Pyrargyrite

Pyrargyrite
General
Category	Sulfosalt
Formula (repeating unit)	Ag3SbS3
Strunz classification	2.GA.05
Crystal system	Trigonal
Crystal class	Ditrigonal pyramidal (3m) (same H-M symbol)
Space group	R3c
Unit cell	a = 11.047 Å, c = 8.719 Å; Z = 6
Identification
Color	Deep red to red gray
Crystal habit	Include prismatic crystals with rhombohedral and scalenohedral faces forming terminations. massive, granular
Twinning	Complex to lammellar
Cleavage	Distinct on {1011}, imperfect on {0112}
Fracture	Conchoidal, uneven
Tenacity	Brittle
Mohs scale hardness	2.5
Luster	Adamantine
Streak	dark cherry red
Diaphaneity	Translucent to nearly opaque
Specific gravity	5.82
Optical properties	Uniaxial (–)
Refractive index	nω = 3.084 nε = 2.881
Birefringence	δ = 0.203
Pleochroism	Distinct
Other characteristics	Darkens upon exposure to light; crystals are frequently striated
References

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

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