CaAlPO4OH

CaAlPO₄OH, also known as calcium aluminum phosphate hydroxide, is a naturally occurring mineral belonging to the phosphate group. It is a rare mineral that typically forms in low-temperature hydrothermal environments, often associated with secondary mineralization in pegmatites and hydrothermal veins. The mineral was first described in 1983 from occurrences in the Ilimaussaq alkaline complex in Greenland, where it was identified as a new species.

Chemically, CaAlPO₄OH consists of calcium (Ca), aluminum (Al), phosphorus (P), oxygen (O), and hydrogen (H). Its structure is characterized by a framework of PO₄ tetrahedra linked with AlO₆ octahedra, with calcium ions and hydroxyl groups (OH) occupying interstitial sites. This arrangement gives it a monoclinic crystal system, though it often appears in granular or massive forms rather than well-defined crystals.

CaAlPO₄OH is typically colorless or white, though impurities may impart pale shades of yellow, pink, or gray. Its transparency is usually low, and it has a vitreous to pearly luster. The mineral is relatively soft, with a Mohs hardness of approximately 4.5 to 5, and exhibits a perfect cleavage along one direction. It is chemically stable under normal conditions but may decompose or alter in strongly acidic or basic environments.

Occurrences of CaAlPO₄OH are limited, with notable localities including the Ilimaussaq complex in Greenland, the Kola Peninsula in Russia, and certain pegmatites in the United States. It is often found in association with other phosphate minerals such as brushite, variscite, and rockbridgeite. Due to its rarity, CaAlPO₄OH is not commonly encountered in collections or commercial applications, and its primary significance lies in its geological and mineralogical study.

Research on CaAlPO₄OH contributes to the understanding of phosphate mineralization processes, particularly in alkaline and hydrothermal systems. Its structural and chemical properties also make it a subject of interest in materials science, where similar phosphate frameworks are explored for potential applications in catalysis, adsorption, and ion exchange.