Allanite-(La)
Allanite (also called orthite) is a sorosilicate group of minerals within the broader epidote group that contain a significant amount of rare-earth elements. The mineral occurs mainly in metamorphosed clay-rich sediments and felsic igneous rocks. It has the general formula A2M3Si3O12[OH], where the A sites can contain large cations such as Ca2+, Sr2+, and rare-earth elements, and the M sites admit Al3+, Fe3+, Mn3+, Fe2+, or Mg2+ among others. However, a large amount of additional elements, including Th, U, Be, Zr, P, Ba, Cr and others may be present in the mineral.
Intro adapted from Wikipedia (CC BY-SA).
Allanite-(La) is an IMA-recognized mineral species in the silicate class, with the chemical formula CaLa(Al₂Fe²⁺)[Si₂O₇][SiO₄]O(OH).
It crystallizes in the monoclinic crystal system (space group P2_1/m). It is a member of the Epidote, allanite structural group.
Key properties: Mohs hardness 6; specific gravity 3.93; Brown streak; Vitreous luster.
IMA symbol Aln-La, IMA status: approved, named in 2003.
Found in barite veins that cut a dolomitic metamorphosed limestone.
It is found associated with Calcite, barite.
The name refers to the La-dominant analog of allanite-(Ce).