Thalfenisite
Thalfenisite is an extremely rare thallium chlor-sulfide mineral that is found as tiny (5 – 30 μm) subhedral to euhedral grains, typically triangular in cross-section, enclosed within chalcopyrite in pentlandite-galena-chalcopyrite ore aggregates. The crystals are visible only with a microscope. The “cabochon” in the picture above is a scientific specimen consisting of a 4.6mm sample in an analytical mount. The sample is a polished chalcopyrite crystal containing a microscopic grain of Thalfenisite. The sample has been analysed using a scanning electron microscope (SEM) and verified to contain Thalfenisite. Thalfenisite is a bronze brown, opaque, soft mineral with a Mohs hardness of 1 – 1.5 and metallic luster. Thalfenisite is very unusual in that it has an extremely high molecular weight of 3,510.71 gm. Compare this to the molecular weights of Diamond (12.01 gm), Quartz (60.08 gm) and Cerussite (267.21 gm).
Thalfenisite’s name reflects its composition of THALlium, FErrum for iron, NIckel and Sulfur (Thal-fe-ni-s-ite). It was discovered in 1979 by N. S. Rudashevsky, A. M. Karpenkov, G. S. Shipova, N. N. Shishkin and V. A. Ryabikin at the Oktyabr’skoe Cu-Ni deposit, Noril’sk, Putoran Plateau, Taimyr Peninsula, Taymyrskiy Autonomous Okrug, Eastern-Siberian Region, Russia. This is the only occurrence of Thalfenisite known to date.
| Chemical Formula: | Tl6(Fe,Ni,Cu)25S26Cl |
| Thallium Iron Nickel Copper Sulfide Chloride (halide-sulfide or chlor-sulfide) | |
| Molecular Weight: | 3,510.71 gm |
| Composition: | Thallium | 34.93 % | Tl | ||
| Iron | 31.82 % | Fe | |||
| Copper | 1.81 % | Cu | |||
| Nickel | 6.69 % | Ni | |||
| Sulfur | 23.75 % | S | |||
| Chlorine | 1.01 % | Cl | |||
| 100.00 % |
| Crystallography: | Isometric – Hexoctahedral |
| Crystal Habit: | As grains, to 5 – 30 μm, in aggregates. These are microscopic crystals visible only with a microscope. |
| Twinning: | None |
| Cleavage: | None |
| Fracture: | n/a |
| Tenacity: | Brittle |
| Mohs Hardness: | 1.0 – 1.5; VHN = 130-164 (100 g load) |
| Density: | 5.26 calculated (g/cm3) |
| Luminescence: | None |
| Radioactivity: | Not Radioactive |
| Color: | Brown, bronze brown, greenish brown. Brown in reflected light, dark brown in oil |
| Transparency: | Opaque |
| Luster: | Metallic |
| Refractive Index: | R: (400) 24.2, (440) 23.6, (480) 23.6, (520) 24.3, (560) 25.6, (600) 27.3, (640) 28.7, (700) 30.3 |
| Birefringence: | 0.000 (opaque) |
| Dispersion: | n/a |
| Pleochroism: | None |
| Anisotrophism: | In reflected light: brown |
