PYRARGYRITE
Pyrargyrite – Ag3SbS3 – is a very rare sulfosalt, an important ore of Ag. In the deposits, its volume varies a lot, only in some cases it is the main Ag ore. It’s called “ruby silver ore”. The mining term “ruby silvers” is used interchangeably for proustite, pyrargyrite, miargyrite and polybasite.
Pyrargyrite is extremely similar to proustite (Ag3AsS3), both macroscopically and microscopically. Pyrargyrite 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 analysis in 1804. The trace, in principle, differentiates proustite from pyrargyrite in hand samples: the pyrargyrite trace is “purplish red” while the proustite trace 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, in the Proustite Group, constituting the Sb analogue of proustite (from As). It is the trigonal dimorph of pyrostilpnite. May contain As.
The possible presence of pitchblende and native arsenic requires special care with silver ores!
1. Characteristics
Crystal system: Trigonal, piramidal ditrigonal.
Color: Deep red or red gray, darkens if exposed to light.
Habit: Prismatic.
Cleavage: {10-11} distinct. {011-2} very poor.
Tenacity: Brittle.
Twinning: Common, after several laws.
Fracture: Irregular, conchoidal.
Mohs Hardness: 2.5
Parting: No.
Streak: Red.
Lustre: Adamantine.
Diaphaneity: Transparent.
Density (g/cm³): 5.82
2. Geology and Deposits
Pyrargyrite is a low-temperature mineral, always very late (one of the last to form), more common in environments rich in Sb and very poor in As (proustite is the opposite!). Occurs in hydrothermal veins, especially in 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 pyrargirite.
It can form supergenically, in the oxidation and enrichment zones, by alteration and oxidation of pre-existing minerals, mainly native silver.
It is common in some high-grade silver deposits hosted in limestone rocks.
3. Mineral Associations
Pyrargyrite occurs with common gangue minerals such as quartz, barite and carbonates (calcite, dolomite, siderite).
Evidently it is associated with other Ag minerals such as native silver, proustite, acanthite, argentopyrite, stephanite, miargyrite, polybasite, argyrodite and others.
Also with other sulfides, some quite common, such as pyrite, pyrrhotite, chalcopyrite, galena, sphalerite, arsenopyrite, nickelline, dyscrasite, famatinite and tetrahedrite-tennantite. With native arsenic, stibarsenic, simpleite, aikinite, breithauptite, safflorite and other minerals from the Ni-Co association.
4. Transmitted Light Microscopy
Refraction indices: nω: 3.084 nε: 2.881
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Distinct pleochroism.
Relief: Very high.
Cleavage: {10-11} distinct and {011-2} very poor.
Habits: Prismatic, idiomorphic crystals may occur, as well as very irregular crystals. It also occurs as a filling of very fine fractures in the host rock.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.203: very high, up to 10th order, with pearly colors, difficult to define.
Extinction: Probably parallel, because it is trigonal.
Elongation sign: No information available.
Twins: They are often present according to many laws, simple or lamellar (polysynthetic).
Zoning: Isolated crystals may show well-developed zonation.
CONVERGENT LIGHT
Character: U(-)
2V angle: No.
Alterations: No information available.
May be confused with: other red and transparent minerals such as proustite, cinnabar, realgar, cuprite and others, but the literature does not mention any reference to these. In fact, the literature offers only a few and sparse references on the behavior of pyrargyrite under Transmitted Light.
5. Reflected Light Microscopy
Sample preparation: the hardness of pyrargyrite is very low, similar to that of proustite, but even so pyrargyrite acquires a good polish easily, and it is possible to almost completely avoid polishing grooves. The polishing hardness of pyrargyrite is lower than the hardness of stephanite and galena, similar or slightly higher than the hardness of miargyrite and pearceite-polybasite and much higher than the hardness of acanthite.
