Ca2Al2SiO6OH2

Ca2Al2SiO6OH2 is the chemical formula for a mineral belonging to the silicate group. This compound is also known by its common name, allanite, or more specifically, parra-allanite when the ytterbium content is significant. Allanite is a complex silicate mineral that typically forms in igneous rocks, particularly those of intermediate to felsic composition like granite and diorite. It can also be found in metamorphic rocks, such as gneisses and schists.

The mineral is characterized by its variable chemical composition, with significant substitutions possible within its crystal structure. Calcium and aluminum are key components, along with silicon and hydroxyl groups (OH). Trace amounts of rare earth elements, including cerium, lanthanum, neodymium, and yttrium, are commonly present and contribute to its radioactivity. Allanite often exhibits a distinct vitreous to greasy luster and can vary in color from black and dark brown to greenish-brown. It is generally opaque to translucent.

Crystallographically, allanite belongs to the monoclinic crystal system. It is often found as prismatic or equant crystals, but also occurs in massive or granular forms. Due to its complex composition and the presence of radioactive elements, allanite can undergo metamictization, a process where radiation damage causes the crystal structure to become disordered. This can affect its physical properties, such as hardness and optical characteristics. Allanite is not typically used for commercial purposes but is of interest to mineralogists and geologists for its role in understanding the geological conditions under which it forms and its utility in geochronology due to the presence of rare earth elements.