TETRADYMITE
Tetradymite – Bi2Te2S – is a rarer sulfide, characteristic of Bi deposits. It is never the main ore mineral in the deposit, but it is common, always in small quantities.
Apparently, tetradymite is more stable when enriched in S, with the formula Bi14Te13S8, compared to stoichiometric tetradymite.
Tetradymite is classified in the Tetradymite Group, being one of the many phases composed of Bi-Te-S. It may contain Au, Cu, Pb, and Se. Twins of 4 individuals are common in well developed crystals. The fact that the mineral marks the paper due to its low hardness, as occurs with graphite, is also diagnostic.
1. Characteristics
Crystal system: Trigonal scalenohedral.
Color: Light steel gray, can be tin-white. Fades to iridescent or dull.
Habit: Generally granular, massive to foliated. Tabular. Rhombohedral crystals and pyramidal prisms.
Cleavage: {0001} perfect.
Tenacity: Sheets are flexible. Sectil.
Twinning: Twins of 4 individuals (from which the name derives) along the {0118} and {0115} planes are common when the crystals are well developed.
Fracture: Irregular.
Mohs Hardness: 1.5 – 2
Parting: No.
Streak: Steel gray.
Lustre: Metallic, dull.
Diaphaneity: Opaque.
Density (g/cm³): 7.2 – 7.9
2. Geology and Deposits
Tetradymite frequently occurs in hydrothermal quartz veins of moderate to high temperature. It also occurs in contact Bi metamorphic and metasomatic deposits. It occurs in chalcopyrite and bornite deposits as well as in chalcocite and quartz veins.
It can be found in subvolcanic metalliferous veins, associated with other tellurides. It also occurs in complex high-temperature pyrite deposits.
3. Mineral Associations
Tetradymite is associated with some defined groups of minerals, as follows:
1. Common gangue minerals from hydrothermal veins, such as quartz and calcite.
2. Common sulfides such as galena, arsenopyrite, chalcopyrite, sphalerite, pyrite, and pyrrhotite.
3. Less common sulfides such as cobaltite, gersdorffite, stibnite, and jamesonite.
4. Cu sulfides such as bornite, digenite, chalcocite, and tetrahedrite.
5. Oxides such as magnetite, rutile, cassiterite, and scheelite.
6. Typical of the paragenesis is the association with Au, Ag, and Te minerals, such as gold, krennerite, calaverite, melonite, altaite, native tellurium, hessite, goldfieldite, and coloradoite.
7. It evidently occurs with a number of other Bi minerals, such as native bismuth, tsumoite, montanite, tellurobismuthite, bismuthinite, cosalite, yecoraite and parkerite.
4. Transmitted Light Microscopy
This does not apply, as tetradymite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Tetradymite is very soft, with a polishing hardness lower than that of galena and bismuthinite. Even so, it acquires a good polish, generally with some polishing scratches, especially when associated with harder gangue minerals. The basal sections are very difficult to polish.
PLANE POLARIZED LIGHT – PPL
Reflection color: White with a cream or light yellow tint.
Compared to the color of pyrite, the color of tetradymite is very similar, but less yellow.
Compared to the color of chalcopyrite, the color of tetradymite is much lighter and whiter.
Compared to the color of galena, the color of tetradymite is yellowish-white.
Compared to the color of tellurobismuthite, the color of tetradymite is slightly darker, discreetly greenish-gray.
Compared to the color of krennerite, the color of tetradymite is grayish-white, with the cream tint less distinct.
Pleochroism: Very faint, in shades of greenish-gray, difficult to see, usually goes unnoticed.
Reflectivity: 53.45 – 56.65%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Moderate anisotropy, with a distinct variation between bluish-gray and yellowish-gray. It can also range from light pinkish-gray to dark brownish-gray.
Internal reflections: No.
May be confused with: Other yellowish-white minerals of the paragenesis.
Pyrite is more yellow, much harder, has a higher relief, poorer polish quality, and is isotropic.
Pyrrhotite has a more brownish color and much stronger anisotropy.
Chalcopyrite has different twinning and a more intense color, but can be very similar.
Tellurobismuthite is very difficult to differentiate from tetradymite.
General Characteristics:
Grain shape: typically anhedral granular, a very characteristic texture. It can be tabular. When crystals develop in cavities, they appear as twins of four individuals (hence the mineral’s name!). It can form idiomorphic acicular crystals with parallel extinction and hexagonal basal sections. In some cases, the crystals are very deformed and folded, forming fusiform grains.
Basal cleavage is almost always visible. Literature differs: some bibliographies report that cleavage is always visible, others that cleavage is frequently visible.
Twinning: Fine lamellar twins can occur, but are rare.
Intergrowths with other tellurides are characteristic. Tetradymite can form intergrowths with bismuthinite. Also with melonite, in which case they can be coarse lamellar. In addition, with gold, bismuth, tellurium, altaite, chalcopyrite, goldfieldite, digenite and bornite.
Substitutions: tetradymite can be replaced by native bismuth in myrmekitic intergrowths with galena.
Exsolutions in tetradymite can be tellurobismuthite, occurring as fine lenticular droplets arranged parallel to (0001) of tetradymite. Bismuthinite exsolutions can also occur.
Exsolutions of tetradymite frequently occur in galena, with the (0001) planes of tetradymite arranged parallel to the (111) planes of galena, generating a pseudo-cleavage (partition) parallel to (111) of galena. Similar features can occur in bornite. It also forms inclusions in pyrrhotite, hessite, and chalcopyrite.
Crusts of tetradymite and galena can form around tellurobismuthite grains.
Inclusions in tetradymite can be sphalerite, pyrrhotite, and coloradoite.
