PROUSTITE

Proustite – Ag₃AsS₃ – is a fairly rare sulfosalt, an important Ag ore. In deposits, its volume varies greatly, and in only some cases is it the main Ag ore. It is called “ruby silver ore” because of its red internal reflections. The mining term “ruby silvers” is used interchangeably for proustite, pyrargyrite, miargyrite, and polybasite.

Proustite is extremely similar to pyrargyrite (Ag₃SbS₃), both macroscopically and microscopically. Pyrgyrite is much more common than proustite, but they occur together. The two minerals were only recognized as different species with the advent of more detailed chemical analyses in 1804. The streak, in principle, differentiates proustite from pyrargyrite in hand specimens: the streak of pyrargyrite is “purplish red” while the streak of proustite is “scarlet red”. Proustite tends to be more transparent, pyrargyrite tends to be more opaque. In some bibliographies, the two minerals are discussed together.

It is classified in the Proustite-Pyrargyrite Series, constituting the As analogue of pyrargyrite. It is the trigonal dimorph of xanthoconite (monoclinic) and may contain Sb.

CAUTION! The possible presence of uraninite and native arsenic requires special care with silver ores.

1. Characteristics

Crystal system: Trigonal scalenohedral.

Color: Blood-red to reddish-gray. When exposed to light, it blurs to black.

Habit: Massive, compact, or in prismatic crystals up to 8 cm. Can form scalenohedrons and rhombohedrons.

Cleavage: {10-11} distinct.

Tenacity: Brittle.

Twinning: Twinning is common, according to 4 different laws, and can form triple twins (“trillings”) and under the microscope shows polysynthetic twins.

Fracture: Conchoidal to irregular.

Mohs Hardness: 2 – 2.5

Parting: No.

Streak: Blood red.

Lustre: Adamantine with red translucency.

Diaphaneity: Transparent. It darkens when exposed to light. Over time, it gives the impression of being opaque, gray in color.

Density (g/cm³): 5.57

2. Geology and Deposits

Proustite is a low-temperature mineral, always very late-forming (one of the last to form), more common in environments rich in As and very poor in Sb (pyrargyrite is the opposite!).

It occurs in hydrothermal veins, especially epithermal ones. It is typical of Pb-Zn-Ag veins, quartz and calcite veins with noble ores (Au and Ag), and Ag-Co-Ni veins. Proustite is much rarer than pyrargyrite.

It can be supergene, in the oxidation and enrichment zone, by alteration and oxidation of pre-existing minerals, mainly native silver.

Proustite is common in some high-grade silver deposits hosted in limestone rocks.

3. Mineral Associations

It occurs associated with common gangue minerals such as quartz, barite, and carbonates (calcite, dolomite).

It also occurs with other Ag minerals such as native silver, acanthite, xanthoconite, argentopyrite, stephanite, chlorargyrite, “freibergite,” and other Ag sulfosalts.

Also with other sulfides, some quite common, such as pyrite, marcasite, chalcopyrite, galena, sphalerite, arsenopyrite, nickeline, and tetrahedrite-tennantite.

With löllingite, native arsenic, gypsum, uraninite, and other Ni-Co minerals.

4. Transmitted Light Microscopy

Refraction indices: n_ω: 3,087 – 3,088 n_ε: 2,792

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Moderate pleochroism in shades of intense red.

Relief: Very high.

Cleavage: {10-11} distinct.

Habits: Solid, compact, or in prismatic crystals up to 8 cm. It can form scalenohedrons and rhombohedrons.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.295 to 0.296, strong colors, difficult to define, pearly.

Extinction: probably parallel, since it is trigonal.

Elongation sign: No information available.

Twins: Common, they can be lamellar (polysynthetic).

Zoning: No information available.

CONVERGENT LIGHT

Character: U(-)

2V angle: No.

Alterations: No information available.

May be confused with: pyrargyrite.

5. Reflected Light Microscopy

Sample preparation: Proustite acquires a good polish without difficulty. Its hardness upon polishing is very low, identical to the hardness of pyrargyrite and similar to the hardness of polybasite and miargyrite, which are the other “ruby silvers”.

The hardness of proustite is greater than the hardness of acanthite and pearceite and less than the hardness of stephanite and galena.

PLANE POLARIZED LIGHT – PPL

Reflection color: Medium gray with bluish tones, almost identical to pyrargyrite. With exposure to light, like all Ag minerals, proustite tarnishes to very dark to black tones.

Compared to the color of pyrargyrite, the color of proustite is bluish and slightly darker, but the differences are too faint to be used for diagnosis.

Pleochroism: Quite distinct, somewhat more intense than in pyrargyrite: white with a slight pink/gray tint, darker, with a somewhat bluish contrast.

Other literature reports that the colors of pleochroism are between yellowish white and bluish gray.

Reflectivity: 27.89 – 29.99%

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy is occasionally strong in dark gray to greenish colors, or between grayish and brownish tones.
The anisotropy may be masked by internal reflections.

Internal reflections: Abundant and intense, always visible, ranging in color from blood-red to brick-red, yellow to orange. They are always double and easy to recognize.

The distribution of these internal reflections is irregular, with areas where they dominate and areas where they are practically absent.

May be confused with: Several other chemically similar minerals, mostly occurring in the same deposits, even exhibit the same behavior under chemical attack.

Pyrargyrite has a much darker streak when spread on paper with a fingernail. This test constitutes the most reliable differentiation between pyrargyrite and proustite.

Miargyrite is lighter and has internal reflections less frequently.

Samsonite is rare and has much weaker anisotropy.

Polybasite usually shows oxidation in air.

Cinnabar and cuprite are also similar, but occur in other paragenesis.

Stephanite is similar in basal sections to proustite and pyrargyrite.

Xanthoconite can also be similar.

General Characteristics:

Grain shape: Proustite generally occurs as irregular grains and in anhedral granular aggregates. Crystals can be prismatic, including long, almost acicular prismatic crystals.

Cleavage is not observed.

Twinning, lamellar (polysynthetic), according to various laws occurs.

Zoning: color zoning may occur.

Envelopes of uraninite may occur around proustite grains.

Envelopes of proustite may occur around native silver.

Inclusions of proustite occur in galena.

Intergrowths occur with native arsenic, pyrargyrite, galena, rathite, and marcasite.

Substitutions: proustite substitutes for native arsenic, native bismuth, freibergite, rammelsbergite, sphalerite, acanthite, pyrargyrite, and native silver.