# Geominerals/Sulfides

Sulfide also sulphide is an inorganic anion of sulfur with the chemical formula S$^{2−}$ or a compound containing one or more S$^{2−}$ ions. Solutions of sulfide salts are corrosive. Sulfide also refers to chemical compounds: large families of inorganic and organosulfur compounds, e.g. lead sulfide and dimethyl sulfide. Hydrogen sulfide (H $_2$S) and bisulfide (SH$^−$ ) are the conjugate acids of sulfide.
The sulfide ion, S$^{2−}$ , does not exist in aqueous alkaline solutions of Na $_2$S. Instead sulfide converts to hydrosulfide:
    S$^{2−}$ + H $_2$O → SH$^−$ + OH$^−$
Upon treatment with an acid, sulfide salts convert to hydrogen sulfide:
    S$^{2−}$ + H$^+$ → SH$^−$
    SH$^−$ + H$^+$ → H $_2$S
Oxidation of sulfide is dependent on the conditions, producing
* elemental sulfur,
* polysulfides,
* polythionates,
* sulfites, or
* sulfates.
Metal sulfides react with halogens, forming sulfur and metal salts.
    8 MgS + 8 I$_2$ → S$_8$ + 8 MgI$_2$
Aqueous solutions of transition metals cations react with sulfide sources (H $_2$S, NaHS, Na $_2$S) to precipitate solid sulfides. Such inorganic sulfides typically have very low solubility in water, and many are related to minerals with the same composition (see below). One famous example is the bright yellow species CdS or "cadmium yellow". The black tarnish formed on sterling silver is Ag$_2$S. Such species are sometimes referred to as salts. In fact, the bonding in transition metal sulfides is highly covalent, which gives rise to their semiconductor properties, which in turn is related to the deep colors. Several have practical applications as pigments, in solar cells, and as catalysts. The fungus Aspergillus niger plays a role in the solubilization of heavy metal sulfides.
Many important metal ores are sulfides. Significant examples include: argentite (silver sulfide), cinnabar (mercury sulfide), galena (lead sulfide), molybdenite (molybdenum sulfide), pentlandite (nickel sulfide), realgar (arsenic sulfide), and stibnite (antimony), sphalerite (zinc sulfide), and pyrite (iron disulfide), and chalcopyrite (iron-copper sulfide).
Dissolved free sulfides (H$_2$S, HS$^−$ and S$^{2−}$) are very aggressive species for the corrosion of many metals such as steel, stainless steel, and copper. Sulfides present in aqueous solution are responsible for stress corrosion cracking (SCC) of steel, and is also known as sulfide stress cracking. Corrosion is a major concern in many industrial installations processing sulfides: sulfide ore mills, deep oil wells, pipelines transporting soured oil, Kraft paper factories.
Microbially-induced corrosion (MIC) or biogenic sulfide corrosion are also caused by sulfate reducing bacteria producing sulfide that is emitted in the air and oxidized in sulfuric acid by sulfur oxidizing bacteria. Biogenic sulfuric acid reacts with sewerage materials and most generally causes mass loss, cracking of the sewer pipes and ultimately, structural collapse. This kind of deterioration is a major process affecting sewer systems worldwide and leading to very high rehabilitation costs.
Oxidation of sulfide can also form thiosulfate (S $_2$O$^{2−}$ $_3$) an intermediate species responsible for severe problems of pitting corrosion of steel and stainless steel while the medium is also acidified by the production of sulfuric acid when oxidation is more advanced.
In organic chemistry, "sulfide" usually refers to the linkage C–S–C, although the term thioether is less ambiguous. For example, the thioether dimethyl sulfide is CH$_3$–S–CH$_3$. Polyphenylene sulfide (see below) has the empirical formula C$_6$H$_4$S. Occasionally, the term sulfide refers to molecules containing the –SH functional group. For example, methyl sulfide can mean CH$_3$–SH. The preferred descriptor for such SH-containing compounds is thiol or mercaptan, i.e. methanethiol, or methyl mercaptan.
Confusion arises from the different meanings of the term "disulfide". Molybdenum disulfide (MoS$_2$) consists of separated sulfide centers, in association with molybdenum in the formal +4 oxidation state (that is, Mo$^{4+}$ and two S$^{2−}$). Iron disulfide (pyrite, FeS$_2$) on the other hand consists of S$^{2−}$ $_2$, or $^−$S–S$^−$ dianion, in association with divalent iron in the formal +2 oxidation state (ferrous ion: Fe$^{2+}$). Dimethyldisulfide has the chemical binding CH$_3$–S–S–CH$_3$, whereas carbon disulfide has no S–S bond, being S=C=S (linear molecule analog to CO$_2$). Most often in sulfur chemistry and in biochemistry, the disulfide term is commonly ascribed to the sulfur analogue of the peroxide –O–O– bond. The disulfide bond (–S–S–) plays a major role in the conformation of proteins and in the catalytic activity of enzymes.

