Allanite
From Freepedia
Allanite is a sorosilicate group of minerals within the broader epidote group that contain a significant amount of rare earth elements. It has the general formula Ca(Ce,La,Y,Ca)Al2(Fe2+,Fe3+)(SiO4)(Si2O7)O(OH). The International Mineralogical Association lists three minerals in the allanite group, each recognized as a unique mineral: allanite-Ce, allanite-La and allanite-Y, depending on the dominant rare earth present.
Allanite, also called orthite, contains up to 20% rare earth elements and is a valuable source of them. Other elements can also substitute in the structure including thorium. The inclusion of thorium and other radioactive elements results in some interesting phenomena. Allanite often has a halo of radiation damage in the minerals immediately adjacent. Also highly radioactive grains of allanite often have their structure disrupted or are metamict.
Allanite is usually black in color, but can be brown to brown-violet. It crystallizes in the monoclinic system and forms prismatic crystals usually disseminated in igneous rocks. It has a Mohs hardness of 5.5 and a specific gravity of 3.3 - 4.2.
It was discovered in 1810 and named for the Scottish mineralogist, Thomas Allan (1777-1833). The type locality is Aluk Island, Greenland.
