Magmatic rare earth element mineralization and secondary fluid-assisted redistribution in the Kamthai carbonatite complex, NW India

An Apatite-Group Praseodymium Carbonate Fluoroxybritholite: Hydrothermal Synthesis, Crystal Structure, and Implications for Natural and Synthetic Britholites

“…Silica-deficient and low-totals britholites observed in some localities are potential hosts for undetected carbonates. 124 − 126 In some cases, apatite–britholite analyses plot consistently below the 1:1 line on a Ca+P–REE+Si plot, 111 , 127 − 130 which could be easily explained by the presence of carbonate substituting for silicate. We expect this to occur mostly in carbonatite-associated fluorapatite.…”

“…Although this mismatch can be often attributed to analytical uncertainties, our geologically reasonable conditions employed in the synthesis described above indicate that an oxybritholite component may contribute to the mismatch. Silica-deficient and low-totals britholites observed in some localities are potential hosts for undetected carbonates. − In some cases, apatite–britholite analyses plot consistently below the 1:1 line on a Ca+P–REE+Si plot, ,− which could be easily explained by the presence of carbonate substituting for silicate. We expect this to occur mostly in carbonatite-associated fluorapatite. , …”

“…The apatite–britholite substitution requires that endmember britholite should contain two Ca apfu and three Ln apfu, limited by three phosphate groups available for substitution by orthosilicate groups. However, there are reports of nonstoichiometric britholite in natural rocks containing REE/Ca > 3/2, where “Ca” includes other divalent cations that commonly substitute on the Ca-dominated M -site such as Sr or Mn, ,,,,− with the nonstoichiometry occasionally exacerbated by presence of phosphate (i.e., nonendmember britholite) . Characterization of these natural britholites is challenging because they often contain a mix of all 14 lanthanides and other monovalent or divalent cations (e.g., Na + , Mn 2+ , Sr 2+ ), and they may be metamict due to the presence of quadrivalent Th. ,,, Additionally, some orthosilicate may be substituted by phosphate or other oxyanions (carbonate, borate, arsenate, or vanadate) .…”