Franckeite: Properties and Occurrences

Franckeite, chemical formula Pb5Sn3Sb2S14, belongs to a family of complex sulfide minerals.  It is a triclinic-pinacoidal mineral containing antimony, iron, lead, sulfur, and tin. It is a rare sulfide mineral. It is closely related to cylindrite. It is found in the sulfide ores in Bolivia and Peru usually with other rare sulfides such as cylindrite, stannite, incaite, and potosiite.

It is named after the mining engineers, Carl and Ernest Francke. It can be found in Bolivia at Poopó in Oruro and at Las Aminas, southeast of Chocaya, in Potosi.

General Information

  • Category: Sulfosalt mineral
  • Formula: Pb5Sn3Sb2S14
  • Crystal system: Triclinic
  • Crystal class: Pinacoidal (1) (same H-M symbol)

Fig: Franckeite

Properties

Franckeite is a sulfosalt. It has an average density of 5.7 and can be both grayish black, blackish gray in color. It occurs in hydrothermal silver-tin deposits in Bolivia and in contact metamorphosed limestone deposit in the Kalkar quarry in California.

  • Color: Grayish black
  • Crystal habit: Typically in spherical, rosette aggregates of thin plates; commonly massive, radiated, or foliated
  • Twinning: Complex
  • Tenacity: Flexible, inelastic; slightly malleable
  • Mohs scale hardness: 2.5 – 3
  • Luster: Metallic
  • Streak: Grayish black
  • Diaphaneity: Opaque
  • Specific gravity: 5.88 – 5.92

Occurrence: In hydrothermal Ag–Sn deposits (Bolivia); in a limestone contact metamorphic deposit (Kalkar quarry, California, USA). It was first described in 1893 as an occurrence in Chocaya, Potosí Department, Bolivia. It occurs with cylindrite, teallite, plagionite, zinkenite, cassiterite, wurtzite, pyrrhotite, marcasite, arsenopyrite, galena, pyrite, sphalerite, siderite, and stannite.

Association: Cylindrite, teallite, plagionite, zinkenite, cassiterite, wurtzite, pyrrhotite, marcasite, arsenopyrite, galena, pyrite, sphalerite, siderite (Bolivia); cassiterite, galena, stannite, teallite, cylindrite (Russia).

 

Information Source: