Discovery of “Meteoritic” Layered Disulphides A CrS 2 ( A = Na, Cu, Ag) in Terrestrial Rock

: For the ﬁrst time, chromium disulphides, known from meteorites, such as caswellsil-verite, NaCrS 2 ; grokhovskyite, CuCrS 2 ; and a potentially new mineral, AgCrS 2 , as well as the products of their alteration, such as schöllhornite, Na 0.3 CrS 2 · H 2 O, and a potentially new mineral with the formula {Fe 0.3 (Ba,Ca) 0.2 } CrS 2 · 0.5H 2 O,


Introduction
In the unusual phosphide-bearing breccia of the pyrometamorphic Hatrurim Complex, discovered in 2019 in the Negev Desert, Israel [1], numerous crystals of caswellsilverite, NaCrS 2 , rare grains of grokhovskyite and a potentially new mineral, AgCrS 2 , as well as products of their alteration-schöllhornite, Na 0.3 CrS 2 •H 2 O-and a potentially new mineral with the simplified formula {Fe 0.3 (Ba,Ca) 0.2 }CrS 2 •0.5H 2 O (thereafter referred to as "mineral X"), were detected.

Materials and Methods
More than 200 samples of phosphide-bearing breccia with fragments of black, weakly altered gehlenite paralava enriched in sulphides were collected during fieldwork in 2019 and 2021 from a small outcrop in the Negev Desert, Israel [1,18].In all, five samples, chromium disulphides, the main object of the investigation, were detected.
The morphology and chemical composition of chromium disulfides and associated minerals were investigated using Phenom XL and Quanta 250 EDS-equipped scanning electron microscopes (Institute of Earth Sciences, University of Silesia, Poland).The mineral chemical composition was measured with a Cameca SX100 electron microprobe analyzer (EMPA, Micro-Area Analysis Laboratory, Polish Geological Institute-National Research Institute, Warsaw, Poland), WDS, accelerating voltage = 15 kV, beam current = 10-20 nA.Natural and synthetic standards were used.
The Raman spectra of the minerals were recorded on a WITec alpha 300R Confocal Raman Microscope (Institute of Earth Science, University of Silesia, Poland) equipped with an air-cooled solid laser (488 nm), a CCD camera operating at −61 • C, and a monochromator with a 600 mm −1 grating.The power of the laser at the sample position was ~4-7 mW.Integration times of 3 s with an accumulation of 20-30 scans were chosen, and the resolution was 3 cm −1 .The monochromator was calibrated using the Raman scattering line of a silicon plate (520.7 cm −1 ).
The reflected spectra of the minerals were measured with the help of the reflectometer Filmetrix coupled with the optical microscope Leica 2700P using objective × 100 and Si Filmetrix standard.

