Sunday, October 20, 2013


Silver is one of the best known of the precious metals and occurs both as “native silver” and as a component in a variety of minerals.  I have a note stuck away somewhere that lists something like 40 different minerals that contain silver in their chemical composition.  Some of these minerals have less than 10% silver (i.e. boleite) while others such as acanthite have nearly 90% Ag (molecular weight).  It appears that even “native” silver is usually mixed with minor amounts of gold (electrum), antimony, arsenic, mercury and often copper (half-breed).  Crystals of “native” silver are quite rare and most often the mineral appears as massive, wiry, or as small gains.

Metallic and adamantine crystals of pyrargyrite on a quartz base with a few solitary and terminated quartz crystals.  Width of specimen ~2.2 cm.
Among the silver minerals there are several that form rather beautiful colored crystals.  For example, boleite is a halide (with Ag) that crystalizes as deep blue cubes.  I have examples of two silver minerals that are both known as “ruby silver”: pyrargyrite (dark ruby silver) and proustite (light ruby silver); both are sulfosalts (rather rare minerals containing two or more metals plus sulfur or selenium).  Pyrargyrite is a silver antimony sulfide (Ag3SbS3) and is isomorphous (both are end members of a solid solution series) with the silver arsenic sulfide proustite (Ag3AsS3).  Both minerals resemble each other however they rarely “mix” with each other.  That is, pyrargyrite contains little As and proustite contains little Sb---there do not seem to be middle members in the solid solution series.   

Photomicrograph showing individual pyrargyrite crystals.  The striations may be twinning planes.  Width of photo ~8 mm.

Pyrargyrite is usually a gray to gray-black mineral with a metallic luster and is quite soft (~2.5); cleavage is indistinct and the fracture conchoidal.  It has a purple-reddish streak.  The name dark ruby silver comes from the fact that very thins pieces of broken minerals show a translucent ruby-red color.  If crystalline, the individuals are hexagonal prisms. With continued exposure to light the crystals continue to darken.

Photomicrograph of pyrargyrite with arrows pointing to ruby-red color of small crystals.  Width of photo ~8 mm.
Proustite is one of the most beautiful minerals that I have observed. My specimen has large prismatic crystals terminated by scalenohedrons (resembling dogtooth spar calcite in shape) and is scarlet to vermillion in color (the light ruby silver).  However, I must keep them covered as they rapidly darken in color upon exposure to light.  Crystals have an adamantine luster, are translucent to transparent, quite soft (~2.5), some distinct cleavage, and a conchoidal fracture.  The vermillion streak will distinguish proustite from pyrargyrite.

Proustite crystals on a calcareous base.  The crystals are actually quite vermillion in color; however, the photograph did not pick up the color.  Width of specimen ~5.1 cm.
Both proustite and pyrargyrite are late-stage, low temperature hydrothermal minerals often occurring with other sulfide minerals in the near-surface oxidized zone.  Apparently both minerals are mined for their silver content.
Both of my specimens were obtained from estate auctions.  The proustite is from Peru (Pacococha District, Huarochiri Province, Lima Department).  MinDat describes the mineralization as fracture filling in Cenozoic volcanics and diabase stocks.  The pyrargyrite is from the historic silver mining area in the Veta Grande District, Zacatecas, Mexico.

I leaned about Joseph Proust (1754-1826) in general chemistry class when trying to understand Proust’s Law, aka The Law of Definite Proportions—a chemical compound always has the same proportion of elements (by mass).  His legacy is further enhanced by having his name attached to the mineral proustite.

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