Cristobalite is a form of silica that solidifies at high temperature. It is chemically identical to quartz, but has a different crystal structure. It is found in volcanic rocks, usually associated with the natural glass rock obsidian. It is known for its brilliant white color and is used in paints, putties and gesso.
In volcanic rock, cristobalite crystals typically appear as small rounded masses. However, it can also occur in angular, octahedral and tetragonal forms. Crystals of octahedral cristobalite are often arranged in a honeycomb pattern and have been nicknamed snowflake obsidian.
Crystobalite is very stable in the temperature range it forms, but at higher temperatures will slowly change to another polymorph that is more stable. This is called a metastable polymorph. The tetragonal phase, b-tridymite is the most stable form at higher temperatures. The octahedral phase, a-cristobalite is the next most stable form. The cubic cristobalite and tetragonal a-cristobalite share the same space group, Fd3m (Peierls symbol F104).
Tridymite and cristobalite have been shown to be relatively toxic to humans. This toxicity has been attributed to the formation of crystalline silica in lung tissues, which results in inhalation of particles that can lead to inflammation and scarring of the lungs. Animal studies have demonstrated that cristobalite and tridymite are more toxic to the respiratory tract than quartz, which is why they have lower American Conference of Governmental Industrial Hygienists threshold limit values (TLV) than quartz.