Fettelite, also known as sanguinite, is a mercury-sulfosalt mineral with the chemical formula Ag16HgAs4S15. The mineral was first described by Wang and Paniagua (1996)[5] who named it after M. Fettel, a German field geologist who collected the first samples from Odenwald.[6] 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.[4]
| Fettelite | |
|---|---|
 Cluster of tiny fettelite crystals in a vug from Copiapó, Atacama Region, Chile | |
| General | |
| Category | Sulfosalt mineral |
| Formula (repeating unit) | Ag16HgAs4S15 (rruff) or [Ag6As2S7]·[Ag10HgAs2S8] (mindat.org) |
| IMA symbol | Ftt[1] |
| Strunz classification | 2.LA.30 |
| Crystal system | Monoclinic |
| Crystal class | Sphenoidal (2) (same H-M symbol) |
| Space group | C2 |
| Unit cell | a = 15.00, c = 15.46 [Å] V = 3014 Å3; Z = 8 |
| Identification | |
| Color | dark violet to red |
| Crystal habit | flakes, hexagonal, micaceous |
| 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 | [2][3][4] |
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.[7] The birefringence of Fettelite is moderate white to grayish brown.[6]