## Chemical sulfides

| Formula | | Melting point (°C) | Boiling point (°C) | CAS number |
| H$_2$S | Hydrogen sulfide is a very toxic and corrosive gas characterised by a typical odour of "rotten egg". | −85.7 | −60.20 | CAS# 7783-06-4 |
| CdS | Cadmium sulfide can be used in photocells. | 1750 | | CAS# 1306-23-6 |
| | Calcium polysulfide ("lime sulfur") is a traditional fungicide in gardening. | | | |
| CS$_2$ | Carbon disulfide is a precursor to organosulfur compounds. | −111.6 | 46 | CAS# 75-15-0 |
| PbS | Lead sulfide is used in infra-red sensors. | 1114 | | CAS# 1314-87-0 |
| MoS$_2$ | Molybdenum disulfide, the mineral molybdenite, is used as a catalyst to remove sulfur from fossil fuels; also as lubricant for high-temperature and high-pressure applications. | | | CAS# 1317-33-5 |
| Cl–CH$_2$CH$_2$–S–CH$_2$CH$_2$–Cl | Sulfur mustard (mustard gas) is an organosulfur compound (thioether) that was used as a chemical weapon in the First World War. | 13–14 | 217 | CAS# 505-60-2 |
| Ag$_2$S | Silver sulfide is a component of silver tarnish. | | | CAS# 21548-73-2 |
| Na$_2$S | Sodium sulfide, as the hydrate, is used in manufacture of Kraft paper and as a precursor to organosulfur compounds. | 920 | 1180 | CAS# 1313-82-2 |
| ZnS | Zinc sulfide is used for lenses and other optical devices in the infrared part of the spectrum. ZnS-doped with silver is used in alpha detectors while zinc sulfide with traces of copper has applications in photoluminescent strips for emergency lighting and luminous watch dials. | | 1185 | CAS# 1314-98-3 |
| C$_6$H$_4$S | Polyphenylene sulfide is a polymer commonly called "Sulfar". Its repeating units are bonded together by sulfide (thioether) linkages. | | | CAS# 26125-40-6 CAS# 25212-74-2 |
| SeS$_2$ | Selenium disulfide is an Antifungal medication used in anti-dandruff preparations, such as Selsun Blue. The presence of the highly toxic selenium in healthcare and cosmetics products represents a general health and environmental concern. | <100 | | CAS# 7488-56-4 |
| FeS$_2$ | Known as "fool's gold", pyrite, is a common mineral. | 600 | | CAS# 1317-66-4 |
Sulfide compounds can be prepared in several different ways:
* Direct combination of elements:
    Example: Fe$_{solid}$ + S$_{solid}$ → FeS$_{solid}$
* Reduction of a sulfate:
    Example: MgSO$_{4solid}$ + 4C$_{solid}$ → MgS$_{solid}$ + 4CO$_{gaseous}$
* Precipitation of an insoluble sulfide:
    Example: M$^{2+}$ + H$_2$S$_{gaseous}$ → MS$_{solid}$ + 2H$^+$$_{aqueoussolution}$

## Abramovites

Abramovite (IMA symbol: Abm) is a very rare mineral of the sulfides and sulfosalt categories, with the chemical formula Pb $_2$SnInBiS $_7$ that occurs as tiny elongated lamellar-shaped crystals, up 1 mm × 0.2 mm in size, and characterized by its non-commensurate structure.
Abramovite is named after the mineralogist Dmitry Vadimovich Abramov (born 1963) of the A.E. Fersman Mineralogical Museum, Russia
It was discovered as fumarole crust on the Kudriavy (Kudryavyi) volcano, Iturup Island, Kuril Islands, Sakhalin Oblast, Far East Region, Russia.
Abramovite is a product of precipitation from fumarolic gases (600 °C [1,112 °F]) in an active stratovolcano.
Abramovite together with Kylindrite and Lévyclaudite form the Kylindrite group of minerals.