Dye the polishing cloth red. There are subtle hardness differences according to different orientations in the mineral. Light etching (corrosion) can be perceived discreetly in polished sections after 10 minutes of exposure: etching causes old polishing grooves to appear and its intensity varies greatly according to grain orientation.
PLANE POLARIZED LIGHT – PPL
Reflection color: Light blue to gray white, gray-blue in various shades. Also classified as “white with a blue-gray tinge”.
Compared with the color of proustite, the color of pyrargyrite is less blue and lighter.
Compared to the color of galena, the color of pyrargyrite is more gray-blue.
Compared to the color of miargyrite, the color of pyrargyrite is distinctly bluish.
Compared to the colors of bournonite and boulangerite, the color of pyrargyrite is gray with a blue to violet tint.
Color indications in the bibliography are contradictory and difficult to assess. It is not possible to differentiate pyrargyrite from proustite by reflection color.
Pleochroism: Quite sharp, in blue-grey or yellow-white to blue-grey colors, with more shades of gray than proustite.
Reflectivity: 26.04 – 31.34%, there are clear variations according to grain orientation.
Bireflectance: Relatively strong.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Strong anisotropy between grays and browns, something like pale gray to dark gray, but this anisotropy is masked by the strong and numerous internal reflections.
Internal reflections: Abundant, numerous and always visible, in crimson red, from light to dark, yellow and orange, often double.
Comparing pyrargyrite grains with proustite grains, provided that they are in the same sections, same sizes and with the same quality of polishing, it can be seen that the red internal reflections of proustite are a little clearer, less pronounced and more frequent than the reflections of pyrargyrite. – but this observation does not lend itself to differentiating the two minerals from each other.
May be confused with: several other minerals that mostly occur in the same deposits.
Proustite has a much lighter streak if smeared on the paper with a fingernail. This test constitutes the safest differentiation between pyrargyrite and proustite
Miargyrite is lighter and has internal reflections more rarely.
Samsonite is rare and has much weaker anisotropy.
Polybasite normally shows oxidation in air.
Cinnabar and cuprite are also similar, but they occur in other paragenesis.
Stephanite is similar in basal sections to proustite and pyrargirite.
Xanthoconite can also be similar.
General Characteristics:
Grain shape: well-formed isolated crystals are often observed, alongside chaotic aggregates with very irregular, intensely intergrown crystals (toothed contacts). Pyrargyrite often occurs as a filler of very fine fractures in both the ore and the host rocks, forming thin films that have the same orientation over long distances. The grain size is very variable, even in the crystals of an individual polished section.
Cleavage is not observed in good quality polished sections.
Twins are very often present according to many laws, simple or lamellar. They occur from growth twins to lamellar pressure twins, especially according to {10-14}. The lamellar twins originated from pressures that the crystals suffered and can be generated artificially.
Zonation, very well developed, can be seen after chemical attack especially in isolated crystals; it is somewhat rarer in aggregates.
Deformations are rare.
Cataclasis very well developed to be seen in some cases.
Inclusions 1: inclusions in Pyrargyrite can be from acanthite, native bismuth, arsenopyrite, pyrite, tetrahedrite, chalcopyrite, sternbergite and canfieldite.
Inclusions 2: pyrargyrite inclusions occur in pyrite, galena, “freibergite”, sphalerite and meneghinite.
Substitutions 1: Substitutions are extremely frequent; almost always pyrargyrite is the mineral that replaces the others. Substituted and penetrated by fractures are especially “freibergite” and other minerals of the tennantite-tetrahedrite series, miargyrite, stephanite and very often galena, in some cases also pyrite and some of the skutterudite zones.
Substitutions 2: pyrargyrite can be substituted for polybasite, owyheeite, native silver, stephanite, dyscrasite and acanthite.
Intergrowths, some of them myrmekitic, occur with several other ores, they can be with proustite, polybasite, tetrahedrite, chalcopyrite, native silver, galena, bournonite, boulangerite, stibnite, acanthite and stephanite.
