Electron microprobe analyses of altered Th-rich britholite-(Ce) from the Saima alkali complex, North China

The Ca-rich analogue of britholite-(Ce) (fenghuanglite) from melanite-urtite of the Saima alkali complex, Liaoning Province, China, was reinvestigated by electron microprobe analyses. Two alteration patterns were observed: a spotted zoning and continuous zoning. Weight loss due to metamictisation ranges from 7 to 11 wt.%. The thorium content is unusually high ranging from 5.80 to 26.19 %. In the continuous zoning samples SiO2 and ThO2 are high, and PO4, CaO and REE2O3 are low relative to the spotted zoning samples. The content of Ca shows a negative correlation with Th. From the spotted zoning samples to the continuous zoning samples A-site vacancies and fourvalent cations increase, divalent and trivalent cations decrease, X-site vacancies increase, and F and O decrease. The main changes in britholite during the alteration processes can be described as: 2P(5+) + 2REE(3+) + 4Ca(2+) = 2Si(4+) + 4Th(4+) + 2 square 3.3 H+ + 7.2H(2)O + (A(10)Si(5.2)P(0.6)Ti(0.1)Al(0.1)O(24)F(2))(bss) = 1.9A(aq) + 0.7HF(aq) + (A(8.1)Si(5.2)P(0.6)Ti(0.1)Al(0.1)O(22.7)F(1.3). 8.5H(2)O)(bss) The species previously identified as "fenghuanglite" is proved to be a britholite solid solution series involving (REE, Ca, Th). In this series most samples are characterized by the: predominant REE atoms and can be determined as Th-rich britholite -(Ce).