## Acanthites

Acanthites crystallize in the Monoclinic system.
Acanthite (IMA symbol: Aca is a form of silver sulfide with the chemical formula Ag $_2$S, is the stable form of silver sulfide below 173 °C (343 °F). Below 173 °C acanthite forms directly. Acanthite is the only stable form in normal air temperature.
Acanthite is a common silver mineral in moderately low-temperature hydrothermal veins and in zones of supergene enrichment. It occurs in association with native silver, pyrargyrite, proustite, polybasite, stephanite, aguilarite, galena, chalcopyrite, sphalerite, calcite and quartz.

## Aguilarites

Aguilarite (International Mineralogical Association (IMA) symbol Agu is an uncommon sulfosalt mineral with formula Ag $_4$SeS. It was described in 1891 and named for discoverer Ponciano Aguilar.
Aguilarite is bright lead-gray on fresh surfaces but becomes dull iron black when exposed to air. The mineral occurs with massive habit, as elongated pseudododecahedral crystals up to 3 cm (1.2 in), or as intergrowths with acanthite or naumannite.
In the late 19th century, Ponciano Aguilar, superintendent of the San Carlos mine in Guanajuato, Mexico, found several specimens of a mineral thought to be naumannite. The samples were given to F. A. Genth for identification, who, along with S. L. Penfield, discovered that it was a new mineral. The mineral was described in the American Journal of Science in 1891 and named aguilarite in honor of Ponciano Aguilar. When the International Mineralogical Association was founded, aguilarite was grandfathered as a valid mineral species.
Aguilarite is uncommon, and forms at relatively low temperatures in hydrothermal deposits rich in silver and selenium but deficient in sulfur. The mineral is known from a number of countries in North and South America, Europe, Asia, and Australasia. Aguilarite occurs in association with acanthite, calcite, naumannite, pearceite, proustite, silver, stephanite, and quartz.
In 2013, aguilarite's chemistry and crystal structure were reexamined. The significant reevaluation of aguilarite did not discredit its status as a valid mineral, but it was established as the selenium analogue of acanthite instead of sulfur-rich naumannite. The sample primarily studied came from the Gem and Mineral Collection of the Department of Geosciences at Princeton University.
The work of Petruk et al. in 1974 formed the basis of knowledge regarding the silver–sulfur–selenium system for about forty years. They indexed their x-ray diffraction patterns of aguilarite on an orthorhombic cell similar to naumannite. Aguilarite is, in fact, monoclinic and is isostructural to acanthite and not naumannite. Petruk et al. may not have been able to resolve closely spaced peaks due to low resolution equipment, making aguilarite appear similar to naumannite. Additionally, a number of inconsistencies in unit cell dimensions in the 1974 work show that aguilarite does not have the same structure as naumannite.
The crystal structure of aguilarite consists of planes nearly parallel to miller index (010) composed of tetrahedrally coordinated nonmetal atoms and AgX $_3$ triangles (where X is a nonmetal). The planes are joined by twofold-coordinated silver atoms.
Aguilarite is part of the acanthite-like solid solution series Ag $_2$S–Ag $_2$S $_{0.4}$Se $_{0.6}$. The mineral comprises the range from 50 atomic percent selenium up to the transition from monoclinic to orthorhombic.

## Aikinites

Aikinite (International Mineralogical Association (IMA) symbol: Aik is a sulfide mineral of lead, copper and bismuth with formula PbCuBiS $_3$, forms black to grey or reddish brown acicular orthorhombic crystals with a Mohs hardness of 2 to 2.5 and a specific gravity of 6.1 to 6.8, was originally found in 1843 in the Beryozovskoye deposit, Ural Mountains, is named after Arthur Aikin (1773–1854), an English geologist.
It has been found in Western Tasmania, in mines located near Dundas, Tasmania.

## Aktashites

Aktashites have the chemical formula: Cu $_6$Hg $_3$As $_4$S $_{12}$.
Aktashite (International Mineralogical Association (IMA) symbol: Ats) is a rare arsenic sulfosalt mineral, the only one known, of hydrothermal origin.
Type Locality: Aktashskoye Sb-Hg deposit, Ulagansky District, Altai Republic, Russia.
Common Impurities: Zn and Sb.
Crystal System: Trigonal.
Morphology: Rarely in crystals resembling trigonal pyramids, to 0.2 mm, which may be zoned with gruzdevite; as xenomorphic grains and granular aggregates.
Geological Setting: Hydrothermal veins.
Geological Setting of Type Material: Uncommon, of hydrothermal origin in complex polymetallic As–Hg-bearing deposits.
Associated Minerals at Type Locality: Tennantite Subgroup, Stibnite, Sphalerite, Realgar, Quartz, Pyrite, Orpiment, Mercurian Tetrahedrite, Luzonite, Enargite, Dickite, Cinnabar, Chalcostibite, Chalcopyrite and Calcite.
Association: Stibnite, chalcostibite, mercurian tetrahedrite, tennantite, luzonite, enargite, cinnabar, chalcopyrite, pyrite, sphalerite, realgar, orpiment, dickite, quartz, calcite.
Isostructural with: Gruzdevite and Nowackiite.
Forms a series with: Gruzdevite, Aktashite-Gruzdevite Series.
Associated Minerals Based on Photo Data: 3 photos of Aktashite associated with Arsiccioite AgHg $_2$Tl(As,Sb) $_2$S $_6$, 1 photo of Aktashite associated with Realgar As $_4$S $_4$.

