Pyrargyrite

Pyrargyrite is a 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 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 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 arsenic, &c. The best crystallized specimens are from St 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.

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 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 the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.