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Home Mineral Information Sulphosalts

Sulphosalts

Sulphosalts are closely related to sulphide minerals in which the sulphur is combined with one or more metalsand one or more semi-metals. The feature which sets sulphosalts apart from sulphides is that the non-metals,in addition to occupying the normal sulphur positions, typically also partially substitute for some of the metalatoms in the metal positions. They form an extensive group of minerals, mostly rare species, noted for some of the most complicated atomicand crystal structures known to mineralogy outside of the silicates. They conform to the generalformula AmBnXp, in which m, n, and p are integers; A may be lead, silver, thallium, or copper; B may be

antimony, arsenic, bismuth, tin, or germanium; and X may be sulfur or selenium. Formerly it was thought that the sulfosalts were salts of hypothetical thioantimonic or thioarsenic acids (e.g.,HSbS2, H18As4S15, H3AsS3). However X-ray diffraction studies show that the atomic structures of many

sulfosalts are based on structural fragments of much simpler compounds such as Galena (PbS) cubes andStibnite (Sb2S3) sheets. No comprehensive theory has been worked out to rationalize many of these curious

compounds. The complexity of many of the structures evidently results from their having crystallized at lowtemperatures and the consequent high degree of ordering of the metal atoms. Syntheses of such compositionsat higher temperature usually result in structures simpler than the complicated low-temperature forms.

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Although sulfosalts are much rarer than the sulfide minerals with which they are often associated, somelocalities are remarkable for the variety of species encountered. At the Lengenbach Mine in Switzerland, forexample, more than 30 individual species have been recognized, 15 of which are not found elsewhere. Most sulfosalts form at low temperature in rock cavities, usually in association with copper–zinc–arsenicsulphide ores. Very often they occur in cavities of calcite and dolomite, as at Lengenbach. Most are dull graywith a metallic lustre, brittle, crystalline, and difficult to tell apart without X-ray diffraction or electron microprobeanalyses. The thallium-bearing sulfosalts are often deep red and transparent, which is also sometimes true ofthe sulfosalts of silver. Although under exceptional circumstances some sulfosalts have been used as silver ores (i.e.,proustite,pyrargyrite, and stephanite), and other species have constituted ores of mercury, arsenic, and antimony(i.e., boulangerite, livingstonite, enargite, and tennantite-tetrahedrite), their economic importance is limited.However, apart from their mineralogical interest, the sulfosalts are studied because their electronic propertiesare related to those of semiconductors. The main mineral groups of the Sulphosalt class are:

ARSENIC sulphosalts - a large group consisting of about 45 minerals in which partialmetal substitution is largely done by arsenic. Nine of these known minerals arecomplexes with silver, including Smithite AgAsS2 and Proustite Ag3AsS3; five are with

copper, including Enargite Cu3AsS4 and Sinnerite Cu6As4S9; and eleven with lead,

including Sartorite PbAsS4 and Jordanite Pb14As6S23. Proustite is relatively common

and has been mined as an ore of silver. However, by far the largest group are thesulphosalts of arsenic with thallium. This group is represented by about 20 minerals,including Ellisite Tl3AsS3, Fangite Tl3AsS4 and Imhofite Tl6As15S25. Most arsenic-

bearing sulphosalts are rare because the minerals oxidise easily, often forming thecorresponding metal arsenate. Arsenic sulphosalts are typically black or grey, exceptfor a few like proustite which are red, and do not form good crystals. Instead, theymainy appear as granular aggregates. Most are soft and all are of hydrothermal origin,forming in conditions where arsenic is present in solution.

ANTIMONY sulphosalts - a group of about 43 minerals in which partial metalsubstitution is largely done by antimony. One of these minerals is a complex with gold,namely Negyagite (Au,Te)3Pb3(Pb,Sb,Bi)3S6; eight with silver including Stephanite

Ag5SbS4 and Pyrargirite Ag3SbS3; twenty one with lead, including Robinsonite

Pb4Sb6S13 and Playfairite Pb16Sb18S43; and eight with thallium, including Rohaite

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TlCu5SbS2 and Parapierrotite Tl(Sb,As)5S8. Nearly all are black or gray, soft, dense and

form granular aggregates rather than well-formed crystals. All are of hydrothermalorigin.