## Alabandites

Alabandite or alabandine (International Mineralogical Association (IMA) symbol: Abd) is a rarely occurring manganese sulfide mineral that crystallizes in the cubic crystal system with the chemical composition Mn$^{2+}$ S and develops commonly massive to granular aggregates, but rarely cubic or octahedral crystals to 1 cm.
Member of the Galena Group.
Other Members of this group: Altaite PbTe, Clausthalite PbSe, Galena PbS, Niningerite (Mg,Fe$^{2+}$ ,Mn$^{2+}$ )S, Oldhamite (Ca,Mg)S.
At ambient pressure, alabandite (α-MnS) is the stable MnS polymorph from room temperature up to the melting point of 1655 °C (Staffansson, 1976; Kang, 2010).
Polymorphism & Series: Dimorphous with rambergite.
Occurrence: May be in large quantities in epithermal polymetallic sulfide veins and especially in low-temperature manganese deposits, an uncommon constituent of a number of meteorites.
Association: Galena (PbS), chalcopyrite (CuFeS $_2$), sphalerite {ZnS), pyrite (FeS $_2$), acanthite (Ag $_2$S), native tellurium, rhodochrosite (MnCO $_3$), calcite, rhodonite (CaMn $_3$Mn[Si $_5$O $_{15}$]), quartz.
Alabandite crystallizes in the cubic crystal system in the space group Fm3m with the lattice parameter a = 5.22 Å and four formula units per unit cell.
Common Impurities: Fe,Mg,Co.

## Alacránites

Alacránite (As $_8$S $_9$) is an arsenic sulfide mineral first discovered in the Uzon caldera, Kamchatka, Russia (International Mineralogical Association (IMA) symbol: Acr.) It was named for its occurrence in the Alacrán silver/arsenic/antimony mine, Pampa Larga, Chile. It is generally more rare than realgar and orpiment. Its origin is hydrothermal, as subhedral to euhedral tabular orange to pale gray crystals that are transparent to translucent. It has a yellow-orange streak with a hardness of 1.5. It crystallizes in the monoclinic crystal system. It occurs with realgar and uzonite as flattened and prismatic grains up to 0.5 mm across.
Polymorphism & Series: Trimorphous with pararealgar and realgar.
Occurrence: In hydrothermal As–S veins (Alacr ́an mine, Chile); in the condensation zone of a hydrothermal Hg–Sb–As system as cement in a sandy gravel (Uzon caldera, Russia); formed at low temperatures in a polymetallic hydrothermal deposit on a submarine seamount (Conical Seamount, Papua New Guinea).
Association: Realgar, orpiment, arsenic, sulfur, stibnite, pyrite, greigite, arsenopyrite, arsenolamprite, sphalerite, acanthite, barite, quartz, calcite (Alacr ́an mine, Chile); realgar, orpiment, uzonite, stibnite, cinnabar, pyrite, sulfur (Uzon caldera, Russia); realgar, pyrite, sphalerite, galena, chalcopyrite, amorphous silica (Conical Seamount, Papua New Guinea).
Geological Setting: in the condensation zone of a hydrothermal Hg–Sb–As system as cement in a sandy gravel (Uzon caldera, Russia); formed at low temperatures in a polymetallic hydrothermal deposit on a submarine seamount (Conical Seamount, Papua New Guinea).
Forms a series with: Bonazziite, Bonazziite-Alacranite Series.

