite, tsygankoite) with few other potentially new species still being under investigation.
In this paper, we describe the new mineral tsygankoite [pronouncing: tsi gan ko ait] while vorontsovite and ferrovorontsovite are described in another paper of the same special issue.
Tsygankoite is named for Mikhail Vladimirovitch Tsyganko (born 25 October 1979), a mineral collector from Severouralsk, Sverdlovskaya Oblast', Northern Urals, Russia. His extensive mineral collection is oriented mostly to the minerals of Northern and Subpolar Urals as well as systematic species served as a base for mineralogical museum "Shtufnoi Kabinet" (Mineral Cabinet), which was founded in 2014 by Mikhail and his friends. Mikhail is the author of the book dedicated to the minerals of Northern Urals [1] where he described in detail the mineralogy of the region and presented the list of all mineral species discovered there so far. The new mineral described here was discovered in the specimens collected by M.V. Tsyganko in June 2015 in the ore stockpile of the Vorontsovskoe deposit. Two other new species from the same deposit (vorontsovite and ferrovorontsovite) were also collected by him.
Both the mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association, under the number 2017-088.
The type specimen is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia under the registration number 5018/1.

Occurrence
The Vorontsovskoe gold deposit is located 0.5 km west of the settlement of Vorontsovka, approximately 13 km south of the city of Krasnotur'insk, Sverdlovskaya Oblast', Northern Urals, Russia (59 • 39 5" N, 60 • 12 56" E). The deposit was discovered in 1985 and is currently operated by Russian mining company Polymetal International PLC for high-grade gold-bearing ore. The ore reserves at 01 January 2017 were estimated at 0.9 million. tr.oz of gold and average content of 2.7 g/t. In 2017, the company produced 118,000 tr.oz. of refined gold.
Genesis of the Vorontsovskoe deposit and its main types of ore remains the subject of discussion in Russian geological literature. According to some of the authors, Vorontsovskoe belongs to the Carlin-type deposits [2,3] while others suggest that it is a part of the complex Vorontsovsko-Peschanskaya ore-magmatic system and breccias with orpiment-realgar cement were formed as a result of fluid explosion [4,5].
The geological location of the deposit and composition of its main mineral types of ore is shortly given in companion paper describing two other new Tl-bearing minerals originating from Vorontsovskoe deposit, vorontsovite and ferrovorontsovite, in the same special issue [6].
Similar to vorontsovite and ferrovorontsovite, tsygankoite was found in the ores of the sulfide-carbonate type. They comprise limestone breccias (calcite-dolomite, up to 85% of volume), main silicate minerals (quartz, feldspars, clinochlore, muscovite, up to 10% of volume), accessory minerals, and sulfide-sulfosalt assemblage. Accessory minerals include apatite, arsenic, barite, gold, russellite, titanite, tilasite, and La-analogue of gasparite-(Ce). Apatite forms chemically zoned crystals: they consist mainly of As-Cl-bearing fluorapatite with some compositions corresponding to hydroxylapatite. The As and Cl contents increase from the core to the rim of the crystals up to the appearance of chlorapatite and turneaureite compositions along the edges.
Tsygankoite should be considered as a very rare mineral. Indeed, its occurrence was so far confirmed in three polished sections only.

Physical and Optical Properties
Tsygankoite occurs as black lath-like elongated crystals up to 0.2 mm across embedded in calcite-dolomite-clinochlore matrix. Tsygankoite either forms individual crystals or is partially replaced by alabandite (Figures 1 and 2).
Tsygankoite should be considered as a very rare mineral. Indeed, its occurrence was so far confirmed in three polished sections only.

Physical and Optical Properties
Tsygankoite occurs as black lath-like elongated crystals up to 0.2 mm across embedded in calcite-dolomite-clinochlore matrix. Tsygankoite either forms individual crystals or is partially replaced by alabandite (Figures 1 and 2).  Tsygankoite is opaque in transmitted light and exhibits metallic luster and black streak. Its tenacity is brittle and fracture-uneven. Cleavage and parting are not observed. It is non-fluorescent. The Vickers hardness (VHN10) is 144 kg/mm 2 (range 131-167 kg/mm 2 ) corresponding to a Mohs hardness of ~3. The density could not be measured because of the absence of suitable heavy liquids and paucity of available material. The calculated density based on the empirical formula (Z = 1) and unit-cell volume obtained from single-crystal X-ray data is 5.450 g cm −3 .
In reflected light, tsygankoite is white. It does not show any pleochroism and internal reflections. The bireflectance is very weak, ∆R = 4.43% (589 nm). Between crossed polars, the mineral Tsygankoite should be considered as a very rare mineral. Indeed, its occurrence was so far confirmed in three polished sections only.

