- Chlorargyrite is a silver halide mineral with the chemical formula AgCl, and it serves as an important secondary ore of silver. It belongs to the halide mineral class and forms through the weathering and oxidation of primary silver-bearing sulfide minerals, such as argentite (Ag₂S) and galena (PbS), in the upper parts of silver ore deposits. The name chlorargyrite is derived from the Greek words chloros (meaning “green”) and argyros (meaning “silver”), referring to its occasional greenish tint and silver content. It was historically known as horn silver, a name that reflects its waxy or horn-like appearance.
- Chlorargyrite crystallizes in the isometric (cubic) crystal system, forming rare cubic or octahedral crystals, though it most commonly occurs in massive, compact, or earthy habits. The mineral is typically colorless, gray, white, yellowish, greenish, or brownish, depending on impurities and exposure to light. It has a resinous to waxy luster, and a white streak, with a Mohs hardness of 1 to 1.5, making it very soft. Its specific gravity is relatively high, around 5.5 to 5.6, due to the silver content. Chlorargyrite is translucent to transparent in fresh samples but tends to darken and become opaque when exposed to light, as silver chloride decomposes to form metallic silver on the surface.
- Chlorargyrite forms as a secondary mineral in the oxidation zones of silver deposits, particularly in arid or semi-arid climates where chloride-bearing solutions are present. It results from the chemical alteration of primary silver sulfide minerals by chloride-rich groundwater or surface waters. Typical environments for chlorargyrite formation include the upper portions of hydrothermal veins, gossans, and the weathered zones of lead-silver ore bodies. It is frequently associated with other secondary silver minerals such as cerargyrite (AgBr), iodargyrite (AgI), native silver, and acanthite (Ag₂S), as well as gangue minerals like limonite, quartz, and calcite.
- Major occurrences of chlorargyrite have been reported from Mexico, Chile, Bolivia, Australia, Germany, and the southwestern United States, notably in Nevada, Arizona, and New Mexico. These localities often contain rich oxidized silver deposits that were mined extensively during the 19th and early 20th centuries. In desert regions, where oxidation and evaporation processes are dominant, chlorargyrite can accumulate as rich surface deposits, sometimes forming large, earthy masses that resemble wax or resin.
- Chemically, chlorargyrite consists of approximately 75% silver and 25% chlorine by weight, making it a high-grade silver ore. It is soluble in ammonia and sodium thiosulfate solutions but insoluble in water. When heated or exposed to strong light, it decomposes to metallic silver, a property that has long been recognized and utilized in silver recovery. Historically, chlorargyrite was an important source of silver before the widespread exploitation of deeper sulfide ores and remains economically valuable in oxidized silver deposits.
- Chlorargyrite is easily identified by its softness, waxy luster, and greenish to yellowish coloration, as well as its tendency to darken upon light exposure. Its high density and reaction to light are diagnostic features. It can be distinguished from similar halide minerals such as cerargyrite (AgBr) and iodargyrite (AgI) by color and chemical tests — for example, cerargyrite tends to be more yellowish, while iodargyrite often has a more brownish tint.
- Historically, chlorargyrite played a central role in early silver mining operations, especially in desert mining districts where oxidation processes produced accessible surface ores. The mineral was mined extensively in places such as Chihuahua (Mexico) and the Southwest United States, providing a major source of silver during the early mining boom. Beyond its economic value, chlorargyrite holds geological importance as an indicator of oxidizing environments and chloride-rich groundwater conditions, offering insights into the geochemical processes that influence secondary mineral formation in arid regions.
