Argentite

Argentite is the high-temperature polymorph of silver sulfide, with the chemical formula Ag2S. It represents the cubic, high-temperature form, while the low-temperature, monoclinic form is known as acanthite. At room temperature, the stable phase of silver sulfide is typically acanthite; argentite can occur only at higher temperatures or when cubic crystals are preserved by rapid cooling.

In terms of structure and appearance, argentite is metallic and opaque, typically gray to black with a metallic luster. The streak is gray to dark, and crystals (when present) are tabular to prismatic, though argentite is more commonly found as small grains or coatings. The hardness is about 2.5 on the Mohs scale, and the specific gravity is high, around 7.0 to 7.3.

Argentite forms in hydrothermal silver-bearing ore veins and in oxidized zones of silver deposits, often as a product of reactions involving native silver and sulfur-bearing solutions. It is commonly found in association with other silver minerals such as galena, sphalerite, and various sulfosalts, as well as with quartz and calcite. Worldwide localities include hydrothermal vein systems and ore deposits where silver sulfide compounds occur; however, at typical surface conditions argentite is rarely preserved, given its tendency to transform to acanthite upon cooling.

Etymology derives from argentum, the Latin word for silver. Argentite has historical significance as an ore-related mineral, though in practice most silver sulfide observed at ordinary temperatures is acanthite rather than argentite.