THORIUM IN CRANDALLITE-GROUP MINERALS: AN EXAMPLE FROM A DEVONIAN BAUXITE DEPOSIT, TIMAN, RUSSIA

Abstract

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 mm to 60—70 mm. Rare grains disseminated among boehmite crystals were also found. Microprobe analyses determined that the ThO 2 content can be as high as 18 wt.%. The mineral composition is intermediate between crandallite CaAl 3 H(PO 4 ) 2 (OH) 6 , goyazite SrAl 3 H(PO 4 ) 2 (OH) 6 , Th-crandallite and svanbergite SrAl 3 PO 4 SO 4 (OH) 6 in the beudantite group. Comparatively high contents of Fe and Si and a very high positive Th and Fe content correlation (r = +0.98) suggest that the formula of the hypothetical Th-bearing end-member is ThFe 3 (PO 4 ,SiO 4 ) 2 (OH) 6 with Th and Si substituting for REE and Prespectively (woodhouseite-type substitution). Another possible substitution is Th ⁴⁺ + Ca ²⁺ ⇋ 2REE ³⁺ (florencite-type). A deficiency of cations in the X site can be explained by either the presence of carbon, undetectable by microprobe, in the crystal lattice or a lack of X-site cations due to radiation damage induced by Th. Some excess of cations in the B site (Al and Fe ³⁺ ) can be explained by the presence of very small boehmite and hematite inclusions on the crandallite grain surfaces. Th-rich crandallite may be the result of alteration of an unidentified silicate mineral from the parent rock with a composition close to the simplified formula Fe ²⁺ ThSiO 4 (OH) 2 .