Occurrence
Monovalent-metal chromium disulphides have been found in phosphide-bearing explosive breccia, forming a small vertical zone 4-5 m wide in layered hydrogrossularbearing rock ("low-temperature Hatrurim") in an outcrop on the Arad-Dead Sea road, Hatrurim Basin, Negev Desert, Israel [1].Aggregates of the Fe-P(±C) system minerals barringerite, Fe 2 P (P-62m, hP9), schreibersite, Fe 3 P, native iron and schreibersite-iron eutectic are widely distributed in the explosive breccia of the pyrometamorphic Hatrurim Complex [1].V-bearing andreyivanovite, FeCrP, and V-Cr-bearing allabogdanite, Fe 2 P (Pnma, oP12), have also been identified in this breccia [1].The super-reduced character of the phosphide association was confirmed by the discovery of osbornite [18], which is extremely unusual for rocks of the Hatrurim Complex, which formed under the oxidizing conditions of the sanidinite facies (700-1400 • C and low pressure) and so are mainly composed of minerals containing trivalent iron [19].Rocks of the pyrometamorphic Hatrurim Complex (Mottled Zone), including larnite, spurrite, and gehlenite rocks and different types of paralavas, are distributed along the Dead Sea rift in the territories of Israel, Palestine, and Jordan [20][21][22][23].The "Classic" genetic hypothesis suggests that rocks of the Hatrurim Complex formed as a result of the burning of the bitumen substance contained in sedimentary protolith [20].The recently proposed "Mud volcanos" hypothesis states that the activation of natural fires and the pyrometamorphic transformation of sedimentary protolith occurred with the participation of methane delivered from gas traps in the tectonically active Dead Sea rift zone [22,24].
The studied breccia consists of clasts of altered sedimentary rock transformed into porous hydrogrossular-bearing rock with relics of high-temperature minerals (pseudowollastonite, iron phosphides, and osbornite) cemented by gehlenite paralava [1,18].As a rule, gehlenite in paralava is intensively replaced by hydrogrossular, which blurs the boundaries between clasts and breccia cement, where minerals of the Fe-P(±C) system concentrate (Figure 1a,b).The rounded aggregates of minerals of the Fe-P(±C) system exhibit a characteristic zonation from the centrum to the rim (barringerite-schreibersite-schreibersiteiron eutectic) [1] or schreibersite-iron (±cohenite) eutectic with relatively large iron segregations featuring rare schreibersite inclusions (Figure 1c).In the schreibersite-iron (+cohenite) eutectic, small drops of native copper (Figure 1d) and daubréelite inclusions (Figure 1e) can be observed.Aggregates of the Fe-P(±C) system minerals intergrow with pyrrhotite with the lamellar exsolution structure of daubréelite (Figure 1f).
phosphide association was confirmed by the discovery of osbornite [18], which is extremely unusual for rocks of the Hatrurim Complex, which formed under the oxidizing conditions of the sanidinite facies (700-1400 °C and low pressure) and so are mainly composed of minerals containing trivalent iron [19].
Rocks of the pyrometamorphic Hatrurim Complex (Mottled Zone), including larnite, spurrite, and gehlenite rocks and different types of paralavas, are distributed along the Dead Sea rift in the territories of Israel, Palestine, and Jordan [20][21][22][23].The "Classic" genetic hypothesis suggests that rocks of the Hatrurim Complex formed as a result of the burning of the bitumen substance contained in sedimentary protolith [20].The recently proposed "Mud volcanos" hypothesis states that the activation of natural fires and the pyrometamorphic transformation of sedimentary protolith occurred with the participation of methane delivered from gas traps in the tectonically active Dead Sea rift zone [22,24].

