ARSENOPYRITE

Arsenopyrite – FeAsS – is a very common sulfide and constitutes an ore of As, but As is usually recovered as a by-product processing ores of other elements.

Arsenopyrite has a high content (46%) in As and a turbulent history of exploration, with over 40 names currently discredited. May contain high levels of Au. It can also contain Ag, Co, Sn, Ni, Sb, Bi, Cu, Pb. When present in Au ores, it makes Au recovery difficult and releases toxic vapors during the process. Mines with arsenopyrite generate acidic and toxic mine effluents.

Arsenopyrite is monoclinic, but twins can give it a pseudo-orthorhombic appearance. Twins are frequent in {100} and {001}, of contact in (101) and (012), forming crosses or “trillings”.

If heated, broken or pulverized, it releases toxic arsenic fumes that smell like garlic. When altered, it presents a greenish color; produces green colors on rocky walls. It is triboluminescent: it releases yellow or orange “sparks” when struck.

1. Characteristics

Crystal system: Monoclinic prismatic.

Color: Steel gray to silver white (may have dark gray colors or iridescent haze films).

Habit: Acicular, prismatic, distorted, striated, compact, granular, columnar, etc. Crystals up to 20 cm.

Cleavage: {110} distinct. Striations paralell to {001}

Tenacity: Brittle.

Twinning: See above.

Fracture: Subconchoidal, irregular.

Mohs Hardness: 5.5 – 6

Parting: No.

Streak: Gray-black to black.

Lustre: Metallic intense.

Diaphaneity: Opaque.

Density (g/cm³): 5.9 – 6.2

 

2. Geology and Deposits

Arsenopyrite occurs in high-temperature hydrothermal veins as one of the first minerals to form with gold and tin. Also in pegmatites, in contact metamorphism rocks and in metasomatic rocks (skarnites).

More rarely in bituminous shales, in alluvium and very rarely in magmatic rocks (granites, basalts).

 

3. Mineral Associations

Arsenopyrite occurs with common gangue minerals such as quartz, fluorite and carbonates (calcite, siderite). With common sulfides such as galena, sphalerite, pyrite, pyrrhotite and marcasite and silicates such as garnet and muscovite.

In the specific paragenesis, there are minerals with Ni, Co and As such as löllingite, nickelline and mackinawite,

Ag minerals such as native silver, acanthite, pyrargyrite, canfieldite and dyscrasite,

Sn minerals such as cassiterite, stannite and franckeite,

Bi minerals such as native bismuth, bismuthinite and tellurides of Bi,

sulfides of Cu such as chalcopyrite, chalcocite, covellite and cubanite,

oxides of W such as wolframite (ferberite) and scheelite.

Also with radioactive minerals (uraninite), native gold, valleriite and cinnabar.

 

4. Transmitted Light Microscopy

Not applicable, as arsenopyrite is completely opaque.

5. Reflected Light Microscopy

Sample preparation: polishing arsenopyrite requires a bit of effort, with a longer lasting polish, but it turns out to be of excellent quality. Its polishing hardness is much lower than that of pyrite and cobaltite, but greater than the hardness of magnetite, pyrrhotite and löllingite.

PLANE POLARIZED LIGHT – PPL

Reflection color: White, slightly yellowish or pink, which is only recognized when the mineral is in the vicinity of others that allow comparisons. Generally, the color impression is pure white, very reminiscent of the white of galena.

Compared to the color of pyrite, the color of arsenopyrite is almost white, as pyrite is cream-yellow with a brown tint.

Compared to the color of löllingite, the color of arsenopyrite is cream.

Compared with the color of safflorite, the color of arsenopyrite is cream.

Compared to the color of galena, the color of arsenopyrite is a very pale yellow.

Compared to the color of sphalerite, the color of arsenopyrite is very pale yellow.

Compared to the native silver color, the color of arsenopyrite is much darker and whiter.

Compared with the color of cobaltite, the color of arsenopyrite is lighter and whiter.

