Sr-Na-REE titanates of the crichtonite group from a fenitized megaxenolith, Khibina alkaline complex, Kola Peninsula, Russia; first occurrence and implications

An extensive, nearly continuous and hitherto unreported solid solution series is observed in Sr-Na-REE titanates in a fenitized megaxenolith from the Khibina alkaline complex. These titanate minerals, related to crichtonite, landauite and davidite-(Ce,La) may represent potentially new minerals of the crichtonite group. They show compositional zoning and are associated with various oxides, albite and alkali feldspars. EPMA results suggest that the 'large cations' (Sr, Na, K and the REE), along with minor Ca, occupy fully the A (M (sub 0) ) site in the structure; in contrast, Ca enters mainly the B (M (sub 1) ) site. The extensive Na (super +) -for-Sr (super 2+) substitution at the A site is coupled with a Ti (super 4+) -for-Fe (super 2+) substitution at the C (M (sub 3) -M (sub 4) -M (sub 5) ) site. The tetrahedral T (M (sub 2) ) site is dominated by Fe. Only the LREE are present in substantial amounts (up to 7.37 wt % REE (sub 2) O (sub 3) ). The presence of up to 1.87 wt % Cr (sub 2) O (sub 3) in these minerals indicates that rocks of mafic affinity were the protolith for the mineralized megaxenolith. The contrasting association of Cr and Nb (up to 1.06 wt % Nb (sub 2) O (sub 3) ) in these minerals undoubtedly involves derivation from two different sources (e.g. mafic Proterozoic rocks and metasomatizing oxidizing fluids of alkaline affinity).