Arsenopyrite

Arsenopyrite is the most common arsenic-bearing mineral in the Earth's crust. It forms in a wide range of hydrothermal environments, from high-temperature magmatic-hydrothermal systems (500-600°C) down to moderate-temperature mesothermal veins (200-400°C). It is particularly abundant in orogenic gold deposits, where gold-bearing fluids travel along deep crustal shear zones. In these systems, arsenopyrite is often the primary host for invisible (refractory) gold, with gold atoms substituted into the crystal lattice at concentrations that can reach hundreds of parts per million.

The mineral crystallizes from sulfur- and arsenic-rich hydrothermal fluids reacting with iron in the wall rocks or carried in solution. In granite-related tin-tungsten deposits, arsenopyrite forms early in the paragenetic sequence alongside cassiterite, wolframite, and quartz. In gold deposits, it may form both as replacement of earlier iron sulfides and as direct precipitation in veins. The arsenic content of arsenopyrite can vary, and its composition is sometimes used as a geothermometer, since the arsenic-to-sulfur ratio shifts systematically with formation temperature.

Arsenopyrite also forms in metamorphic rocks, particularly in sulfidic black shales and schists that have been heated to greenschist facies conditions (350-500°C). In these settings, it develops from the reaction of pre-existing pyrite with arsenic released from organic matter during metamorphism.

Arsenopyrite is recognized by its silver-white to steel-gray metallic luster on fresh surfaces, its distinctive monoclinic crystal habit forming pseudo-orthorhombic prisms with a diamond-shaped cross section, and the characteristic garlic odor produced when it is struck with a hammer or scratched with a steel tool. This smell is from arsenic compounds released by friction. The mineral is harder than most sulfides at 5.5-6 on the Mohs scale.

Distinguish from pyrite (paler brass-yellow color, cubic crystal system, no garlic odor when struck), from marcasite (lighter yellow, orthorhombic, lower hardness), and from lollingite (FeAs₂, which is arsenic-rich but sulfur-free, slightly higher specific gravity). Arsenopyrite's silver-white color is cooler and more steel-like than the warm yellow of pyrite. The diamond-shaped crystal cross sections and striated prism faces are characteristic. Twinning is common and often produces star-shaped or cruciform penetration twins.