Pyrargyrite

Sulfosalt mineral of silver and antimony
(repeating unit)Ag3SbS3IMA symbolPyg[1]Strunz classification2.GA.05Crystal systemTrigonalCrystal classDitrigonal pyramidal (3m)
(same H-M symbol)Space groupR3cUnit cella = 11.047 Å, c = 8.719 Å; Z = 6IdentificationColorDeep red to red grayCrystal habitInclude prismatic crystals with rhombohedral and scalenohedral faces forming terminations. massive, granularTwinningComplex to lammellarCleavageDistinct on {1011}, imperfect on {0112}FractureConchoidal, unevenTenacityBrittleMohs scale hardness2.5LusterAdamantineStreakdark cherry redDiaphaneityTranslucent to nearly opaqueSpecific gravity5.82Optical propertiesUniaxial (–)Refractive indexnω = 3.084 nε = 2.881Birefringenceδ = 0.203PleochroismDistinctOther characteristicsDarkens upon exposure to light; crystals are frequently striatedReferences[2][3][4]

Pyrargyrite is a sulfosalt mineral consisting of silver sulfantimonite, Ag3SbS3. Known also as dark red silver ore, ruby blende, garnete blende or ruby silver, it is an important source of the metal.

It is closely allied to, and isomorphous with, the corresponding sulfarsenide known as proustite or light red silver ore. Ruby silver or red silver ore (German Rotgültigerz) was mentioned by Georg Agricola in 1546, but the two species so closely resemble one another that they were not completely distinguished until chemical analyses of both were made.

Both crystallize in the ditrigonal pyramidal (hemimorphic-hemihedral) class of the rhombohedral system, possessing the same degree of symmetry as tourmaline. Crystals are perfectly developed and are usually prismatic in habit; they are frequently attached at one end, the hemimorphic character being then evident by the fact that the oblique striations on the prism faces are directed towards one end only of the crystal. Twinning according to several laws is not uncommon. The hexagonal prisms of pyrargyrite are usually terminated by a low hexagonal pyramid or by a drusy basal plane.

The color of pyrargyrite is usually greyish-black and the lustre metallic-adamantine; large crystals are opaque, but small ones and thin splinters are deep ruby-red by transmitted light, hence the name, from the Greek pyr and argyros, "fire-silver" in allusion to color and silver content, given by E. F. Glocker in 1831. The streak is purplish-red, thus differing markedly from the scarlet streak of proustite and affording a ready means of distinguishing the two minerals. The Mohs hardness is 2.75, and the specific gravity 5.85. The refractive indices (nω = 3.084 nε = 2.881) and birefringence (δ = 0.203) are very high. There is no very distinct cleavage and the fracture is conchoidal. The mineral occurs in metalliferous veins with calcite, argentiferous galena, native silver, native arsenic, etc. The best crystallized specimens are from Sankt Andreasberg in the Harz, Freiberg in Saxony, and Guanajuato in Mexico. It is not uncommon in many silver mines in the United States, but rarely as distinct crystals; and it has been found in some Cornish mines.

Pyrargyrite silver ore from the Comstock Lode, Storey Co., Nevada, US

Although the red silver ores afford a good example of isomorphism, they rarely form mixtures; pyrargyrite rarely contains as much as 3% of arsenic replacing antimony, and the same is true of antimony in proustite. Dimorphous with pyrargyrite and proustite respectively are the rare monoclinic species pyrostilpnite or fireblende (Ag3SbS3) and xanthoconite (Ag3AsS3): these four minerals thus form an isodimorphous group.

References

  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ Pyrargyrite in the Handbook of Mineralogy
  3. ^ Pyrargyrite on Mindat.org
  4. ^ Pyrargyrite data on Webmineral

 This article incorporates text from a publication now in the public domainChisholm, Hugh, ed. (1911). "Pyrargyrite". Encyclopædia Britannica. Vol. 22 (11th ed.). Cambridge University Press. pp. 685–686.

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