DYSCRASITE
Dyscrasite – Ag3Sb – is a rare alloy that occurs in some Ag ores. It constitutes silver ore, but is generally not the main ore mineral in the deposit.
Ores with dyscrasite contain native arsenic, several other As minerals, and may contain radioactive minerals. In addition, dyscrasite may contain Hg. Therefore, it is an ore that requires caution!
1. Characteristics
Crystal system: Orthorhombic pyramidal.
Color: White as silver. Tarnishes to lead gray, yellowish, or black.
Habit: Pyramidal, prismatic, or tabular crystals. Granular, foliated, massive.
Cleavage: {001} distinct. {110} imperfect. Striations may occur.
Tenacity: Sectil, but brittle.
Twinning: It always exhibits twinning by {110}, generating pseudohexagonal or “V” shaped forms.
Fracture: Irregular.
Mohs Hardness: 3.5 – 4
Parting: No.
Streak: Silver white.
Lustre: Metallic
Diaphaneity: Opaque
Density (g/cm³): 9.71
2. Geology and Deposits
Dyscrasite typically occurs in hydrothermal vein deposits connected to intrusive rocks, especially Ni-Co-Ag-As veins. It is generally not a common mineral in this type of paragenesis, but it can occur abundantly and, in this case, constitutes an important Ag ore.
It can apparently form in some cases in the secondary enrichment zone.
In many cases, dyscrasite occurs as a silver-bearing mineral in bornite and galena in the form of tiny grains. Generally, the grains are too small for identification under a microscope.
3. Mineral Associations
It is associated with some common gangue minerals such as quartz and calcite.
It also occurs with other Ag minerals with or without Sb, such as native silver, acanthite, proustite, pyrargyrite, stromeyerite, miargyrite, and allargentum. It is associated with Sb minerals with or without As (native antimony, tetrahedrite, breithauptite, and stibarsene) and minerals with As (native arsenic, löllingite, cobaltite, and safflorite).
Some common sulfides may be associated, such as pyrite, galena, and chalcopyrite. Also with actinolite.
4. Transmitted Light Microscopy
This does not apply, as dyscrasite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Dyscrasite achieves an excellent polish, but polishing grooves will always occur. Its hardness upon polishing is greater than that of galena, native bismuth, and native silver, approximately similar to that of native antimony, and less than that of native arsenic and chalcopyrite.
PLANE POLARIZED LIGHT – PPL
Reflection color: White, often with a yellowish tint. It tarnishes to lead gray, to yellowish, or to black.
Compared to the color of native antimony, the color of dyscrasite is slightly more yellow, more creamy.
Compared to the color of galena, the color of dyscrasite is creamy white.
Compared to the color of native silver, the color of dyscrasite is slightly more grayish.
Pleochroism: Weak to very weak, between white and creamy-white.
Reflectivity: 59.67 – 63.00%, very high, but lower than native silver and native antimony that ocurr with dyscrasite.
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Weak to moderate anisotropy in diffuse shades of brownish-gray.
Internal reflections: No.
May be confused with: Other soft, white minerals from the paragenesis.
Native antimony tarnishes much more slowly. Dyscrasite tarnishes in a few days.
Native silver is isotropic, has slightly lower reflectivity, and a slightly more yellowish tone.
Native bismuth is much softer and exhibits lamellar twinning.
Native arsenic has much lower reflectivity.
General Characteristics:
Grain shape: Dyscrasite forms idiomorphic crystals with prismatic, square, or rhombic cross-sections. Anhedral granularity may occur. Equidimensional grains may form.
Cleavage is not visible.
Polishing grooves are always present, inevitable, and diagnostic, associated with other mineral features.
Twinning, irregular, zigzag may occur. It always forms twins of 3 individuals (“trillings”) in pseudohexagonal patterns. These twins are visible by the extinction pattern of basal sections to Crossed Nicols.
Zoning does not occur.
Deformations may sometimes be observed.
Intergrowths with native arsenic may occur.
Substitutions: Dyscrasite can substitute niqueline and maucherite.
Alterations: sometimes the alteration removes Sb from the periphery of the crystals, resulting in a pyrostilpnite (Ag3SbS3) rim in the dyscrasite grains.
Dyscrasite inclusions occur in galena, cobaltite, pyrite, and native arsenic.
