Eucryptite

Eucryptite is a rare lithium aluminum silicate mineral with the chemical formula LiAlSi2O6. It occurs in lithium-rich pegmatites and high-temperature metamorphic rocks and forms when Li-bearing fluids crystallize with aluminosilicate minerals. The mineral is known in two polymorphic forms, alpha- and beta-eucryptite, which differ in structure and stability.

Beta-eucryptite is especially noted for exhibiting negative thermal expansion along the c-axis over a range of temperatures, a property of interest for tuning thermal behavior in ceramic composites. The two forms can interconvert with temperature and pressure.

Eucryptite typically occurs as colorless to pale yellow or pinkish crystals or masses with a vitreous luster. It has a hardness around 6 on the Mohs scale and a relatively low to moderate specific gravity (~2.5–2.7).

Crystals are commonly found in pegmatites associated with quartz, spodumene, lepidolite, and feldspars, and in some metamorphic lithium-rich rocks. It is considered a minor, accessory mineral and is rarely mined for industrial use.

Because of its lithium content and the unusual thermal expansion behavior of beta-eucryptite, the mineral is of interest to researchers studying lithium-bearing silicates and thermal expansion-engineered materials. Eucryptite samples are typically studied for crystallography and phase-transition behavior rather than as ore.