| Fettelite | |
|---|---|
Cluster of tiny fettelite crystals in a vug from Copiapó, Atacama Region, Chile - (size: 2x2 mm) |
|
| General | |
| Category | Mineral |
| Chemical formula | Ag24HgAs5S20 |
| Strunz classification | 02.LA.30 |
| Crystal symmetry | trigonal |
| Unit cell | a = 15.00 Å, c = 15.46 Å, V 3014 ų; Z = 3 |
| Identification | |
| Color | dark violet to red |
| Crystal habit | flakes, hexagonal, micaceous |
| Crystal system | Trigonal |
| Twinning | intimately twinned with six twin domains |
| Cleavage | perfect |
| Fracture | subconchoidal |
| Mohs scale hardness | 3.5 |
| Luster | metallic |
| Streak | dark vermillion |
| Diaphaneity | subopaque to opaque |
| Specific gravity | 6.29 |
| Optical properties | Biaxial, anisotrophism weak with strong red internal reflections |
| Refractive index | N(calc) = 1.74 |
| Birefringence | moderate white to brownish gray |
| References | [1] [2] |
Fettelite is a mercury-sulfosalt mineral with the chemical formula Ag24HgAs5S20. The mineral was first described by Wang and Paniagua (1996)[3] who named it after M. Fettel, a German field geologist who collected the first samples form Odenwald.[4] It was first collected in the Nieder-Beerbach mine, 10 km south of Darmstadt, Odenwald, Germany. Its normal occurrence is in hydrothermal veins, which can cut gabbro-diorite intrusives. It is closely related to other rare minerals like dervillite, daomanite, vaughanite and criddleite which are also found in the same type locality as fettelite.[5]
Fettelite occurs as clusters of hexagonal flakes. These flakes can get up to 0.2 mm across and around 5-10 µm thick. In more complex hexagonal tablets, somewhat larger sub parallel aggregates can be measured.[6] The birefringence of Fettelite is moderate white to grayish brown.[4]