Thorium in crandallite-group minerals: an example from a Devonian bauxite deposit, Timan, Russia

摘要

A Th-rich mineral of the crandallite group has been investigated from the weathering profile of the Schugorsk bauxite deposit, Timan, Russia. It occurs within thin (up to 0.5 mm) organic-rich veinlets together with ‘leucoxene’ in the form of small shapeless grains which vary in size from 1–2 µm to 60–70 µm. Rare grains disseminated among boehmite crystals were also found. Microprobe analyses determined that the ThO₂ content can be as high as 18 wt.%. The mineral composition is intermediate between crandallite CaAl₃H(PO₄)₂(OH)₆, goyazite SrAl₃H(PO₄)₂(OH)₆, Th-crandallite and svanbergite SrAl₃PO₄SO₄(OH)₆ in the beudantite group.