Fahlore

Fahlore is a traditional mineral term used for the copper sulfosalt minerals in the tetrahedrite-tennantite group. These minerals form a continuous solid solution between tetrahedrite, Cu12Sb4S13, and tennantite, Cu12As4S13, with substitutions of zinc, iron, and other elements. The general formula can be written as Cu12(Sb,As)4S13, with additional cation sites displaced by Zn, Fe, Mn, and others.

Fahlore minerals are typically metallic in luster and range in color from black to gray or bronze. They crystallize in the isometric (cubic) system and have a Mohs hardness of about 3 to 3.5. The streak is grayish, and the minerals are often opaque and brittle, occasionally showing tarnish on exposure to air.

Occurrence and ore significance: Fahlore minerals occur primarily in hydrothermal vein deposits and are commonly associated with sulfide minerals such as sphalerite, galena, and pyrite, as well as carbonate gangue like calcite and quartz. They are an important source of copper and can contain silver as a byproduct. The composition is highly variable, which makes precise identification by chemistry important in ore deposits.

Etymology and usage: The name fahlore derives from German fahl, meaning dull or pale, reflecting the ore’s appearance. The term is largely historical and is often used to describe the copper sulfosalt component of the tetrahedrite-tennantite series. In modern mineral nomenclature, these species are typically referred to by their end-members or as tetrahedrite-tennantite solid solutions.