Allanite

Allanite is a group of dark, rare-earth-bearing sorosilicate minerals in the epidote family. It occurs chiefly in igneous and metamorphic rocks and can incorporate uranium and thorium.

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Epidote group: A mineral group of sorosilicates with related structures and compositions, including epidote, clinozoisite, and allanite. Allanite is the rare-earth-rich branch of this mineral family.
Crystal chemistry: The study of how chemical composition relates to the structures and properties of crystals. Allanite’s variable composition follows from substitutions among its crystallographic sites.
Uranium–lead dating: A radiometric dating method that uses uranium decay to lead in minerals. Uranium-bearing allanite can preserve dates of crystallization or later geological events.
Allanite-(Ce): The cerium-dominant species of allanite, a rare-earth-bearing epidote-group mineral. It is the most common member of the allanite group.
Sorosilicate: A class of silicate minerals built from pairs of linked silicon–oxygen tetrahedra. Allanite’s paired tetrahedra define its broader structural class.
Solid solution: A single crystalline phase in which elements substitute for one another within a shared structure. Substitution produces compositional variation across allanite species.
Geochronology: The science of determining the ages and timing of geological events. Allanite is one mineral used to establish geological timescales.
Allanite-(La): The lanthanum-dominant species of allanite in the epidote group. It illustrates how the dominant rare earth distinguishes allanite species.
Monoclinic crystal system: A crystal system with three unequal axes, one pair of which meets at a non-right angle. Most common allanite species crystallize in this system.
Metamictization: The loss of long-range crystal order caused by radiation damage in a mineral. Uranium and thorium decay can progressively disrupt allanite’s structure.