Ferroallanite

Ferroallanite is a sorosilicate mineral belonging to the allanite group and the epidote supergroup. Its chemical formula is Ca(Fe2+,Al)Al2(SiO4)(Si2O7)O(OH). It is an iron-rich variety of allanite, distinguished by a higher proportion of iron in its ferrous (Fe2+) state compared to magnesium. Ferroallanite typically occurs as dark brown to black prismatic or acicular crystals, often found in metamorphic rocks such as gneisses and schists, as well as in some igneous rocks like granites and syenites.

The mineral is characterized by its relatively high specific gravity and a Mohs hardness of 5.5 to 6. It exhibits variable optical properties depending on its chemical composition and the degree of alteration. Ferroallanite is often associated with other common rock-forming minerals like quartz, feldspars, micas, and amphiboles. It can also be found alongside other members of the epidote group.

While not a commercially significant ore mineral, ferroallanite is of interest to mineralogists and geologists for its role in understanding geological processes and the geochemistry of metamorphic and igneous environments. Its presence can indicate specific conditions of temperature and pressure during rock formation. Like other allanite minerals, ferroallanite is slightly radioactive due to the presence of trace amounts of thorium and uranium.