KERMESITE
Kermesite – Sb2S2O – also known as “red antimony”, is a relatively common oxide. It is not of great importance as an ore, only being part of polymetallic ores containing Sb.
Kermesite crusts and coatings can be confused with metastibnite.
1. Characteristics
Crystal system: Triclinic pinacoidal, pseudomonoclinic.
Color: Crimson red or dark red, it can be black.
Habit: Acicular, fibrous, prismatic // a [010], frequently in radial aggregates. Irregular grains.
Cleavage: {100} perfect.
Tenacity: Sectil, fibers are flexible.
Twinning: No.
Fracture: No information available.
Mohs Hardness: 1 – 1.5
Parting: by {001}
Streak: Brownish red.
Lustre: Adamantine
Diaphaneity: Transparent.
Density (g/cm³): 4.5 – 4.6
2. Geology and Deposits
Kermesite occurs in Sb deposits, formed as an alteration product (partial oxidation) of stibnite, valentinite, or stibiconite.
It is commonly found in gossans of Sb+Ag deposits.
It is found in a significant number of locations, but always in small quantities.
3. Mineral Associations
It is associated with typical gangue minerals such as quartz and calcite.
It occurs with the characteristic Sb paragenesis: schafarzikite, senarmontite, stibioconite, stibnite, valentinite, pääkkönenite, parapierrotite, and native antimony.
It also ocurrs with gypsum, realgar, proustite, pyrargyrite, and sphalerite.
4. Transmitted Light Microscopy
Refraction indices: nα: 2,720 nβ: 2,740 nγ: 2,740
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: It has a cherry-red color, with pleochroism in shades of cherry-red.
Relief: Very high.
Cleavage: {100} perfect, only visible in larger crystals, not in fibers and needles.
Habits: Granular, long prismatic, acicular, fibrous.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: maximum birefringence of 0.020, corresponding to colors up to the end of the 1st order: gray, white, yellow, orange, red, and blue.
Extinction: Probably oblique.
Elongation sign: No information available.
Twins: No.
Zoning: No.
CONVERGENT LIGHT
Character: B(+)
2V angle: 70º.
Alterations: No information available.
May be confused with: No information available.
5. Reflected Light Microscopy
Sample preparation: Kermesite acquires a fairly reasonable polish despite its low hardness and typical fibrous habit.
PLANE POLARIZED LIGHT – PPL
Reflection color: Medium gray.
Pleochroism: Distinct, in gray with shades of brownish-gray or greenish-gray.
Reflectivity: 24.22 – 29.22 %
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Strong anisotropy in shades of gray with violet to greenish hues. Or violet to blue-green; the literature differs on the colors.
When the needles are very thin, this anisotropy is difficult to visualize.
Internal reflections: Intense, very beautiful red, with a violet hue, mainly in fine-grained, acicular, and fibrous aggregates. Rarer in coarse-grained aggregates, but also with the same color.
May be confused with: Proustite and pyrargyrite, which have the same anisotropy and the same internal reflections, but these have higher reflectivity and never present with an acicular or fibrous habit.
The association with stibnite and Sb sulfosalts is very characteristic.
General Characteristics:
Grain shape: occurs in irregular grains or in acicular crystals arranged in radial aggregates. Also with isolated acicular crystals.
Cleavage is visible in some grains, provided they are larger. It is not visible in fibers and needles.
Substitutions of stibnite by kermesite, although kermesite is a product of stibnite alteration, are not observed.