fasettin

Fasettin is a rare and poorly documented mineral that belongs to the class of sulfosalts, a group of compounds containing both sulfur and arsenic. Its chemical formula is Pb₁₆As₆S₂₇Cl₂, indicating it consists primarily of lead, arsenic, sulfur, and chlorine. Fasettin was first identified in 1980 in the Tsumeb mine, Namibia, where it was found as a secondary mineral in the oxidized zone of a hydrothermal lead-zinc deposit. The name "fasettin" derives from the German word *fasettig*, meaning "faceted," in reference to its crystal habit.

Fasettin typically occurs as small, transparent to translucent crystals with a cubic or octahedral shape, often exhibiting a glassy luster. Its color ranges from colorless to pale yellow or grayish-white, though specimens may appear slightly tinted due to impurities. The mineral is brittle with a Mohs hardness of approximately 2.5, making it relatively soft compared to many other sulfosalts. Fasettin is chemically unstable in air and may decompose over time, particularly in humid conditions, leading to a loss of luster or alteration into other arsenic-bearing minerals.

Due to its rarity and the presence of arsenic, fasettin is not commonly collected by mineralogists or hobbyists. It is primarily of academic interest, studied for its unique crystal structure and composition. The Tsumeb mine remains the only known locality for fasettin, though similar sulfosalts with comparable chemical compositions have been reported in other hydrothermal deposits. Fasettin’s study contributes to the understanding of secondary mineralization processes in lead-zinc ore deposits.