KËSTERITE
Kësterite – Cu2ZnSnS4 – is an extremely rare sulfide. It does not constitute an ore.
It is classified within the Stannite Group, in the Kësterite–Stannite series. Its iron (Fe) analogue is ferrokësterite. The synthetic form of kësterite is abbreviated as CZTS (“copper zinc tin sulfide”). The term “kësterite” is sometimes used to encompass both the mineral and the synthetic material, and may also include CZTSe, which contains selenium (Se) instead of sulfur (S).
1. Characteristics
Crystal system: Tetragonal bisphenoidal, pseudocubic.
Color: Greenish black.
Habit: Pseudocubic crystals, such as octahedrons.
Massive
Cleavage: No.
Tenacity: No information available.
Twinning: No.
Fracture: No information available.
Mohs Hardness: 4.5
Parting: No information available.
Streak: Black.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 4.54 – 4.59
2. Geology and Deposits
Kësterite is characteristic of high-temperature hydrothermal quartz-sulfide veins in tin deposits, where it is associated with cassiterite veins.
3. Mineral Associations
It occurs associated with some common minerals such as quartz, muscovite, and calcite.
It also occurs with common sulfides such as pyrite, chalcopyrite, arsenopyrite, sphalerite, tennantite, chalcocite, digenite, and covellite.
In its specific paragenetic assemblages, in addition to sulfides, it is found with cassiterite, stannite, stannoidite, beryl (including the varieties aquamarine [bluish] and goshenite [colorless]), amblygonite, montebrasite, planerite, acanthite, herzenbergite, varlamoffite, malachite, and mushistonite.
It may also be associated with cryolite.
4. Transmitted Light Microscopy
Not applicable, as kesterite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: The hardness of kesterite is similar to the hardness of stannite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Gray with an olive tint.
Compared to the color of stannite, the color of kesterite is very similar – slightly darker and lacking any brown tones.
Compared to the color of mawsonite, the color of kesterite is gray.
Pleochroism: No.
Reflectivity: 23.20 – 24.15%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic; may exhibit weak anomalous anisotropy.
Internal reflections: Very rare.
May be confused with: Stannite, but stannite is strongly anisotropic.
General Characteristics:
Exsolutions of kesterite can occur within stannite, as the two minerals form a solid solution at temperatures above 680ºC.
Inclusions of chalcopyrite may be present.
Intergrowths of stannite and kesterite can occur, forming a network of lamellae with a “parquet” (checkerboard-like) pattern.
Replacement of kesterite by cassiterite can occur.