## Aleksites

Aleksite (International Mineralogical Association(IMA) symbol: Alk) is a rare lead bismuth tellurium sulfosalt mineral with formula PbBi $_2$Te $_2$S $_2$.
Member of the Aleksite Group.
Geological Setting: hydrothermal origin in sulfide-quartz veins.
Associated Minerals at Type Locality: Galena, Gold, Altaite, Tetradymite, Tsumoite, Rucklidgeite and Quartz.
Other Members of this group: Babkinite Pb $_2$Bi $_2$(S,Se) $_3$, Kochkarite PbBi $_4$Te $_7$, Poubaite PbBi $_2$(Se,Te,S) $_4$, Rucklidgeite PbBi $_2$Te $_4$ and Saddlebackite Pb $_2$Bi $_2$Te $_2$S $_3$.

## Alloclasites

Alloclasite, formula: (Co,Fe)AsS, is a sulfosalt mineral (International Mineralogical Association (IMA) symbol: Acl). It is a member of the arsenopyrite group crystallizes in the monoclinic system, typically forms as columnar to radiating acicular prismatic clusters, is an opaque steel-gray to silver-white, with a metallic luster, a black streak, is brittle with perfect cleavage, a Mohs hardness of 5 and a specific gravity of 5.91–5.95.
It was first described in 1866 for an occurrence in Romania. Its name is derived from Greek for "other" and "to break," in reference to its distinct cleavage which distinguished it from the similar appearing mineral marcasite.
The mineral is monoclinic in the P2$_1$ space group.

## Argentites

Argentite is the stable form of Ag $_2$S above 173 °C (343 °F). Or, 177 °C or 179 °C. As argentite cools below that temperature its cubic form is distorted to the monoclinic polymorph of acanthite.
The International Mineralogical Association has decided to reject argentite as a proper mineral.
The name "argentite" sometimes also refers to pseudomorphs of argentite: specimens of acanthite which still display some of the outward signs of the cubic crystal form, even though their actual crystal structure is monoclinic due to the lower temperature. This form of acanthite is occasionally found as uneven cubes and octahedra, but more often as dendritic or earthy masses, with a blackish lead-grey color and metallic luster.
Argentite belongs to the galena group, is perfectly sectile and has a shining streak; hardness 2.5, specific gravity is 7.2–7.4, occurs in mineral veins, and when found in large masses, as in Mexico and in the Comstock Lode in Nevada, forms an important ore of silver, mentioned in 1529 by G. Agricola, but the name argentite was not used till 1845 and is due to W. Haidinger, with old names for the species are Glaserz, silver-glance and vitreous silver, where a related copper-rich mineral occurring e.g. in Jalpa, Zacatecas, Mexico, is known as jalpaite.

## Arsensulfurites

Arsensulfurites have the chemical formula: (S,As) $_8$.
Crystal System: Amorphous.
Arsensulfurite is a variety of Sulfurite.
An arsenic-rich (10-33 mas.% As) sulfurite. Originally described from Papandagan volcano, Java, Indonesia.

## Berryites

Berryites have the chemical formula Cu $_3$Ag $_2$Pb $_3$Bi $_7$S $_{16}$.
Environment: "In quartz veins with other sulfides and sulfosalts, and in siderite-rich cryolite."
Geological Setting: "Hydrothermal veins"
Association: Emplectite, aikinite, cuprobismutite, cupropavonite (Colorado, USA); galena, chalcopyrite, sphalerite, quartz (Nordmark, Sweden); galena, cosalite, ourayite, matildite, aikinite (Ivigtut, Greenland); aikinite, matildite, benjaminite, quartz, barite (Tary Ekan deposit, Kazakhstan).
"Structurally related to litochlebite and watkinsonite."

## Cinnabars

Cinnabar or cinnabarite (red mercury(II) sulfide (HgS), native vermilion), is the common ore of mercury. Its color is cochineal-red, towards brownish red and lead-gray. Cinnabar [may be] found in a massive, granular or earthy form and is bright scarlet to brick-red in color. Generally cinnabar occurs as a vein-filling mineral associated with recent volcanic activity and alkaline hot springs. Cinnabar is deposited by epithermal ascending aqueous solutions (those near surface and not too hot) far removed from their igneous source.

## Covellites

Covellite [CuS] has been found in veins at depths of 1,150 meters, as the primary mineral. Covellite formed as clusters in these veins reaching one meter across. Covellite is a hexagonal form of CuS. Covellite is a chalcogen.

## Cubanites

Cubanite is a yellow mineral of copper, iron, and sulfur, CuFe$_2$S$_3$. Cubanite occurs in high temperature hydrothermal deposits with pyrrhotite and pentlandite as intergrowths with chalcopyrite. It results from exsolution from chalcopyrite at temperatures below 200 to 210 °C. It has also been reported from carbonaceous chondrite meteorites.