Pleochroism: Weak, only visible in intergranular contacts.

Reflectivity: 50.36 – 51.21% – very bright.

It rises to 53.6% if it contains Co.

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Strong anisotropy, varying according to the section between strong blue (“Prussian Blue”, resembles jeans) and brown (with a strong orange tinge), can be green.

Internal reflections: No.

May be confused with:

a series of similar minerals that are associated with arsenopyrite. When in very small grains, arsenopyrite can be confused with the minerals of the Pyrite Group and Marcasite.

See table at the end of this sheet!

Safflorite has pleochroism and is softer, with other habits (radial diamond crystals).

Löllingite is very similar, but its reflection color has a slight bluish tinge.

Rammelsbergite has very similar anisotropy colors.

Glaucodote has similar anisotropy, but the colors are fainter.

Gudmundite is another very similar mineral, but it is softer and pleochroism stronger.

Marcassite has a more yellowish reflection color.

General Characteristics: 

Grain shape: there is a strong tendency to idiomorphy. Polygonal outlines are frequent, with well-defined edges and perfect vertices, which can evolve into perfect rhombs in basal sections. Usually the crystals are small, with sizes of less than one centimeter. It can be skeletal, granular, subhedral, elongated into spherulites, fine-grained or very fine-grained. Idioblastic texture is possible.

Cleavage only is noticeable when the crystals are slightly altered.         

Twins are very common; they can be simple or lamellar. Lamellae can occur in two systems. Mimetic twins are possible. Cross twins produce radial clusters, macroscopically and microscopically.

Cataclasis is very common, concentrated in zones, whose fractures may contain other minerals.

Zonation can occur and can be confused with mimetic twins.   

Exsolution lamellae of glaucodote may occur.

Substitutions 1: Arsenopyrite is replaced by chalcopyrite, sphalerite, cassiterite, tetrahedrite, stannite, ilmenite, chalcocite, acanthite and other silver ores, in addition to magnetite and löllingite.

Substitutions 2: Arsenopyrite replaces pyrite, cobaltite, löllingite, molybdenite, wolframite and galena, more rarely magnetite.

Inclusions 1: Inclusions in arsenopyrite can be gold and molybdenite, as well as gold in fractures.

Inclusions 2: Arsenopyrite inclusions occur in pyrite, tetrahedrite and pyrrhotite.

Intergrowths with other minerals, oriented or not, occur frequently. They can be with löllingite, safflorite, cobaltite, pyrite, tetrahedrite, galena, pyrrhotite and skutterudita.

Alteration of arsenopyrite produces soluble compounds with or without limonite.          

Isotropic borders can be generated as a function of neighboring pitchblende or brannerite grains.

Basal section of arsenopyrite in PPL. These rhombs or similar polygonal shapes, always with very well-defined boundaries and perfect vertices, are typical for arsenopyrite.
Arsenopyrite in XPL+2º, showing the two anisotropy colors and a twin, very typical for this mineral. Above, hematite. Below, gangue minerals with milky internal reflections.
Arsenopyrite (white), sphalerite (light grey) with unmixings of chalcopyrite (yellow) and gangue (dark grey) in PPL. Arsenopyrite always shows a white color very similar to galena (but galena is isotropic).
Tarnished arsenopyrite in XPL, showing the mimetic twin lamellae typical of the mineral, but not seen in freshly polished sections.
Tarnished arsenopyrite in PPL, showing the typical twins of the mineral that are visible later in XPL due to the different anisotropy colors.
Arsenopyrite 01: Cluster of arsenopyrite crystals in PPL. Typical is the white, somewhat yellow color, as well as the tendency of crystals to form polygons, sometimes rhombs (basal sections).
Arsenopyrite 01: The same crystals as in the previous image, now in XPL. The blue and orange anisotropy colors are very diagnostic.
In XPL, arsenopyrite showing one of its anisotropy colors (yellow-orange), cataclastic texture and lamellar twins.
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