|
Pyrargyrite |
Chemical Formula |
Ag3SbS3 |
Species |
Sulfides |
Crystal System |
Trigonal |
Mohs Scale |
2 |
Specific Gravity |
5.855 |
Color |
Deep red or red gray |
Streak |
Purplish red |
Luster |
Adamantine |
Refractive Index |
n = 3.084 n = 2.881 |
Diaphaneity |
Translucent |
Cleavage |
Distinct/GoodDistinct on very imperfect on |
Fracture |
Irregular/Uneven, Conchoidal |
Crystal Habit:Prismatic crystals |
Geological Setting:Hydrothermal veins as a late stage, low temperature mineral; also formed by secondary processes. |
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.