Cerargyrite

Cerargyrite, also known as horn silver, is a mineral composed of silver chloride. Its chemical formula is AgCl. It is a rare halide mineral of silver. The name comes from the Greek words kerastos (meaning "horn") and argyros (meaning "silver"), referring to its horn-like appearance and silver content. Cerargyrite is typically colorless when pure, but often appears gray, greenish, or yellowish due to impurities. It is known to darken upon exposure to light, a process similar to the way photographic film is exposed. This light sensitivity is a defining characteristic of the mineral.

The mineral is usually found in the oxidized zones of silver deposits, particularly in arid or semi-arid environments where weathering is prominent. It forms through the alteration of other silver-bearing minerals. Notable occurrences include the Comstock Lode in Nevada, USA, and various locations in Mexico, Chile, and Australia. Cerargyrite has been historically important as a source of silver, although its rarity and relatively low silver concentration compared to other ores mean it is not a primary source in modern mining. Its geological significance lies in its formation within specific weathering environments and its role as an indicator mineral for silver mineralization.