Fuloppite group - a group of five antimony sulphosalts classified separately becausethey share certain structural features. The crystal structure is based on sheets ofgalena bonded by layers of antimony. Minerals in this group are Fuloppite Pb3Sb8S15,

Plagionite Pb5Sb8S17, Heteromorfite Pb7Sb8S19, Semseyite Pb9Sb8S21 and Rayite

Pb8(Ag,Tl)2Sb8S21. Their structures differ by the thickness of a galena sheet in the

structure. Fuloppite has the thinnest such sheet. All are soft, moderately dense andblack or gray. All are of hydrothermal origin and crystalise in the monoclinic system.

Cylindrite group - a group of six antimony sulphosalts based on lead, tin and ironcontent. Examples are Cylindrite Pb3FeSn4Sb2S14, Franckeite Pb5FeSn3Sb2S14 and

Potosiite Pb6FeSn2Sb2S14. Cylindrite has a unique crystal habit whereby it forms

perfectly cylindrical crystals. The crystals are actually coiled sheets that give theappearance of having been rolled into tubes or cylinders. Under pressure the sheets,often become uncoiled. The majority of these minerals contain their elements inmultiple oxidation states. The formula for Franckeite could therefore be written as

(Pb2+,Sn2+)6Fe2+Sn4+2Sb3+

2S2-14. All minerals in the group occur as dark gray

metallic grains or crystals. All are of hydrothermal origin.

Pyrargyrite group - a sub-group forming a solid solution series between arsenic andantimony sulphosalts which contains two silver minerals Pyrargyrite Ag3SbS3 and

Proustite Ag3AsS3. Both occur in low-temperature veins and are deep red, making

identification from each other difficult. There may be a solid solution series betweenthe two but this has not been shown conclusively.

Tetrahedrite group - a sub-group forming a solid solution series between arsenic andantimony sulphosalts Tetrahedrite (Cu,Fe)12Sb4S13 (Sb end-member) and Tennantite

(Cu,Fe)12As4S13 (As end-member). Copper is the main metal in each but other

minerals, including iron and zinc, substitute extensively. Minerals in this group areamong the commonest sulphosalts. Their metallic grey appearance, brittleness andhighly characteristic tetrahedral crystals (hence the name) make them easy to identify.However they are not easily distinguished individually without complex tests. Other

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minerals in this group include Jamesonite Pb4FeSb6S14, Zinkenite Pb6Sb14S27,

Boulangerite Pb5Sb4S11 and Stephanite Ag5SbS4.

Enargite group - a sub-group forming a solid solution series between arsenic andantimony sulphosalts Enargite Cu3AsS4, an arsenic-rich copper sulphosalt and

Famatinite Cu3SbS4 the antimony-rich analogue. However the two minerals can

exchange some of their nonmetallic elements so that Enargite can contain up to 6% ofSb and Famatinite up to 10% As in place of Sb. Both occur together in mediumtemperature hydrothermal veins.

Bournonite group - a sub-group forming a solid solution series between arsenic andantimony sulphosalts Bournonite PbCuSbS3 and Seligmannite PbCuAsS3. There is an

incomplete solid solution between them. Although rarely found together, both occur inmedium temperature hydrothermal veins with other sulfosalts and sulphides.Bournonite is one of the most common sulphosalts and can be a major source of leadand copper. It is commonly associated with galena and sphalerite. The Pb:Cu ratio isabout 1:1. It has a tendency to form very distinct crystals consisting of cyclic twinnedaggregates. Their shape resembles cog-wheels and thus bournonite is often calledcog-wheel ore. Seligmannite is relatively rare and typically occurs with othersulhphides in Dolomite.

Colusite group - a group of complex sulphosalts which also contain arsenic, antimonyand germanium. Examples include Colusite Cu26VAs6S32, Germanocolusite

Cu26V(Ge,As)6S32 and Stibiocolusite Cu26V2(Sb,Sn,As)6S32. Vanadium can be replaced

by iron, copper, tin or zinc. It is likely that a series of solid solutions exists based onthe partial or total substitution of these metallic elements, as well as the antimony,arsenic sulphur and germanium. Most of the minerals are rare and occur in associationwith vanadium deposits. All are isostructural and crystallise in the isometric system.

