Formula: Fe²⁺(Pb,Sn²⁺)₆Sn⁴⁺₂Sb₂S₁₄

Species:

Colour: Grey-black, black; iridescent at times

Lustre: Metallic

Hardness: 2½ – 3

Specific Gravity: 5.90

Crystal System: Triclinic

Member of: Franckeite Group

Name: Named in honor of brothers Johann Heinrich Karl (Carl) Francke (1832–1907) and Ernst Otto Francke (1838-1913), mining engineers, for the promotion of the mineralogy, geology and paleontology of the Bolivian highlands. They supplied the first specimens for study.

Type Locality: Chocaya mine, Chocaya-Animas (Animas-Chocaya), Sud Chichas Province, Potosí – Bolivia

Forms a homologous pair with cylindrite.