STIBNITE
Stibnite – Sb2S3 – also known as antimonite, is a relatively rare sulfide, yet it is the most common sulfide of antimony (Sb) and serves as the primary ore mineral for the element.
Stibnite is typically very pure. Its long prismatic crystals form radiating or randomly arranged clusters. Stibnite is dimorphous with metastibnite and forms a solid-solution series with bismuthinite. It is frequently subject to pseudomorphic replacement by stibiconite (yellow) and cervantite; sometimes this replacement is only partial. A variety exists in which sulfur is replaced by selenium. Small fragments melt in a candle flame, producing a blue-green color.
As an antimony sulfide, it is potentially toxic and should be handled with care.
1. Characteristics
Crystal system: Orthorhombic bipyramidal.
Color: From steel-gray to lead-gray, or bluish lead-gray, often tarnishing to black or developing an iridescent film.
Habit: Typically long-prismatic. Radial, reticular, massive, granular, capillary, crustiform. Deformed crystals common. Crystals up to 65 cm.
Cleavage: {010} perfect, {100} and {110} imperfect. Deep striations parallel to the elongation.
Tenacity: Slightly sectile. Highly flexible in thin plates, but not elastic.
Twinning: Polysynthetic, rare.
Fracture: Subconchoidal.
Mohs Hardness: 2, somewhat anisotropic.
Parting: No.
Streak: Silvery-gray. The scattered powder is deep red.
Lustre: Metallic, intense on crystalline faces.
Diaphaneity: Opaque. Very thin, deep-red transparent plates.
Density (g/cm³): 4.6
2. Geology and Deposits
It typically occurs in hydrothermal veins spanning a wide temperature range—from very high to quite low—making it impossible to distinguish between intrusive-hydrothermal and extrusive origins.
It also occurs in hydrothermal replacement deposits; these are rarer, though some are of economic importance. It may also be found in hot spring deposits.
Stibnite deposits are usually small; large deposits are rare.
3. Mineral Associations
It is associated with common gangue minerals such as carbonates (calcite, dolomite, ankerite, and siderite), silica (quartz, chalcedony), barite, and fluorite.
In general, it is associated with Fe sulfides (pyrite, marcasite, bravoite), Cu sulfides (enargite, tetrahedrite), Pb sulfides (galena), sulfosalts, Pb and Sb sulfides and oxides (kermesite, cervantite, stibiconite, famatinite, stibarsen, vrbaite, livingstonite, and berthierite), As sulfides (arsenopyrite, orpiment, pararealgar, and lorandite), native elements (gold, antimony, sulfur), and Ag minerals.
It is also associated with cinnabar, realgar, vaesite, pitchblende, wolframite, molybdenite, and scheelite.
4. Transmitted Light Microscopy
This does not apply, as stibnite is opaque. Literature states that very thin plates are transparent in deep red—an observation that is difficult to make.
5. Reflected Light Microscopy
Sample preparation: Despite its low hardness, stibnite is very easy to polish. Excessive force or overly coarse abrasives should be avoided during the preparation of the polished section to prevent the formation of an excessively thick surface film with significant structural alterations—alterations which are, incidentally, highly characteristic of stibnite.
PLANE POLARIZED LIGHT – PPL
Reflection color: White to light gray and grayish-white with brown tones.
Pleochroism: Very strong: // a is pale gray-white, // b is pale gray with an olive tint (the darkest color of the three) and // c is pure white, very light. The frequent twins facilitate the observation of pleochroism.
Reflectivity: 30 – 45%
Bireflectance: Distinct.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Very strong anisotropy, ranging from gray-brown, gray-white, and leather-brown to white-brown-pink and gray-blue (or from bluish-gray to pinkish-brown); the colors reported in the literature vary.
Internal reflections: No.
May be confused with: many other minerals. A careful examination is required for correct identification, especially if the crystals are not prismatic and lack the twinning typical of stibnite. The following minerals are very similar:
Chalcostibite (bluish to pinkish color, exhibits cleavage, weaker pleochroism).
Bournonite (weaker anisotropy)
Jamesonite (weaker pleochroism and anisotropy, no deformation twin lamellae)
Bismuthinite (more yellowish color, weaker anisotropy)
Berthierite (in pleochroism, one of the colors is brown, similar to magnetite).
General Characteristics:
Grain shape: the primary grain shape is characteristically elongated, acicular-radiate, or occurs as subhedral, cleavable masses. Subsequent recrystallization may transform the grains into very coarse-grained, anhedral granular aggregates. Fine-grained masses are rare. Grain contacts range from simple polygonal boundaries to “jagged” or somewhat myrmekitic interfaces.
Cleavage is rarely visible in high-quality polished sections due to the mineral paste that forms on the surface during polishing; cleavage becomes visible only upon incipient alteration.
Extinction is parallel.
Polysynthetic twinning in the (010) to (001) zone is very common, though sometimes deformed. Deformation (pressure) lamellae with undulose extinction—which are easily observed—may also occur.
Deformation features are almost always present.
Translation gliding in elongated grains is possible and can be mistaken for polysynthetic twinning or deformation features.
Cataclasis is frequent.
Zoning is very common and frequently indicates that crystal morphology changed significantly during growth.
Exsolutions does not occur.
Intergrowths with berthierite, boulangerite, pyrargyrite, and kermesite are possible.
Replacement is uncommon in stibnite. In the oxidation zone, stibnite may be pseudomorphosed—preserving all morphological details—by antimony oxide or antimony ochre (a mixture of cervantite and stibiconite). It may also be replaced by realgar or orpiment.
