THE MINERAL FRANCKEITE

Chemistry: (Pb, Sn)6FeSn2Sb2S14, lead Tin Iron Antimony Sulfide.
Class: Sulfides
Subclass: Sulfosalts
Uses: A very minor ore of lead and tin and as a mineral specimen.
Specimens

Franckeite is a rare sulfide mineral. It is found in the sulfide ores in Bolivia and Peru usually with other rare sulfides such as cylindrite, stannite, incaite and potosiite. Franckeite is a sulfosalt. Normally semi-metal ions such as antimony will substitute for the sulfur atoms, but in the sulfosalts they substitute for the metal ions and bond with the sulfurs.

The formula of franceite points out the difference between the two different tin ions in the mineral. If the formula was written with the oxidation states, it would look like this:

(Pb⁺²,Sn⁺²)6Fe⁺²Sn⁺⁴2Sb⁺³2S^-214.

As can be seen there are two different tin ions. One is in a positive two (+2) oxidation state and the other is in a positive four (+4) oxidation state. The oxidation states also show the positive charge on the antimonies, a condition found in sulfosalts.

PHYSICAL CHARACTERISTICS:

Color is gray-black to gray-white.
Luster is metallic.
Transparency crystals are opaque.
Crystal System is triclinic; bar 1
Crystal Habits include accicular sprays and crusts.
Hardness is 2.5 - 3
Specific Gravity is approximately 5.8 - 6.0 (above average for metallic minerals)
Streak is gray-black.
Associated Minerals include quartz, galena and pyrite as well as rare sulfides such as cylindrite, stannite, incaite and potosiite.
Notable Occurrences are limited to Las Animas District, Bolivia and Hulcani Mine, Peru.
Best Field Indicators are crystal habit, locality, color and density.