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Occurs in Pinal County. Common in Arizona.

Andalusite. Al Copley photo
Andalusite. Al Copley photo

Chemical Formula: Al2SiO5

Specific Gravity: 3.15-3.18

Luster: Vitreous to pearly

Hardness: 7-7.5

Andalusite is a mineral known for its prismatic crystals and pleochroism. It forms in high-temperature metamorphic environments and can provide information about geological conditions and processes. While not as common as some other minerals, andalusite’s distinctive features make it interesting to both scientists and mineral enthusiasts.

Next Pinal County mineral: Andradite

MinDat Andalusite

Andalusite is a mineral that belongs to the aluminosilicate group. It is known for its distinctive prismatic crystals and its ability to exhibit a phenomenon called “pleochroism,” where the mineral appears to change color when viewed from different angles. The name “andalusite” is derived from the Andalusia region of Spain, where the mineral was first discovered.

The chemical formula for andalusite is Al2SiO5, indicating its composition of aluminum (Al), silicon (Si), and oxygen (O). Andalusite is part of a group of minerals known as “sillimanite group,” which also includes kyanite and sillimanite.

Andalusite typically forms in high-temperature metamorphic environments, where rocks experience significant heat and pressure. It is often associated with rocks that have undergone regional or contact metamorphism, such as schists and gneisses. Andalusite crystals can develop elongated prismatic shapes and are often found embedded within these metamorphic rocks.

One of the remarkable properties of andalusite is its pleochroism, which means that the mineral can display different colors when viewed from different angles. This optical phenomenon is due to variations in the absorption of light by the crystal lattice in different directions. Andalusite’s pleochroism can range from pale green to brown to yellow, depending on the orientation of the crystal.

Geologically, the presence of andalusite in metamorphic rocks provides insights into the temperature and pressure conditions under which the rocks formed. Andalusite can also serve as an indicator of the degree of metamorphism that a particular region has experienced.

Due to its unique properties and attractive crystal form, andalusite is sometimes used as a gemstone or a collector’s mineral. However, it is not as widely known or used as some other minerals like quartz or feldspar.