Figure 10a
Figure

Raman Investigation of Layered Chromium Disulphides
In the Raman spectra of caswellsilverite, there are two strong bands from Cr-S vibrations typical for the spectra of synthetic NaCrS 2 : 316 (A 1 ) and 252 cm −1 (E g ) [25].An orientation effect is observed: the band at 316 cm −1 polarizes, and its intensity drops by a factor of three times at polarization of the laser beam perpendicular to the direction of the flattening crystal (Figure 11a,b).To avoid artefacts in the spectra of the studied minerals, we also obtained their spectra after the laser-induced heating in air.The Raman spectrum of thermally changed caswellsilverite is related to sodium chromate.The strongest band in the spectrum at about 850 cm −1 is related to A 1 vibrations in (CrO 4 ) 2- [26][27][28][29] (Figure 11c).
The bands in the Raman spectrum of the potentially new mineral AgCrS 2 (Figure 13) had a weak intensity that can be explained by its surface quality due to high instability in the ambient conditions (Figure 8c).There are also bands in the spectrum related to Cr-S vibration: 644, 320 (A 1 ), 279(?) и 250 (E g ) cm −1 [25,[32][33][34].
We measured the Raman spectra of schöllhornite and "mineral X" and their products as a result of thermal change under the laser beam (Figure 14).It should be emphasised that in no case did we observe active vibrational modes from OH/H 2 O.
tions typical for the spectra of synthetic NaCrS2: 316 (A1) and 252 сm −1 (Eg) [25].An orientation effect is observed: the band at 316 cm −1 polarizes, and its intensity drops by a factor of three times at polarization of the laser beam perpendicular to the direction of the flattening crystal (Figure 11a,b).To avoid artefacts in the spectra of the studied minerals, we also obtained their spectra after the laser-induced heating in air.The Raman spectrum of thermally changed caswellsilverite is related to sodium chromate.The strongest band in the spectrum at about 850 cm −1 is related to A1 vibrations in (CrO4) 2- [26][27][28][29] (Figure 11c).The Raman spectra of grokhovskyite were measured on lamellar crystal in two orientations (Figure 12).The spectra featured weak bands at 315/319 cm −1 (A1) and 253/251 cm −1 (Eg), which are typical for synthetic CuCrS2 [30,31].The bands in the Raman spectrum of the potentially new mineral AgCrS2 (Figure 1 had a weak intensity that can be explained by its surface quality due to high instability the ambient conditions (Figure 8c).There are also bands in the spectrum related to Cr vibration: 644, 320 (A1), 279(?) и 250 (Eg) cm −1 [25,[32][33][34].The bands in the Raman spectrum of the potentially new mineral AgCrS2 (Figure 13) had a weak intensity that can be explained by its surface quality due to high instability in the ambient conditions (Figure 8c).There are also bands in the spectrum related to Cr-S vibration: 644, 320 (A1), 279(?) и 250 (Eg) cm −1 [25,[32][33][34].We measured the Raman spectra of schöllhornite and "mineral Х" and their products as a result of thermal change under the laser beam (Figure 14).It should be emphasised that in no case did we observe active vibrational modes from OH/Н2О.
In the schöllhornite spectrum (Figure 14c), there were two strong bands at 336 and 276 cm −1 related to vibrations A1 and Eg in the (CrS2) -layers, and there was a strong band near 467 cm −1 , which may correspond to the S-S bond [34].An effect of the dimerization of sulphur was noted in NaCr2/3Ti1/3S2 disulphide as a result of the migration of Cr to the Na-vacancies [35].It is interesting that schöllhornite, thermally affected by the Raman microscope laser beam, was replaced by escolaite, Cr2O3 (Figure 14e).

