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The crystal structures of natural Nb-rich chevkinite-(Ce) from the Biraya rare-metal deposit, Russia, crystallizing in space groups  C 2/ m  and  P 2 1 / a , were solved and refined to  R  1  = 0.03 and  R  1 = 0.07, respectively, from data collected with a single-crystal diffractometer. X-ray photoelectron spectroscopy was used to determine the oxidation states of the following ions: Ce 3+ , Fe 2+ , Fe 3+ , Ti 4+ , and Ti 3+ . Typically, Nb-rich chevkinite-(Ce) has space group  C 2/ m , but specimens with space group  P 2 1 / a  were also identified. While XPS shows that both  C 2/ m  and  P 2 1 / a  crystal structures contain Ti 4+  and Ti 3+ , it also indicates that Ti 2+  may occur in the  P 2 1 / a  phases. The general formula for the Nb-rich chevkinite-(Ce) having space group  C 2/ m  is (Ce 3+ ,La 3+ ,Nd 3+ ,Pr 3+ ,Ca 2+ ,Na + ) ∑4 Fe 2+ (Fe 3+ ,Nb 5+ ,Al 3+ ) ∑2  (Ti 4+ ,Ti 3+ ,Nb 5+ ,Mg 2+ ) ∑2 (Si 2 O 7 ) 2 O 8  and with  P 2 1 / a  is (Ce 3+ ,La 3+ ,Nd 3+ ,Pr 3+ ,Ca 2+ ,Na + ) ∑4 (Fe 2+ ,Mg 2+ ,Ti 2+ ) ∑1  (Fe 3+ ,Nb 5+ ,Al 3+ ) ∑2 (Ti 4+ ,Ti 3+ ,Nb 5+ ) ∑2 (Si 2 O 7 ) 2 O 8 .

american mineralogist

Cation ordering, valence states, and symmetry breaking in the crystal-chemically complex mineral chevkinite-(Ce): X-ray diffraction and photoelectron spectroscopy studies and mechanisms of Nb enrichment