| Fettelite | |
| Category: | Sulfosalt mineral |
| Formula: | Ag16HgAs4S15 (rruff) or [Ag<sub>6</sub>As<sub>2</sub>S<sub>7</sub>]·[Ag<sub>10</sub>HgAs<sub>2</sub>S<sub>8</sub>] (mindat.org) |
| Imasymbol: | Ftt[1] |
| Strunz: | 2.LA.30 |
| System: | Monoclinic |
| Class: | Sphenoidal (2) (same H-M symbol) |
| Symmetry: | C2 |
| Unit Cell: | a = 15.00, c = 15.46 [Å] V = 3014 Å3; Z = 8 |
| Color: | dark violet to red |
| Habit: | flakes, hexagonal, micaceous |
| Twinning: | intimately twinned with six twin domains |
| Cleavage: | perfect |
| Fracture: | subconchoidal |
| Mohs: | 3.5 |
| Luster: | metallic |
| Refractive: | N(calc) = 1.74 |
| Opticalprop: | Biaxial, anisotrophism weak with strong red internal reflections |
| Birefringence: | moderate white to brownish gray |
| Streak: | dark vermillion |
| Gravity: | 6.29 |
| Diaphaneity: | subopaque to opaque |
| References: | [2] [3] [4] |
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 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]