andalusitekyanitesillimanite

Andalusite, kyanite, and sillimanite are polymorphs of aluminum silicate, meaning they share the same chemical formula (Al2SiO5) but have different crystal structures and physical properties. This difference in structure arises from the varying pressure and temperature conditions under which they form during metamorphism.

Andalusite is typically found in low-pressure, relatively low-temperature metamorphic rocks, such as hornfels formed by contact metamorphism. It often occurs as prismatic or stubby crystals, sometimes exhibiting a characteristic cross-shaped pattern due to inclusions of graphite. Andalusite is relatively stable and is a common accessory mineral.

Kyanite is stable at high pressures and low to moderate temperatures, commonly found in regional metamorphic rocks like schists and gneisses. It usually forms bladed or columnar crystals, often in radiating aggregates. Kyanite's hardness varies depending on the crystallographic direction, being harder along its length than across it.

Sillimanite is stable at high temperatures and moderate to high pressures, typically forming in high-grade metamorphic environments such as granulites. It occurs as fine, needle-like or fibrous crystals, often appearing in parallel or radiating arrangements within metamorphic rock matrices. Sillimanite is a key indicator of significant thermal and pressure events in the Earth's crust. The geological context and associated mineral assemblages are crucial for distinguishing between these three polymorphs and understanding the metamorphic history of a rock.