BISMUTH sulphosalts - a group of about 50 minerals in which partial metalsubstitution is largely done by bismuth. The group sub-divided into six sub-groupsbased on metal content and crystal system. All are hydrothermal in origin. Thesubgroups are Matildite, Pavonite, Emplektite, Aikinite, Aschamalmite and Eclaritegroups.

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Matildite group - contains three rare bismuth sulphosalt minerals which form a solidsolution series based on substitution of sulphur by selenium and tellurium. These areMatildite AgBiS2, Bohdanowichite AgBiSe2 and Wolynskite AgBiTe2. All three crystallise

in the trigonal system and all are hydrothermal in origin.

Pavonite group - contains seven bismuth sulphosalt minerals based on substitution ofsilver and copper. These include Pavonite Ag(Ag,Cu)Bi6S10, Benjaminite (Ag,Cu)3Bi7S12

and Mummeite Ag2CuPbBi6S13. All crystallise in the isometric system. Most of the

minerals occur as dull gray grains with a metallic lustre. They are thus difficult toidentify. Most are soft but dense. All are of hydrothermal origin.

Emplectite group - contains eleven rare bismuth sulphosalts based on copper withoccasional substitution by nickel, lead and iron. Examples include Emplectite CuBiS2,

Muckeite CuNiBiS3, Paderaite Cu6Pb2AgBi11S22, Kupcikite (Cu,Fe)4Bi5S10 and

Pertovicite Cu3HgPbBiSe5. The group minerals appear to have a structural unit

BiS45- which is tetrahderal. These complex ions are linked by copper and other metal

ions holding the structure together. Another way of thinking about this group is asdouble sulphides where the formula for empectite could be written as CuS-BiS. Most ofthe minerals occur as dull gray grains with a metallic lustre. They are thus very difficultto identify. Most are rhombohedral although some members like Hydrusite Cu8Bi12S22

are isometric. All are of hydrothermal origin.

Aikinite group - contains ten bismuth sulphosalts based on lead and copper content.Examples are Aikinite PbCuBiS3, Friedrichite Pb5Cu5Bi7S18, Emilite Pb2.7Cu2.7Bi5.3S12

and Krupkaite CuPbBi3S6. All are black or gray. They occur as dull grains with a metallic

lustre. All crystallise in the rhombohedral system and all are of hydrothermal origin.Aikinite forms a solid solution series with Bismuthinite Bi2S4 through the partial

substitution of lead and copper for some of the bismuth.

Aschamalmite group - contains about eight bismuth sulphosalts based on lead. Mosthave partial substitution of the bismuth by other related non-metals such as arsenic orantimony. Examples include Aschamalmite Pb6Bi2S9, Kirkiite Pb10Bi3As3S19 and

Ustarasite Pb(Bi,Sb)6S10. Like most other bismuth sulphosalts these minerals are black

or gray, quite dense and are all of hydrothermal origin. About half the group adopt the

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isometric crystal system whilst the other half are orthorhombic. Most are very rare andmost occur as dark metallic grains in matrix.

Eclarite group - contains eleven complex lead bismuth sulphosalt minerals. Thecomplexity arises from the partial substitution of lead by other metals including copper,iron and silver. Examples include Eclarite Pb9(Cu,Fe)Bi12S28, Eskimoite Pb10Ag7Bi15S36

and Vikingite Pb8Ag5Bi13S30. Most crystallise in the isometric system and occur as

black or gray grains in matrix. All are of hydrothermal origin.

Lillianite group - a group of nine sulphosalts based on the partial substitution ofbismuth and antimony, on the one hand, and lead and silver on the other. Examplesinclude Andorite PbAgSb3S6, Fizelyite Pb14Ag5Sb21S48, Lillianite Pb3Bi2S6 and

Gustavite PbAgBi3S6. A solid solution series exists between Lillianite and Gustavite. All

are dark gray and occur mostly s metallic grains in matrix. All are of hydrothermalorigin.

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