Stability fields: This chart shows the temperature / pressure ranges where andalusite, kyanite, and sillimanite are stable. Andalusite, kyanite, and sillimanite all share the chemical composition of Al2SiO5. However, they have different crystal structures. Their crystal structure differs because they form under extremely different conditions of temperature and pressure. The phase diagram at left summarizes the…

Andalusite forms during the regional metamorphism of shale. It is found in schist and gneiss at some present and ancient convergent plate boundaries where the rocks have been exposed to the temperatures and pressures needed for its formation. In these rocks, andalusite is often associated with kyanite and sillimanite. Andalusite also forms during the contact metamorphism of shale. In this situation,…

Andalusite has a number of useful physical properties. It has the ability to withstand high temperatures without alteration. For that reason it is used to make high-temperature ceramics and refractories. The white porcelain of some spark plugs is made using andalusite. Andalusite is one of a small number of minerals that commonly forms prismatic crystals…

Chiastolite is a variety of andalusite that contains black particles of graphite arranged in geometric patterns. The graphite is pushed aside by crystal growth within a rock that is being metamorphosed. As growth occurs, the particles become concentrated at crystal interfaces. The result can be a cross-shaped pattern within the mineral – similar to the “cross-stone” shown in the accompanying…

Andalusite is an aluminosilicate mineral with a chemical composition of Al2SiO5. It forms at low to medium metamorphic temperatures and pressures where argillaceous rocks (such as shale) are exposed to regional and contact metamorphism. Andalusite is a rock-forming mineral most commonly found in schist, gneiss, and hornfels. Minor amounts are also found in granite and granitic pegmatite. The most important use of andalusite is in making heat-resistant bricks and…