pyrargyrite

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A mineral having composition Ag₃SbS₃. The mineral occurs as prismatic crystals and in massive form and in disseminated grains. The hardness is 2.5 on Mohs scale and specific gravity is 5.85. The luster is adamantine and the color a deep ruby red to black, giving it the name dark ruby silver. Pyrargyrite is an important silver ore when it is found in veins associated with proustite and other silver minerals. It has been mined as silver ore at Chanarcillo, Chile; Freiberg, Germany; Guanajuato, Mexico; and Cobalt, Ontario, Canada. See also Proustite.

Environment

Low-temperature silver veins.

Often well crystallized; being hemimorphic, the upper faces differ from those of the lower end, although since crystals are seldom doubly terminated, this phenomenon is not usually apparent. Most often truncated at the tip with gently sloping rhombohedron faces above steep scalenohedrons. Also massive.

Deep red to almost black. Luster adamantine; hardness 2Ɖ; specific gravity 5.8-5.9; streak purplish red; fracture conchoidal; cleavage 2 rhombohedral (1 good). Brittle.

Silver antimony sulfide (59.9% Ag, 22.3% Sb, 17.8% S). Surprisingly low arsenic substitution for antimony, even though it often occurs with proustite.

Fuses readily into round shiny globule. Antimony sublimate forms white ring close to assay, best seen immediately after first melting. Continued heating creates a dull gray malleable button, which gives good test for silver (hydrochloric acid added to nitric acid solution).

Usually the crystal form and dark red color are a sufficient guide. Pyrargyrite could be confused with dark proustite, but the antimony sublimate around the pyrargyrite button reveals a difference. Cuprite gives copper tests; zincite is infusible. In general, the mineral associations suggest silver minerals whenever we encounter pyrargyrite or proustite.

Pyrargyrite is a late mineral in the silver veins, and probably is also formed secondarily by hot volatile compounds reworking earlier deposits. Specimens are usually discovered only in the early stages of a silver-mining operation, when it is sometimes an important ore of silver. Although fine specimens are often destroyed to quickly obtain their silver value, the specimen value of good crystals is, as a rule, far greater. The best examples have come from Germany, in the Harz and Saxony. In France good crystals were found at Sainte-Marie-aux-Mines, Alsace, and at Chalanches, Isère. Chanarcillo, Chile, better known for its proustite, was a great source, as was Colquechaca, Bolivia. Some Mexican mines, as at Guanajuato, have enough pyrargyrite for it to be an ore. It is not abundant in the U.S. Castrovirreyna, Peru, has lately been a source of respectable examples.

Known as dark ruby-silver ore (dunkles Rotgültigerz) , distinguished as a rule by its deeper hue from the light ruby-silver ore, proustite (lichtes Rotgültigerz) . Both, on exposure to light, have a tendency to darken and develop a dull coating that dims their luster. If the specimen is sturdy enough to stand it, a light brushing with soap and water will remove much of the coating. A quick dip in a silver-polishing solution has been found effective but may be risky to the luster.

• Mineral compounds

Pyrargyrite
General
Category	Sulfosalt
Chemical formula	Ag3SbS3
Strunz classification	02.GA.05
Identification
Color	dark red to red-black
Crystal habit	Include prismatic crystals with rhombohedral and scalenohedral faces forming terminations. There is no perpendicular mirror plane and therefore a hemimorphic crystal can be seen, in some rare examples, with differing terminations at the top and bottom of the crystal. Typical crystals are poorly formed and modified heavily by secondary faces. Also found massive.
Crystal system	trigonal; 3m
Cleavage	Sometimes distinct in three directions forming rhombohedrons
Fracture	conchoidal
Mohs scale hardness	2.5
Luster	adamantine
Streak	dark cherry red
Specific gravity	approximately 5.8
Refractive index	translucent to nearly opaque
Other characteristics	darkens upon exposure to light; crystals are frequently striated

Pyrargyrite is a sulfosalt mineral consisting of silver sulfantimonide, Ag₃SbS₃. Known also as dark red silver ore 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 Rotgiiltigerz) 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, &c. 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, USA

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

• Mindat
• Webmineral data
• Amethyst Galleries' Mineral Gallery

 This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press. 

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