Genesis and Alteration of Chromium Disulphides in Pyrometamorphic Rock
Highly reducing conditions in the terrestrial pyrometamorphic combustion process is a rare phenomenon that leads to the appearance of minerals typical for meteorites [1,18,39]."Meteoritic" minerals, mainly phosphides, form at the contact facies of black, reduced pyrrhotite-bearing Hatrurim Complex paralavas intruded into the country rocks containing carbonaceous matter, which play the role of reductant [1,18].It should be emphasised that yellow-green, brown oxidized paralavas with a mineral composition close to that of black paralava are widespread in the Hatrurim Complex, especially in the Hatrurim Basin, and contain mainly Fe 3+ -bearing minerals [19].
Combustion processes during pyrometamorphism of a large area, as in the case of the Hatrurim Complex, determine the formation of a significant volume of reducing gases (CH 4 , H 2 , H 2 S, CO, NO) as a result of the pyrolytic decomposition of organic matter (bitumen, oil) contained in the sedimentary protolith.The crystallization of highly reduced phases proceeds along the paths of flow of reducing gases.For example, small crystals of oldhamite, CaS, formed on the walls of micron-sized channels penetrating spurrite marble of the Hatrurim Basin [40].Sometimes reducing gases have a significant effect on pyrometamorphic rocks, which is expressed in the crystallization of rock-forming oldhamite in larnite rock (Figure 15a) by the reaction: CaO + H 2 S gas = CaS + H 2 O gas .In larnite rock, oldhamite is associated with Fe 3+ -bearing minerals such as brownmillerite, Ca 2 FeAlO 5 (Figure 15a), and cannot be an indicator of the reduction conditions for the entire rock volume.Caswellsilverite crystallizes in the central porous parts of paralava together with pyrrhotite (Figure 2b), whereas in non-porous fragments of paralava, small iron drops form (Figure 2a) that can indicate that the primary iron melt is enriched in sulphur carried by combustion gases.Experimental studies indicate that caswellsilverite (and grokhovskyite) form in paralava at relatively higher oxygen activity 0 ≤ ΔIW< −2 in comparison with oldhamite [42].This suggests that super-or high-reduction conditions (ΔIW ≈ −6-−2) at the contact zone of paralava with clasts of altered country rock change within a distance of a few centimetres (the central parts of paralava zones) to the reduction conditions near the Fe/FeO (ΔIW ≈ 0) buffer.Caswellsilverite and pyrrhotite crystallize from sulphide melt mosaically distributed in paralava between previously crystallized silicates (Figure 2b,c).Sodium, which is necessary for сaswellsilverite genesis, is probably introduced into the same portion of the sulphide melt as a result of the replacement of flamite, Ca2-x (Na,K)x(Si1-xPx)O4 by rankinite, Ca3Si2O7 [18].In experiments, NaCrS2 crystals were obtained from alkaline polysulfide melt at temperatures below 1000 °C.It was also shown that they decompose slowly in the atmosphere at room temperature and are relatively quickly oxidized at temperatures above 1000 °C with the formation of NaCrO2 and Cr2O3 crystals [43].Both types of caswellsilverite high-temperature alteration products we observed to form due to the thermal effect of the Raman probe (Figures 11 and 14).
In a previous analysis of the conditions of the genesis of Cr-V-bearing phosphides (barringerite, allabogdanite, andreyivanovite) in the same breccia, it was suggested that high-reduction conditions are necessary for the enrichment of Fe(+P) melt by Cr and V [1].Additionally, we noted a local enrichment of Fe(±P,C) melt by Cu (Figure 1d).All these observations can be applied to sulphide melts, the crystallization of which between previously formed gehlenite crystals (Figure 2b,c), on solidified Fe drops (Figure 10b) and wall cavities (Figure 5a) led to the formation of Fe-monosulfide with a higher Cr(+Cu) concentration.Later, monosulphide transformed into lamellar polysynthetic aggregates of pyrrhotite with the grokhovskyite exsolution structures (Figure 5b).The local enrichment of sulphide melt by Ag led to the crystallization of a potentially new mineral AgCrS2 (Figure 8).
Hexagonal octahedral layers (CrS2) 1-, between which M-sites of the monovalent cations Ag, Cu, and Na set, are present in the structures of layered chromium disulphides, МCrS2 (Figure 16), [44][45][46][47][48].The sodium is at the octahedral coordination, whereas Cu and Ag are in the deformed tetrahedra.There are two types of tetrahedral site: α-and β-(Figure 16b) [45][46][47][48][49]. Ordered disulphide forms at temperatures below ∼500 °С as a result of the occupation of the first type of site.In CuCrS2, an effect of some Cr moving into the space between disulphide layers was observed [45] (Figure 16c).In considering the super-and high-reduced mineral associations in pyrometamorphic rocks, two main forms of "meteorite" mineral formation should be taken into account: (1) mineral formation reactions following the short-distance transport of reacting components on the contact of hot paralava and country rock containing the reductant (carbonaceous matter); (2) mineral formation as a result of reducing gases reacting with minerals of the early "clinker" association.
The generation of gehlenite paralava with "meteoritic" chromium disulphides took place at the combustion foci at a high temperature (probably higher than 1500 • C) and low pressure [1].An intrusion of paralava into brecciated clay-carbonate sedimentary rock containing phosphatised and graphitized organic matter as well as iron oxides caused the formation of mineral aggregates of the Fe-P(+C, Cr, V) system on the boundary of paralava and country rock (Figure 1a-c) as a result of high-temperature carbothermal reduction reactions [1,18].On iron phosphide aggregates presented by barringerite, and schreibersite, the rim of the Fe-schreibersite eutectic was formed.This is where the monosulphide phase, which later transforms into lamellar pyrrhotite and daubréelite aggregates (Figures 1f and 8d), was detected.In rare cases, phosphides and pyrrhotite associate with osbornite, TiN-a mineral-indicator of the super-reducing conditions (f O 2 < iron-wüstite buffer ∆IW ≈ −6) [18,41].