## Galenas

Galena in the image on the right is the metallic cuboidal crystal atop a matrix. Galena is PbS, 50 atomic % lead and 50 atomic % sulfur. Each cubic unit cell contains four PbS molecules in a face-centered cubic lattice.

## Gallites

Gallite (CuGaS$_2$) is 25 at % gallium.

## Germanites

The sample of germanite on the right has a composition of Cu$_{26}$Fe$_4$Ge$_4$S$_{32}$. Generally, germanite has a composition closer to Cu$_3$(Ge, Ga, Fe, Zn) (S,As)$_4$. "This sample also contains tennantite."

## Heazlewoodites

Heazlewoodite, Ni$_3$S$_2$, is a rare sulfur-poor nickel sulfide mineral found in serpentinitized dunite. It occurs as disseminations and masses of opaque, metallic light bronze to brassy yellow grains which crystallize in the trigonal crystal system. It has a hardness of 4, a specific gravity of 5.82. Heazlewoodite was first described in 1896 from Heazlewood, Tasmania, Australia.
Heazlewoodite is formed within terrestrial rocks by metamorphism of peridotite and dunite via a process of nucleation. Heazlewoodite is the least sulfur saturated of nickel sulfide minerals and is only formed via metamorphic exsolution of sulfur from the lattice of metamorphic olivine.
Heazlewoodite forms from sulfur and nickel which exist in pristine olivine in trace amounts, and which are driven out of the olivine during metamorphic processes. Magmatic olivine generally has up to ~4000 ppm Ni and up to 2500 ppm S within the crystal lattice, as contaminants and substituting for other transition metals with similar ionic radii (Fe$^{2+}$ and Mg$^{2+}$).
During metamorphism, sulfur and nickel within the olivine lattice are reconstituted into metamorphic sulfide minerals, chiefly millerite, during serpentinization and talc carbonate alteration.
When metamorphic olivine is produced, the propensity for this mineral to resorb sulfur, and for the sulfur to be removed via the concomitant loss of volatiles from the serpentinite, tends to lower sulfur fugacity.
In this environment, nickel sulfide mineralogy converts to the lowest-sulfur state available, which is heazlewoodite.
Heazlewoodite is known from few ultramafic intrusions within terrestrial rocks. The Honeymoon Well ultramafic intrusive, Western Australia is known to contain heazlewoodite-millerite sulfide assemblages within serpentinized olivine adcumulate dunite, formed from the metamorphic process.
The mineral is also reported, again in association with millerite, from the ultramafic rocks of New Caledonia.

## Lorándite

Lorándite can have the formula TlAsS$_2$.

