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

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 mu m to 60-70 mu m. Rare grains disseminated among boehmite crystals were also found. Microprobe analyses determined that the ThO (sub 2) content can be as high as 18 wt.%. The mineral composition is intermediate between crandallite CaAl (sub 3) H(PO (sub 4) ) (sub 2) (OH) (sub 6) , goyazite SrAl (sub 3) H(PO (sub 4) ) (sub 2) (OH) (sub 6) , Th-crandallite and svanbergite SrAl (sub 3) PO (sub 4) SO (sub 4) (OH) (sub 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 (sub 3) (PO (sub 4) ,SiO (sub 4) ) (sub 2) (OH) (sub 6) with Th and Si substituting for REE and P respectively (woodhouseite-type substitution). Another possible substitution is Th (super 4+) +Ca (super 2+) <-->2REE (super 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 (super 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 (super 2+) ThSiO (sub 4) (OH) (sub 2) .