In the studied phosphide-bearing breccia, a "meteoritic" sulphide, oldhamite, CaS, rarely encountered as small rounded inclusions in pseudowollastonite from the paralava contact zone, crystallizes from melt (Figure 15b), and can be an indicator of the reducing conditions.The investigation of sulphide genesis in "mercurian melt" showed that oldhamite is stable at about ∆IW ≈ −2 [42].
Caswellsilverite crystallizes in the central porous parts of paralava together with pyrrhotite (Figure 2b), whereas in non-porous fragments of paralava, small iron drops form (Figure 2a) that can indicate that the primary iron melt is enriched in sulphur carried by combustion gases.Experimental studies indicate that caswellsilverite (and grokhovskyite) form in paralava at relatively higher oxygen activity 0 ≤ ∆IW< −2 in comparison with oldhamite [42].This suggests that super-or high-reduction conditions (∆IW ≈ −6-−2) at the contact zone of paralava with clasts of altered country rock change within a distance of a few centimetres (the central parts of paralava zones) to the reduction conditions near the Fe/FeO (∆IW ≈ 0) buffer.Caswellsilverite and pyrrhotite crystallize from sulphide melt mosaically distributed in paralava between previously crystallized silicates (Figure 2b,c).Sodium, which is necessary for caswellsilverite genesis, is probably introduced into the same portion of the sulphide melt as a result of the replacement of flamite, Ca 2-x (Na,K) x (Si 1-x P x )O 4 by rankinite, Ca 3 Si 2 O 7 [18].In experiments, NaCrS 2 crystals were obtained from alkaline polysulfide melt at temperatures below 1000 • C. It was also shown that they decompose slowly in the atmosphere at room temperature and are relatively quickly oxidized at temperatures above 1000 • C with the formation of NaCrO 2 and Cr 2 O 3 crystals [43].Both types of caswellsilverite high-temperature alteration products we observed to form due to the thermal effect of the Raman probe (Figures 11 and 14).
In a previous analysis of the conditions of the genesis of Cr-V-bearing phosphides (barringerite, allabogdanite, andreyivanovite) in the same breccia, it was suggested that high-reduction conditions are necessary for the enrichment of Fe(+P) melt by Cr and V [1].Additionally, we noted a local enrichment of Fe(±P, C) melt by Cu (Figure 1d).All these observations can be applied to sulphide melts, the crystallization of which between previously formed gehlenite crystals (Figure 2b,c), on solidified Fe drops (Figure 10b) and wall cavities (Figure 5a) led to the formation of Fe-monosulfide with a higher Cr(+Cu) concentration.Later, monosulphide transformed into lamellar polysynthetic aggregates of pyrrhotite with the grokhovskyite exsolution structures (Figure 5b).The local enrichment of sulphide melt by Ag led to the crystallization of a potentially new mineral AgCrS 2 (Figure 8).
The content of water in "mineral X" and caswellsilvertite was calculated on the basis Low-temperature alterations of layered chromium disulphides are exclusively reflected in changes to the composition and structure of the monovalent cation layer, whereas the hexagonal octahedral layer (CrS 2 ) 1− stays practically unaltered (Tables 2, 3, 5 and 6).Hydrated products of synthetic NaCrS 2 were experimentally studied [49], and later they were discovered in meteorites as the natural minerals schöllhornite, Na 0.3 CrS 2 •H 2 O [12], phases of A and B type ≈ (Na,K) 0.07-0.12CrS 2 •nH 2 O [14] and cronusite, Ca 0.2 CrS 2 •(H 2 O) 2 [13].Caswellsilverite and grokhovskyite in gehlenite paralava are replaced by the potentially new "mineral X" with high Fe content.This process proceeds simultaneously with pyrrhotite oxidation (Fe source) and through an intermediate phase of schöllhornite-type (Tables 2 and 3; Figures 2d, 3a and 5b)."Mineral X" has a variable (non-stoichiometric) composition (Figures 2, 3, 5 and 6), but nevertheless, its composition can be described by a non-idealized formula {Fe 0.3 (Ba,Ca) 0.2 }CrS 2 •0.5H 2 O, whose charge can be balanced only if all Fe is represented by the Fe 2+ cation or the Fe 3+ (OH) − complex.The appearance of Fe 2+ in Fe 3+ -hydroxide aggregates replacing pyrrhotite is hardly probable, and this mineral needs further investigation.Schöllhornite was found at the central part of pseudomorphs of "mineral X" after caswellsilverite (Figure 9) and also as thin zones between the "mineral X" rim and the caswellsilverite core (Figure 2d).The sum of cations (Na+Sr+Ca+Ba+Fe) in the intermedium layer of caswellsilverite varied from 0.14 (Na = 0.09) to 0.35 (Na = 0.27) apfu (Table 5).
The content of water in "mineral X" and caswellsilvertite was calculated on the basis of microprobe analyses as a difference of a total of 100%.The water content of "mineral X" is lower than that of caswellsilverite or cronusite.This can be connected both with the conditions of the microprobe analyses and with genuinely low water concentrations in altered layered sulphides in a hot desert climate.
In conclusion, the necessary conditions for the appearance of "meteoritic" chromium disulphides in terrestrial rock are high chromium content, high temperatures up to ~1500 • C, low pressure, and high reducing formation conditions, i.e., conditions usually realized in the processes of meteorite genesis.