## Millerites

Millerite is a nickel sulfide mineral, NiS. Millerite is a common metamorphic mineral replacing pentlandite within serpentinite ultramafics. It is formed in this way by removal of sulfur from pentlandite or other nickeliferous sulfide minerals during metamorphism or metasomatism.
Millerite is also formed from sulfur poor olivine cumulates by nucleation. Millerite is thought to form from sulfur and nickel which exist in pristine olivine in trace amounts, and which are driven out of the olivine during metamorphic processes. Magmatic olivine generally has up to ~4000 ppm Ni and up to 2500 ppm S within the crystal lattice, as contaminants and substituting for other transition metals with similar ionic radii (Fe$^{2+}$ and Mn$^{2+}$ ).
During metamorphism, sulfur and nickel within the olivine lattice are reconstituted into metamorphic sulfide minerals, chiefly millerite, during serpentinization and talc carbonate alteration. When metamorphic olivine is produced, the propensity for this mineral to resorb sulfur, and for the sulfur to be removed via the concomitant loss of volatiles from the serpentinite, tends to lower sulfur fugacity.
This forms disseminated needle like millerite crystals dispersed throughout the rock mass. Millerite may be associated with heazlewoodite and is considered a transitional stage in the metamorphic production of heazlewoodite via the above process.
"Millerite, NiS, fractured under high vacuum and reacted with air and water has been analyzed by X-ray photoelectron spectroscopy (XPS). The pristine millerite surface gives rise to photoelectron peaks at binding energies of 853.1 eV (Ni 2p3/2) and 161.7 eV (S 2p), thus resolving ambiguities concerning binding energies quoted in the literature. Air-reacted samples show the presence of NiSO $_4$ and Ni(OH) $_2$ species. There is evidence for polysulfide species (S$^{2-􏰄}$ $_n$, where 2 􏰀≤ n 􏰀≤ 8) on air-oxidized surfaces. These may occur in a sub-surface layer or may be intermixed with the Ni(OH) $_2$ in the oxidized layer. The NiSO $_4$ species at the millerite surface occur as discrete crystallites whereas the Ni(OH) $_2$ forms a thin veneer covering the entire millerite surface. The NiSO $_4$ crystallites form on the surface of millerite but not on surfaces of adjacent minerals. Surface diffusion of Ni$^{2+}$ 􏰃 and SO$^{2−}$ $_4$ across the millerite surface [may] be responsible for the transport and subsequent growth of NiSO $_4$ crystallites developed on millerite surfaces. [It] is clear that Ni and SO$^{2−}$ $_4$ does not diffuse onto surfaces of adjacent minerals in sufficient quantity to form crystallites [...]. XPS results for water-reacted surfaces show little difference from the vacuum fractured surfaces with the exception that minor amounts of polysulfide and hydroxy nickel species are present. Similar reaction products to those formed in air [NiSO $_4$ and Ni(OH) $_2$] are believed to be produced, but these are removed from the millerite surface by dissolution, leaving behind a sulfur-enriched surface (polysulfide) and hydroxyl groups chemisorbed to nickel ions at the millerite surface.”
"The presence of NiSO $_4$ can be explained through oxidation of the sulfide ion in millerite to sulfate by molecular oxygen according to the following scheme:
NiS +􏰃 2O $_2$ → NiSO $_4$
In fact, it is most likely that the salt is hydrated. The presence of water in the O 1s spectrum supports the suggestion. The free energy of formation of hydrated NiSO $_4$ species is about 300 to 400 kcal/mol more negative than anhydrous NiSO $_4$, the difference being largest for the greatest degree of hydration. Even without hydration, the oxidation of NiS to NiSO $_4$ by molecular oxygen has a [reaction (rxn)] 􏰉∆G $_{rxn}$ 􏰅= -􏰄162.6 kcal/mol. Therefore, the oxidation of NiS to NiSO $_4$ is thermodynamically favored and should occur provided it is kinetically favored.”
"Coincident with formation of the hydroxy nickel surface complex is the formation of polysulfides. The nickel that reacts with the water and oxygen of ambient air is no longer bonded to sulfide. This sulfide is therefore available to react with other near-surface species, including other sulfide ions, which may lead to the formation of polysulfides (including disulfide) according to the following reaction scheme:”
nNiS 􏰃+ (n-􏰄1)H $_2$O 􏰃+ (n-􏰄1)/2O $_2$ → Ni$^{2􏰃􏰄+}$ $_0$ - S$^{2+}$ $_n$ + S$^{2−}$ $_n$ +􏰃 (n􏰄-1)Ni(OH) $_2$,
"where 2 ≤􏰀 n ≤􏰀 8. The designation Ni$^{2􏰃􏰄+}$ $_0$ - S$^{2+}$ $_n$ is used to denote polysulfide bonded to nickel in the lattice at the millerite surface. The Ni(OH) $_2$ and polysulfide may exist as separate, thin layers on the millerite surface with the Ni(OH) $_2$ presumably forming the overlayer. Alternatively, the polysulfides may be intermixed with the Ni(OH) $_2$ in the oxidized overlayer.”

## Orpiments

The mineral orpiment at right is a source of yellow and orange pigments and has the chemical formula As $_2$S $_3$.

## Patrónites

Patronite is the vanadium sulfide mineral with the chemical formula VS $_4$. The material is usually described as V$^{4+}$ (S$^{2−}$ $_2$) $_2$. Structurally, it is a "linear-chain" compound with alternating bonding and nonbonding contacts between the vanadium centers. The vanadium is octa-coordinated, which is an uncommon geometry for this metal.
The mineral was first described in 1906 for an occurrence in the Minas Ragra vanadium mine near Junín, Cerro de Pasco, Peru, was named for Peruvian metallurgist Antenor Rizo-Patron (1866–1948) the discoverer of the deposit. At the type locality in Peru it occurs in fissures within a red shale likely derived from an asphaltum deposit. Associated minerals include, native sulfur, bravoite, pyrite, minasragrite, stanleyite, dwornikite, quartz and vanadium bearing lignite. It has also been reported from the Yushkinite gorge on the Middle Silova-Yakha River on the Paikhoi Range of the polar Urals of Russia and from the Tsumeb mine in Namibia.

