Al2SiO4OH2

Al2SiO4OH2 is the chemical formula for the mineral known as andalusite. Andalusite is an aluminum silicate mineral that crystallizes in the orthorhombic system. It is one of three polymorphs of aluminum silicate, the other two being kyanite and sillimanite. These polymorphs share the same chemical composition but differ in their crystal structure, and thus in their physical properties.

Andalusite is typically found in metamorphic rocks, particularly those formed under relatively low pressure and high temperature conditions. It is often associated with schists and gneisses. The mineral commonly occurs as prismatic crystals, which can be stubby or elongated. Its color can range from colorless to white, gray, yellow, reddish-brown, or even greenish. A distinctive variety of andalusite is chiastolite, which contains dark, carbonaceous inclusions arranged in a cross-like pattern within the crystal, a result of inclusions during crystal growth.

The hardness of andalusite is typically between 6.5 and 7.5 on the Mohs scale, making it relatively hard. Its specific gravity is around 3.1 to 3.2. Andalusite is generally resistant to weathering. Due to its hardness and attractive colors, it is sometimes used in jewelry, although it is not as commonly faceted as some other gemstones. It also finds industrial applications, particularly in the production of refractories and high-temperature ceramics, due to its stability at elevated temperatures.