CORDYLITE-(CE) - A CRYSTAL-CHEMICAL INVESTIGATION OF MATERIAL FROM 4 LOCALITIES, INCLUDING TYPE MATERIAL

Six cordylites from the four known localities [Narssarssuk (Greenland) , Mont St. Hilaire (Canada), Bayan Obo (China), and Kola Peninsula (Ru ssia)] were investigated by electron microprobe and X-ray single-cryst al structure determination, and a seventh sample was investigated by X -ray methods only. The material studied included type cordylite. The i dealized formula for cordylite is redefined as (Na1-xCax/2)BaCe2(CO3)( 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 t o 5.10, c congruent to 23.10 Angstrom) agree very well among themselve s and with the published structures of ''baiyuneboite-(Ce)'' and unnam ed (Ca(0.5)square(0.5))BaCe2(CO3)(4)F. Cordylite has a sheet structure formed of (001) layers of [Ba] [CO3] [Ce,CO3] [Na,F] [Ce,CO3] [CO3] [ BA] stacked along [001]. The interatomic distances are as expected, wi th the exception of unshielded Na-F distances of 2.94 Angstrom; unlike the carbonate groups in basnasite and synchysite, the carbonate group s in cordylite-(Ce) are parallel (001). Investigation of type cordylit e showed that the formula proposed by Flink, i.e., BaCe3(CO3)(3)F-2, i s to be modified to that given above, with x congruent to 0 for Flink' s material. Baiyuneboite-(Ce), a mineral preciously approved by the IM A CNMMN but later withdrawn because of potential similarities with cor dylite, is confirmed here as being essentially identical to type cordy lite.