ALTAITE
Altaite – PbTe – is a relatively rare sulfide. Due to this rarity, it does not have great economic importance, despite Te being a strategic semi-metal with many uses, especially in the construction of photovoltaic panels.
There are only three deposits on the planet where Te is obtained as a primary ore; most Te (~85%) is obtained as a “by-product” in the exploitation of Porphyry Copper-type Cu ores.
It is classified in the Galena Group and may contain Ag, Au, Cu, Fe, Se, and S. It forms a solid solution with galena.
Altaite is very similar to galena, including its perfect cubic cleavage. Its determination in polished sections requires experience and auxiliary methods. Diagnostic features include cleavage and high reflectivity.
1. Characteristics
Crystal system: Cubic hexaoctahedral.
Color: White from tin to yellowish white. Fades to bronze yellow and blue.
Habit: Typically solid or granular, rarely cubes or octahedrons up to 1 cm.
Cleavage: Perfect cubic {100}.
Tenacity: Sectil.
Twinning: No.
Fracture: Subconchoidal.
Mohs Hardness: 2 – 3
Parting: No.
Streak: Black.
Lustre: Intense metallic.
Diaphaneity: Opaque.
Density (g/cm³): 8.2
2. Geology and Deposits
Altaite is typical of epithermal hydrothermal vein deposits of Au-Ag-Te.
It can occur in skarns associated with porphyry copper-type deposits.
It is also found in greenstone belts, associated with gold mineralization, in Archean world-class gold deposits.
3. Mineral Associations
It occurs associated with common gangue minerals such as quartz (including chalcedony variety) and carbonates (calcite, dolomite, siderite, cerussite).
It occurs with common sulfides such as pyrite, pyrrhotite, chalcopyrite, arsenopyrite, sphalerite and galena.
The specific paragenesis comprises several native elements such as gold, silver, bismuth, antimony and tellurium. Then, minerals with these elements, combined with each other and with others such as Pd, Co, Hg, Pt, Sn, Cu, Pb, Se and Bi: electrum, tellurium-antimony, sylvanite, petzite, calaverite, coloradoite, nagyagite, hessite, krennerite, frodite, niggliite, mattagamite, aguilarite, taimyrite, melonite, montbrayite, tetradymite and sobolevskite.
Altaite also occurs with a range of Cu minerals, such as chalcocite, bornite, cubanite, covellite, enargite, talnakhite, boulangerite, bournonite, tetrahedrite-tennantite, rickardite, and moolhoekite.
Other associated minerals are magnetite, jamesonite, cobaltite, molybdenite, and djerfisherite.
4. Transmitted Light Microscopy
This does not apply, as altaite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Altaite polishes easily. Its hardness upon polishing is low, approximately the same as or slightly less than the hardness of galena. The hardness of altaite is slightly greater than the hardness of petzite and tellurium bismuthite.
PLANE POLARIZED LIGHT – PPL
Reflection color: White to light gray with a greenish tint.
Compared to galena, altaite is lighter and whiter, with a faint yellowish or greenish tint. It does not have the pinkish-brown tint of galena.
Compared to calaverite, altaite is light gray.
Compared to sylvanite, altaite is light gray, slightly greenish.
Compared to tellurium bismuthite, altaite is bluish-white.
Compared to petzite, altaite is white, much lighter.
Compared to gold, altaite is light bluish.
Pleochroism: No.
Reflectivity: 66,83%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic; any anisotropic effects are due to problems in the polishing process.
Internal reflections: No.
May be confused with: galena, which is very similar; care is needed.
General Characteristics:
Grain shape: The grains are typically very small and anhedral, forming aggregates.
Cubic cleavage is often visible, similar to that of galena, but less distinct.
Triangular polishing pits are uncommon, but can occur.
Polishing scratches: Large amounts of polishing scratches are difficult to avoid, especially in the presence of harder minerals such as quartz in the gangue.
Twinning does not occur.
Myrmekites of altaite in other sulfides such as krennerite and aguilarite can occur.
Intergrowths occur of altaite with galena, tetradymite, tellurium bismuthite, and native tellurium.
Oriented intergrowths of altaite with galena occur, which can be seen by the continuity of the cleavage of the galena grains into the altaite grains. Altaite also forms oriented intergrowths with nagyágite.
Inclusions 1: Altaite occurs as inclusions in bornite, hessite, and coloradoite, or it is arranged along the grain boundaries of these minerals. Altaite inclusions in galena are common; they can be small idiomorphic grains or rounded grains arranged parallel to the (100) of galena.
Inclusions 2: Altaite may contain inclusions of calaverite, melonite, and krennerite.
Borders of altaite, with hessite and galena, can form around pyrite grains.
Alteration to an opaque, yellowish-brown, strongly anisotropic material can occur. The alteration proceeds along the cleavage planes into the mineral and, within it, progresses in an oriented manner.
