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

Russian Research Geological Institute (VSEGEI), Sredny Pr. 74, St Petersburg 199106, Russia

^* E-mail: leonid_mordberg{at}vsegi.ru

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.

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 ThFe₃(PO₄,SiO₄)₂(OH)₆ with Th and Si substituting for REE and P respectively (woodhouseite-type substitution). Another possible substitution is Th⁴⁺ + Ca²⁺ 2REE³⁺ (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 Fe³⁺) 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 Fe²⁺ThSiO₄(OH)₂.

KEYWORDS: bauxite, weathering, crandallite, woodhouseite, goyazite, svanbergite, florencite, thorium, Devonian, Timan, Russia

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