Andalusite (Al2SiO5): properties, formation, and uses

Characteristics

Andalusite typically appears in shades ranging from colorless to shades of pink, brown, yellow, green or gray. It is a relatively hard and dense mineral with a moderate specific gravity and noticeable pleochroism — colors change when the crystal is viewed from different directions. A distinctive variety called chiastolite contains dark, cross-shaped inclusions of carbon or graphite that produce a visible cross pattern on polished sections. Many gem-quality andalusites are faceted to showcase their color range and pleochroic effects.

• Chemical formula: Al2SiO5
• Crystal system: orthorhombic
• Notable variety: chiastolite (cross-bearing andalusite)

Geologic significance and formation

Andalusite commonly forms during low‑pressure, moderate‑to‑high temperature metamorphism such as contact metamorphism around intrusions. Because it is one of a trio of Al2SiO5 minerals that are stable under different pressure–temperature conditions, andalusite serves as an important index mineral for reconstructing the pressure and temperature history of metamorphic rocks. In general terms, kyanite indicates higher pressures, sillimanite higher temperatures, and andalusite conditions of lower pressure; transitions among these polymorphs record metamorphic changes.

Occurrence and uses

Andalusite is found worldwide in metamorphic rocks such as schist and gneiss and in contact aureoles. Aside from its interest to geologists, andalusite has practical uses: gem‑quality crystals are cut for jewelry, and more commonly, andalusite is a raw material for refractory and ceramic applications because it yields mullite when fired. The chiastolite variety has historical and cultural uses as amulets and ornamental stones. For more mineralogical data see detailed mineral descriptions and for metamorphic context see metamorphic mineral guides.

Notable facts

As an index mineral, andalusite provides direct clues to the metamorphic environment of its host rock. Its conversion to sillimanite or kyanite under changing pressure–temperature conditions makes it useful for petrologists studying crustal processes. The visually striking chiastolite with its dark cross remains a popular collector's specimen and a memorable demonstration of mineral inclusions.