Quick Facts
Formation & Origin
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.
Identification Guide
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.
Spotting Fakes
Arsenopyrite is rarely faked because it has limited commercial value as a gemstone and moderate collector value. The primary identification challenge is distinguishing it from other metallic sulfide minerals. The garlic smell test (striking or scratching the mineral) is reliable but should be done outdoors or in a well-ventilated area due to the release of arsenic compounds. Do NOT inhale directly over the scratch. The hardness test is useful: arsenopyrite is significantly harder than most sulfides and will scratch glass, which pyrite will also do, but arsenopyrite's cooler silver color and diamond-shaped crystals set it apart. TOXICITY WARNING: Arsenopyrite contains approximately 46% arsenic by weight. Always wash hands thoroughly with soap and water after handling. Never store arsenopyrite specimens where they might contact food preparation surfaces. Do not lick or taste. Weathering arsenopyrite produces scorodite and other arsenic-bearing secondary minerals that can contaminate water. If arsenopyrite specimens develop a yellowish-green powdery coating, this is likely scorodite, which is water-soluble arsenic. Handle with gloves and store in sealed containers. Keep away from children.
Cultural & Metaphysical Traditions
Presented as cultural traditions, not scientific evidence
Arsenopyrite has virtually no metaphysical tradition. Its high toxicity and metallic nature place it firmly in the domain of mineral collecting and geological study rather than spiritual practice. In medieval alchemy, arsenic minerals were associated with transformation and were used (dangerously) in various experimental processes. Some modern crystal reference books associate arsenopyrite with personal power and determination due to its connection to gold deposits, but these are recent attributions without historical basis. Due to its arsenic content, arsenopyrite should never be used in crystal elixirs, placed on skin for extended periods, or handled without washing hands afterward.
Where It's Found
Classic tin mining district with abundant arsenopyrite
World-class crystals in tungsten-tin vein system
Major global source, associated with gold deposits
Sharp crystals in polymetallic tin-silver veins
Associated with silver and cobalt mineralization
Price Guide
Good to Know
Scratch test: At hardness 5.75, Arsenopyrite resists scratching from a knife but can be scratched by quartz. Best for pendants and earrings rather than rings.
Sources: Found in 5 notable locations worldwide, from England to Canada.
Heft test: With a specific gravity of 5.90-6.20, Arsenopyrite feels surprisingly heavy for its size. This weight is actually a useful identification tool.
Related Minerals
Iron sulfide cousin, similar structure without arsenic
Iron diarsenide, arsenic-rich relative without sulfur
Iron arsenate, common weathering product of arsenopyrite