Peprossiite-(Ce), a new mineral from Campagnano (Italy): the first anhydrous rare-earth-element borate

Peprossiite-(Ce) occurs as light yellow platy and well cleaved grains filling the open spaces between sanidines, in a «sanidinite» ejectum within the pyroclastics of the Sacrofano volcanic centre, Sabatini complex, Roman perpotassic province, at Campagnano (Latium, Italy). Associated minerals are sanidine, aegirine-augite, a hellandite-group mineral, zircon, titanite and magnetite. Peprossiite-(Ce) is optically uniaxial, n E 1.711 (5) and n O 1.703(2), non pleochroic. The empirical formula derived from ion probe analysis for B and WDS microprobe analysis is (Ce 0.40 La 0.32 Pr 0.09 Nd 0.05 Th 0.02 Ca 0.09 )Σ 0.97 Al 2.04 [B 3.00 O 8.97 (OH) 0.03 ], or ideally (Ce,La)Al 2 B 3 O 9 . D meas 3.45(5) g.cm −3 , D cale 3.47 g.cm −3 . The mineral is hexagonal, P62m, with a = 4.610(1), c = 9.358(7)A, Z = 1. The most intense lines of the powder pattern (Gandolfi camera) are [as d in A - I/I o (hkl)] 3.67 - 100 (101), 3.04 - 100 (102), 2.458 - 75 (103), 2.308 - 50 (110), 2.020 - 50 (104), 1.953 - 50 (201), 1.855 - 50 (113), 1.835 - 50 (202). The crystal structure consists of boron-oxygens tetrahedral sandwiches geometrically similar to those present in micas with REE as interlayer cation. The genesis is related to a final stage of deposition from pneumatolytic to hydrothermal fluids circulating through the trachytic mother-rock prior to its explosive ejection into pyroclastics