Petalite, also known as castorite, is a lithium aluminium phyllosilicate mineral LiAlSi₄O₁₀, crystallizing in the monoclinic system. Petalite is a member of the feldspathoid group. It occurs as colourless, grey, yellow, yellow grey, to white tabular crystals and columnar masses. Occurs in lithium-bearing pegmatites with spodumene, lepidolite, and tourmaline. Petalite is an important ore of lithium, and is converted to spodumene and quartz by heating to ~500 °C and under 3 kbar of pressure in the presence of a dense hydrous alkali borosilicate fluid with a minor carbonate component. The colorless varieties are often used as gemstones.