CUBANITE
Cubanite – CuFe2S3 – is a rarer sulfide that is part of Cu sulfide ores. Locally, it is an important ore. Crystals are highly accepted in the collector mineral market.
It has high sensitivity to variations in temperature, pressure, and composition. When heated above 200-210ºC, it irreversibly transforms into isocubanite (cubic), with which it is therefore dimorphic. It is weakly magnetic (ferromagnetic), with very strong magnetic susceptibility parallel to the b-axis, but almost non-magnetic parallel to the a- and c-axes.
Twins are very common along the {110} plane, in pairs, also as clusters of 4 crystals (fourlings) and 6 crystals (sixlings), in this case pseudohexagonal.
1. Characteristics
Crystal system: Orthorhombic bipyramidal.
Color: Brass yellow to bronze yellow, a color between pyrrhotite and chalcopyrite.
Habit: Usually found as lamellae in chalcopyrite. Rare, prismatic, thick tabular crystals, up to 3 cm.
Cleavage: No, but there are deep striations parallel to [010] on the faces of the crystals.
Tenacity: No information available.
Twinning: See above.
Fracture: Conchoidal.
Mohs Hardness: 3.5
Parting: By {110} and {1-30}.
Streak: Gray-black.
Lustre: Intense metallic.
Diaphaneity: Opaque.
Density (g/cm³): 4,03 – 4,18
2. Geology and Deposits
Cubanite is characteristic of high-temperature (>250ºC) hydrothermal sulfide deposits and, therefore, an excellent geological thermometer. It typically occurs with pentlandite and pyrrhotite in differentiated sulfide magmas. It also makes up the sulfide content of common eruptive rocks.
Cubanite occurs in pegmatites, pneumatolytic veins, and contact metasomatism deposits, especially those formed at great depths by silicate replacement and correlated impregnations. It occurs in deposits formed at any temperature, provided that these later underwent high temperatures due to regional or contact metamorphism.
It has rarely been found in meteorites (carbonaceous chondrites).
3. Mineral Associations
It is associated with common gangue minerals such as quartz and carbonates (calcite, dolomite, siderite).
Its association with common sulfides such as chalcopyrite, pyrite, pyrrhotite, pentlandite, arsenopyrite, galena, tetrahedrite, molybdenite, and sphalerite is very typical.
Less common associated sulfides are alabandite, cobaltite, linnaeite, stannite, and mackinawite.
Also with oxides such as magnetite, ilmenite, wolframite, hematite, and cassiterite.
It is associated with rarer minerals such as moolhoekite, paolovite, cabriite, sperrylite, violarite, and silver.
4. Transmitted Light Microscopy
This does not apply, as cubanite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Cubanite generally acquires an excellent polish without any difficulty. In some cases, such as with some pyrrhotites, a pseudo-structure forms that reflects old polishing grooves and is difficult to remove. Its hardness upon polishing is medium, slightly greater than the hardness of chalcopyrite, less than the hardness of sphalerite, and much less than the hardness of pyrrhotite and magnetite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Cream; a bronze color with a high white content. Creamy gray, pale brown, peach, yellowish-white, pinkish-brown (literature differs).
Compared to the color of pyrrhotite, the color of cubanite is very similar, somewhat more yellow, less pinkish, and more gray.
Compared to the color of chalcopyrite, the color of cubanite is pinkish-gray.
Pleochroism: No.
Reflectivity: 38.62 – 42.18%
Bireflectance: Very light in various shades of cream-grey, pale brown and peach. Sections parallel to (001) have less sharp bireflectance.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Very distinct, highly diagnostic, from light blue to dark blue, brownish, much less distinct in basal sections.
This anisotropy is not as strong as that of pyrrhotite.
Sections perpendicular to (001) show, upon examination very carefully, very thin lamellae resulting from the polysynthetic structure of the cubanite lamellae.
Internal reflections: No.
May be confused with: several other minerals from the paragenesis.
Pyrrhotite is very similar, but has much greater hardness.
Magnetite has a very similar color, but much higher hardness.
Enargite has lower reflectivity.
Mainly the lamellae of cubanite are very characteristic. The hardness of cubanite is similar to that of chalcopyrite and sometimes the distinction can be difficult, but the much greater anisotropy of cubanite is diagnostic. Low-quality sections of chalcopyrite sometimes show old polishing grooves filled with material from later polishings, which can be interpreted as thin lamellae of cubanite. Very small demixing bodies can be difficult to recognize, as they occur in chalcopyrite, pyrrhotite, cubanite and valleriite, practically with the same appearance.