Abstract

Six cordylites from the four known localities [Narssarssuk (Greenland), Mont St. Hilaire (Canada), Bayan Obo (China), and Kola Peninsula (Russia)] were investigated by electron microprobe and X-ray single-crystal structure determination, and a seventh sample was investigated by X-ray methods only. The material studied included type cordylite. The idealized formula for cordylite is redefined as (Na (sub 1-x) Ca (sub x/2) )BaCe 2 (CO 3 ) 4 F, 0<x<1, with Ce for the sum of REE; the SrO content may reach about 5.7%. All of our structure refinements (P6 3 /mmc; a congruent to 5.10, c congruent to 23.10 Aa) agree very well among themselves and with the published structures of "baiyuneboite-(Ce)" and unnamed (Ca (sub 0.5[]0.5) ) BaCe 2 (CO 3 ) 4 F. Cordylite has a sheet structure formed of (001) layers of [Ba] [CO 3 ] [Ce,CO 3 ] [Na,F] [Ce,CO 3 ] [CO 3 ] [Ba] stacked along [001]. The interatomic distances are as expected, with the exception of unshielded Na-F distances of 2.94 Aa; unlike the carbonate groups in basnasite and synchysite, the carbonate groups in cordylite-(Ce) are parallel (001). Investigation of type cordylite showed that the formula proposed by Flink, i.e., BaCe 2 (CO 3 ) 3 F 2 , is to be modified to that given above, with x congruent to 0 for Flink's material. Baiyuneboite-(Ce), a mineral previously approved by the IMA CNMMN but later withdrawn because of potential similarities with cordylite, is confirmed here as being essentially identical to type cordylite.

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