Physical and Optical Properties
Tsygankoite occurs as black lath-like elongated crystals up to 0.2 mm across embedded in calcite-dolomite-clinochlore matrix. Tsygankoite either forms individual crystals or is partially replaced by alabandite (Figures 1 and 2).  Tsygankoite is opaque in transmitted light and exhibits metallic luster and black streak. Its tenacity is brittle and fracture-uneven. Cleavage and parting are not observed. It is non-fluorescent. The Vickers hardness (VHN10) is 144 kg/mm 2 (range 131-167 kg/mm 2 ) corresponding to a Mohs hardness of ~3. The density could not be measured because of the absence of suitable heavy liquids and paucity of available material. The calculated density based on the empirical formula (Z = 1) and unit-cell volume obtained from single-crystal X-ray data is 5.450 g cm −3 .
In reflected light, tsygankoite is white. It does not show any pleochroism and internal reflections. The bireflectance is very weak, ∆R = 4.43% (589 nm). Between crossed polars, the mineral Tsygankoite is opaque in transmitted light and exhibits metallic luster and black streak. Its tenacity is brittle and fracture-uneven. Cleavage and parting are not observed. It is non-fluorescent. The Vickers hardness (VHN 10 ) is 144 kg/mm 2 (range 131-167 kg/mm 2 ) corresponding to a Mohs hardness of~3. The density could not be measured because of the absence of suitable heavy liquids and paucity of available material. The calculated density based on the empirical formula (Z = 1) and unit-cell volume obtained from single-crystal X-ray data is 5.450 g·cm −3 .
In reflected light, tsygankoite is white. It does not show any pleochroism and internal reflections. The bireflectance is very weak, ∆R = 4.43% (589 nm). Between crossed polars, the mineral is strongly anisotropic with rotation tints varying from light grey to dark grey to black. Quantitative reflectance measurements were performed in air relative to a WTiC standard by means of a Universal Microspectrophotometer UMSP 50 (Opton-Zeiss, Germany) ( Table 1).

Chemical Composition and Chemical Properties
Preliminary chemical analyses using a scanning electron microscope CamScan 4D equipped with INCA Energy microanalyzer (EDS mode, 20 kV, 5 nA and beam diameter 5 µm) showed the presence in tsygankoite of Mn, Tl, Hg, Pb, Sb, As, and S. No other elements with atomic numbers higher than 8 were detected.
Tsygankoite showed to be totally insoluble in acids, aqua regia, and alkalis.

X-Ray Crystallography
A small crystal fragment of tsygankoite having 0.032 mm × 0.017 mm × 0.007 mm across was carefully extracted from the polished section used for EMPA. This grain was used for collection of X-ray data utilizing a Rigaku SuperNova single-crystal diffractometer equipped with the Atlas S2 detector. The instrument is equipped with a Mo micro-focus X-ray tube, working at 50 kV and 0.8 mA (40 W), providing a beam spot of 120 µm. Omega scan mode produced 272 frames in three runs with exposition time of 500 s per frame. Unit-cell parameters were refined from 514 observed reflections. The following data were obtained: tsygankoite is monoclinic, space group C2/m, with a = 21.362 (4)  Due to the lack of suitable material, we present calculated powder pattern of tsygankoite only. The theoretical d hkl and relative intensities were calculated using PowderCell program [7] from the structure model. Data are given in Table 3.

