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 Baiyuneboite-(Ce), a mineral previously approved by the IMA CNMMN but later withdrawn because of potential similarities with cordylite, is confirmed here as being essentially identical to type cordylite.

First Page Preview

First page PDF preview
You do not currently have access to this article.