Fettelite

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	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]