## Pentlandites

This massive sulfide specimen on the right consists of brassy gray-brown pyrrhotite (Fe $_{(1-x)}$S - imperfect iron monosulfide) with brighter brassy-colored patches of pentlandite ((Ni,Fe) $_9$S $_8$ - nickel iron sulfide), plus a network of grayish to black patches of magnetite (Fe $_3$O $_4$ - iron oxide).
Pentlandite is an iron–nickel sulfide with the chemical formula (Fe,Ni) $_9$S $_8$. Pentlandite has a narrow variation range in Ni:Fe but it is usually described as having a Ni:Fe of 1:1. It also contains minor cobalt, usually at low levels as a fraction of weight.

## Pyrites

The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS$_2$. This mineral's metallic luster and pale brass-yellow hue have earned it the nickname fool's gold because of its superficial resemblance to gold. Pyrite is the most common of the sulfide minerals [on Earth]. Pyrite is usually found associated with other sulfides or oxides in quartz veins, sedimentary rock, and metamorphic rock, as well as in coal beds, and as a replacement mineral in fossils. Despite being nicknamed fool's gold, pyrite is sometimes found in association with small quantities of gold. Gold and arsenic occur as a coupled substitution in the pyrite structure. In the Carlin–type gold deposits, arsenian pyrite contains up to 0.37 wt% gold.

## Realgars

Realgar an arsenic sulfide mineral of 1.5-2.5 Mohs hardness is used to make red-orange pigment.
Realgar is a sulfide mineral. But, with equal atomic numbers of sulfur and arsenic, it may act as a pnictide.
This piece on the right is from the less well-known Royal Reward Mine of Washington.

## Sulfurites

Formula: S, amorphous crystal system.
A mixture of metastable allotropes of native sulphur slowly reverts to crystalline alpha-sulphur; mixture formed when liquid sulphur is rapidly quenched (as when a sulphur flow runs into water). The composition of the mixture depends on the temperature and age of the liquid when quenched and is usually dominated by long helical chain molecules that are the cause of the plasticity of the sulphur, with about 3 atoms per spiral. Initially elastic, the mixture soon gets brittle due to crystallization of the S $_8$ contained in the mixture.
Geological Setting: Where liquid native sulphur has flowed from volcanoes.

## Troilites

This is a macro photograph of an etched surface of the Mundrabilla meteorite, a small piece of the approximately 3.9 billion-year-old meteorite that was first discovered in Western Australia in 1911. Two more giant chunks, together weighing about 17 tons, were found in 1966. Researchers can learn much from this natural crystal growth experiment since it has spent several hundred million years cooling, and would be impossible to emulate in a lab. This single slice, taken from a 6 ton piece recovered in 1966, measures only 2 square inches. The macro photograph shows a metallic iron-nickel alloy phase of kamcite (38% Ni) and taenite (6% Ni) at bottom right, bottom left, and top left. The darker material is an iron sulfide (FeS or troilite) with parallel precipitates of duabreelite (iron chromium sulfide (FeCr $_2$S $_4$).

## Violarites

Violarite (Fe$^{2+}$Ni$_2$$^{3+}$S$_4$) is a supergene sulfide mineral associated with the weathering and oxidation of primary pentlandite nickel sulfide ore minerals.
Violarite is formed by oxidisation of primary sulfide assemblages in nickel sulfide mineralisation. The process of formation involves oxidation of Ni$^{2+}$ and Fe$^{2+}$ which is contained within the primary pentlandite-pyrrhotite-pyrite assemblage.
Violarite is produced at the expense of both pentlandite and pyrrhotite, via the following basic reaction;
Pentlandite + Pyrrhotite --> Violarite + Acid
    (Fe,Ni)$_9$S$_8$ + Fe$_{(1-x)}$S + O$_2$ → Fe$^{2+}$Ni$_2$$^{3+}$S$_4$ + H$_2$SO$_3$
Violarite is also reported to be produced in low-temperature metamorphism of primary sulfides, though this is an unusual paragenetic indicator for the mineral.
Continued oxidation of violarite leads to replacement by goethite and formation of a gossanous boxwork, with nickel tending to remain as impurities within the goethite or haematite, or rarely as carbonate minerals.
Violarite is reported widely from the oxidised regolith above primary nickel sulfide ore systems worldwide. It is of particular note from the Mount Keith dunite body, Western Australia, where it forms an important ore mineral.
It is also reported from open cast mines around the Kambalda Dome, and Widgiemooltha Dome, in association with polydymite, gaspeite, widgiemoolthalite and hellyerite, among other supergene nickel minerals.

## Hypotheses

* Most minerals on Earth are oxides.