Allanite
Also known as Orthite and a member of the Epidote group of minerals (see https://bit.ly/2HqqNnM) is this mineral, rich in a variety of rare earth elements (see https://bit.ly/2HQxOgU). It is formed when clay rich sediments when heated and baked by metamorphism or in silica rich igneous rocks, where the magma has distilled and concentrated rare earth elements from the depths of the crust,often leaching wide areas of rock to concentrate their rare earth contents into a much smaller volume of magma.
The compositions are variable as one site on the crystal lattice can accommodate a wide variety of atoms, including the rare earths and radioactive elements like thorium (which can leave a halo of colourful radiation damage around Allanite crystals in the mother rock, see https://bit.ly/2HtCAxq). Like zircon, which also incorporates radioactive elements in its crystal structure, the radiation can damage the crystallinity by destroying the lattice until the mineral becomes amorphous, without regular structure, a condition known as metamict. The group has thus been divided into cerium, lanthanum and yttrium rich end members, and the mineral can contain as much as 20% of these metals that are vital for many high tech industries. As a result of this Allanite is often referred to as a trashcan mineral, into which all the bits and bobs that can't be dumped into minerals during crystallisation end up.
Colour is usually black, though brown and violet also exist, though as in the 8.6 x 4.9 x 2.6 cm specimen here from the Rossing Uranium Mine in Namibia crystals are often coated in a yellowish alteration product, probably the iron oxide Limonite. Despite its relative hardness of 6 on Mohs scale, it is not faceted for collectors, though it has a party trick of glowing red hot at low temperatures of 95 degrees Celsius. It was first discovered in Greenland in 1810 and named after a Scottish mineralogist. Like other radioactives, it should be stored away from other minerals as it can affect their colour by altering their crystal lattice to create or remove colour centres, and one should take basic hygiene precautions when and after handling them. Occurrences include the Ural Mountains of Russia, Sweden, France, Madagascar, the Eifel District of Germany, Spain, Canada and various localities in the USA.
Loz
Image credit: Rob Lavinsky/iRocks.com
http://www.galleries.com/Allanite https://www.mindat.org/min-125.html
