Franckeite Locality: San José Mine, Oruro City, Cercado Province , Oruro Department , Bolivia ( Locality at mindat.org ) Size: 3.3 x 2.5 x 1.5 cm. This specimen is a very rich, incredibly rare, superb quality, crystallized specimen of the triclinic lead, tin, iron, antimony sulfosalt Franckeite consisting of somewhat "sword"-shaped crystals oriented into spherical aggregates forming the shape of a flower. — Photo: Robert M. Lavinsky · CC BY-SA 3.0
Franckeite
A lead-tin-antimony sulfosalt closely related to cylindrite, described in 1893 from Chocaya, Bolivia.
Intro adapted from Wikipedia (CC BY-SA).
Franckeite is an IMA-recognized mineral species in the sulfosalt class, with the chemical formula Pb₂₁.₇Sn₉.₃Fe₄.₀Sb₈.₁S₅₆.₉.
It crystallizes in the triclinic crystal system (space group C-1). It is a member of the Cylindrite structural group.
Key properties: Mohs hardness 2.5–3; specific gravity 8.98; Grayish black streak; Metallic. Anisotropism: Weak. R1-R2: (400) 38.2-39.4, (420) 37.9-39.2, (440) 37.6-39.0, (460) 37.3-38.7 luster.
IMA symbol Fke, grandfathered status — known before the IMA’s founding, named in 1893.
Found in hydrothermal Ag-Sn deposits (Bolivia); in a limestone contact metamorphic deposit (Kalkar quarry, California, USA).
It is found associated with Cylindrite, teallite, plagionite, zinkenite, cassiterite, wurtzite, pyrrhotite, marcasite, arsenopyrite, galena, pyrite, sphalerite, siderite (Bolivia).
The mineral was named to honor the mining engineers Carl and Ernest Francke.