Figure 1 .
Figure 1.(a) Polished fragment of breccia, on the boundary of altered country rock clasts (brown hue) and hydrated gehlenite paralava (grey) there is a large amount of mineral aggregate of the Fe-P(±C) system (white).(b) Polished breccia fragment with black weakly altered gehlenite paralava.

Figure 4 .
Figure 4. Reflectance spectra of chromium disulphides and pyrrhotite from gehlenite paralava.The approximate orientation of crystals during measurements is shown by rectangles on the right side of the figure.

Figure 11 .
Figure 11.(a,b) Raman spectra of caswellsilverite in two orientations: perpendicular (a) and parallel (b) to the polarized incident laser beam.Spots of Raman spectra measurement are shown by white circles in reflected light images.(c) Raman spectrum obtained after thermal laser effect.

Figure 11 .Figure 12 .
Figure 11.(a,b) Raman spectra of caswellsilverite in two orientations: perpendicular (a) and parallel (b) to the polarized incident laser beam.Spots of Raman spectra measurement are shown by white circles in reflected light images.(c) Raman spectrum obtained after thermal laser effect.Minerals 2023, 13, x FOR PEER REVIEW 14 of

Figure 12 .
Figure 12.Raman spectra of grokhovskyite in two orientations: perpendicular (a) and parallel (b) to the polarized incident laser beam Points of spectra measurements are marked by white circles in reflected light images.

Figure 12 .
Figure 12.Raman spectra of grokhovskyite in two orientations: perpendicular (a) and parallel (b) to the polarized incident laser beam Points of spectra measurements are marked by white circles in reflected light images.

21 Figure 16 .
Figure 16.Structures of trigonal synthetic layered chromium disulphides, projection on (010): (a)Caswellsilverite analogue[44]; (b) analogue of potentially new mineral AgCrS2[44], chromium occupies tetrahedral sites of the β-type, empty circles show vacancy sites of the α-type; (c) analogue of grokhovskyite with vacancies in Cr-disulphide layer, fully occupied Cu-tetrahedral sites and additional Cr-sites (deformed octahedron, green balls) in cation layer[45].Atoms/polyhedra are shown in green for Cr; dark blue for Cu; light blue for Ag; light yellow for Na; and yellow for S.

Figure 16 .
Figure 16.Structures of trigonal synthetic layered chromium disulphides, projection on (010): (a) Caswellsilverite analogue [44]; (b) analogue of potentially new mineral AgCrS 2[44], chromium occupies tetrahedral sites of the β-type, empty circles show vacancy sites of the α-type; (c) analogue of grokhovskyite with vacancies in Cr-disulphide layer, fully occupied Cu-tetrahedral sites and additional Cr-sites (deformed octahedron, green balls) in cation layer[45].Atoms/polyhedra are shown in green for Cr; dark blue for Cu; light blue for Ag; light yellow for Na; and yellow for S.

Author
Contributions: E.V.G. and I.O.G. discovered the layered chromium disulfides, the main idea of the article, performed the optical and Raman investigation and wrote the article.Y.V. carried out the fieldwork, geological description and verification of the data.G.Z. performed the microprobe analyses.All authors have read and agreed to the published version of the manuscript.Funding: The investigations were supported by the National Science Center of Poland Grant [grant number 2021/41/B/ST10/00130].

Table 4 .
Chemical composition of Ag analogue of grokhovskyite.

Table 4 .
Chemical composition of Ag analogue of grokhovskyite.

Table 4 .
Chemical composition of Ag analogue of grokhovskyite.