illeistite

Illeistite is a very rare mineral composed of calcium, strontium, uranium, thorium, titanium, niobium, tantalum, and oxygen. It is a complex oxide and belongs to the pyrochlore supergroup. The mineral's chemical formula is typically written as (Ca,Sr)2(U,Th)2Ti2(Nb,Ta)2O14, with variations in the ratios of these elements.

Illeistite was first discovered in the Khibiny Massif, Kola Peninsula, Russia. It has since been found in other rare-element pegmatites and alkaline intrusions around the world, though always in very small quantities. Its formation is associated with the late-stage crystallization of highly evolved magmas, often in environments rich in incompatible elements like uranium and rare earths.

The mineral typically occurs as small, euhedral to subhedral crystals, often found within larger rock-forming minerals. Its color can range from reddish-brown to dark brown and black. Illeistite is relatively hard, with a Mohs hardness of around 5.5, and has a high specific gravity due to the presence of heavy elements like uranium and thorium.

Due to its rarity and complex composition, illeistite is of interest to mineralogists and geochemists studying the processes of rare-element mineralization and the behavior of radioactive elements in geological systems. It is not considered an economically significant mineral.