Description of Crystal Structure and Discussion
The crystal structure of tsygankoite was refined to R = 0.0607 for 957 observed reflections, with criterion I > 3σ(I). The crystal structure of tsygankoite was solved from diffraction data using SHELXT [8] and refined using the least-squares algorithm of the Jana2006 program [9]. Data collection and refinement details are listed in Table 4, atom coordinates and displacement parameters along with the bond-valences are listed in Table 5, and selected bond-lengths in Table 6. The CIF file for tsygankoite is available in the Supplementary Materials as file S1.   The crystal structure of tsygankoite contains 11 independent cation sites (four of which are mixed sites which were refined with one set of coordinates each) and 12 distinct sulfur sites. It consists of an alternation of two thick layer-like arrays; both are typical for sulfosalt structures.
The array based on PbS-archetype (i.e., with a PbS-like topology) contains a Hg site with a typical bond scheme, a Tl site, the octahedrally coordinated Mn1 site, and two pure Sb sites, labeled Sb1 and Sb2. The alternating array, based on the SnS-archetype (an array with a more pronounced steric role of lone electron pairs), with a general bond scheme represented by that observed in SnS [10], contains four distinct Sb sites, all of which statistically mix with either heavier cations (refined as Pb or Tl component) or with arsenic. Embedded in this array is a thallium site (refined as "Tl3"), and a Mn 2+ site, this being apical to the second array. Interesting is the close spatial relationship of large Tl + and small Mn 2+ polyhedra in the former array and along its contact with the latter array.
The PbS-like portions represent a sequence of interconnected rods, four (100) PbS planes thick, ideally three polyhedra broad but heavily overlapped (rod-based sulfosalt structures were defined in Makovicky [11]). The contact/overlap portions of two adjacent rods are formed by the coordination octahedron of Hg, surrounded by two Mn octahedra and two bicapped trigonal coordination prisms of Tl; paired columns of Sb coordination pyramids form the cores of rods, with a lone electron space in between the pyramidal pairs ( Figure 3). The PbS-like portions represent a sequence of interconnected rods, four (100)PbS planes thick, ideally three polyhedra broad but heavily overlapped (rod-based sulfosalt structures were defined in Makovicky [11]). The contact/overlap portions of two adjacent rods are formed by the coordination octahedron of Hg, surrounded by two Mn octahedra and two bicapped trigonal coordination prisms of Tl; paired columns of Sb coordination pyramids form the cores of rods, with a lone electron space in between the pyramidal pairs ( Figure 3). The contact of rods differs from those which were known when Makovicky [11] classified rod-based sulfosalt structures. In the present case, the rods are interconnected via three atomic (100)PbS planes (Figure 3), whereas at that time, only rod interconnections/overlaps via one or two such planes were known. The only hitherto published situation close to the here-described one occurs in the PbS-archetype layers of the crystal structure of rouxelite Cu2HgPb22Sb28S64(O,S)2 [12], in which two adjacent rod-like bulges periodically occur, interconnected and amalgamated via octahedrally coordinated Hg site.
The general orientation of the PbS-based layers is (510) regarding a pseudo-tetragonal subcell which can be defined from Figure 3, and it is (522) regarding a cubic PbS-like subcell which can be defined as an alternative in this layer (Figure 4). The contact of rods differs from those which were known when Makovicky [11] classified rod-based sulfosalt structures. In the present case, the rods are interconnected via three atomic (100) PbS planes (Figure 3), whereas at that time, only rod interconnections/overlaps via one or two such planes were known. The only hitherto published situation close to the here-described one occurs in the PbS-archetype layers of the crystal structure of rouxelite Cu 2 HgPb 22 Sb 28 S 64 (O,S) 2 [12], in which two adjacent rod-like bulges periodically occur, interconnected and amalgamated via octahedrally coordinated Hg site.
The general orientation of the PbS-based layers is (510) regarding a pseudo-tetragonal subcell which can be defined from Figure 3, and it is (522) regarding a cubic PbS-like subcell which can be defined as an alternative in this layer (Figure 4). The layer based on the SnS-archetype represents a step-like cut-out of an SnS-like, antimony-based sulfide slab, limited by the overall (410)SnS planes (Figure 4). It consists of a set of double-ribbons which are diagonal to the overall orientation of this slab (Figure 3), tightly bonded by Sb-S bonds in the ribbon interior, and contain planar interspaces which house lone electron pairs, spaced between them. In subsurface portions of double-ribbons, one site is occupied by thallium (Tl3), whereas the step tips are formed by an octahedron of Mn (Mn2). The distribution of shorter and longer bonds in the central portions of this layer differs from that in SnS and is made possible only by lengthening of cation-sulfur bonds due to the partial substitutions of Sb by (Pb,Tl), as described in detail further below. The Sb3-Sb6 distance is 4.196 Å. The complicated match of the two-layer arrays proceeds via Mn octahedra, followed by a space for lone electron pairs of Sb from the PbS-like layer, Tl from the SnS-like layer, and a trigonal coordination prism of Tl (Tl2).
Manganese octahedra are fairly regular, especially the Mn2 site (Mn-S distance from 2.536 to 2.636 Å). The asymmetrically positioned Mn1 site is somewhat less regular (from 2.539 to 2.728 Å). This compares very well with the Mn octahedra in MnSb2S4: the orthorhombic modification [15] has octahedral Mn-S distances between 2.537 and 2.692 Å, the monoclinic modification [16] has two sites with ranges 2.546-2.643 Å and 2.611-2.614 Å, respectively. The layer based on the SnS-archetype represents a step-like cut-out of an SnS-like, antimony-based sulfide slab, limited by the overall (410) SnS planes ( Figure 4). It consists of a set of double-ribbons which are diagonal to the overall orientation of this slab (Figure 3), tightly bonded by Sb-S bonds in the ribbon interior, and contain planar interspaces which house lone electron pairs, spaced between them. In subsurface portions of double-ribbons, one site is occupied by thallium (Tl3), whereas the step tips are formed by an octahedron of Mn (Mn2). The distribution of shorter and longer bonds in the central portions of this layer differs from that in SnS and is made possible only by lengthening of cation-sulfur bonds due to the partial substitutions of Sb by (Pb,Tl), as described in detail further below. The Sb3-Sb6 distance is 4.196 Å. The complicated match of the two-layer arrays proceeds via Mn octahedra, followed by a space for lone electron pairs of Sb from the PbS-like layer, Tl from the SnS-like layer, and a trigonal coordination prism of Tl (Tl2).
Manganese octahedra are fairly regular, especially the Mn2 site (Mn-S distance from 2.536 to 2.636 Å). The asymmetrically positioned Mn1 site is somewhat less regular (from 2.539 to 2.728 Å). This compares very well with the Mn octahedra in MnSb 2 S 4 : the orthorhombic modification [15] has