Peprossiite-(Ce), a new mineral from Campagnano (Italy): the first anhydrous rare-earth-element borate
Ventura, Giancarlo Della; Parodi, Gian Carlo; Mottana, Annibale; Chaussidon, Marc
published: Mar 4, 1993
manuscript accepted: Apr 23, 1992
manuscript received: Oct 9, 1990
ArtNo. ESP147050501007, Price: 29.00 €
Abstract 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. Peprossπte-(Ce) is optically uniaxial, nE 1.711 (5) and nO 1.703(2), non pleochroic. The empirical formula derived from ion probe analysis for B and WDS microprobe analysis is (Ce0.40La0.32Pr0.09Nd0.05Th0.02Ca0.09)Σ0.97Al2.04[B3.00O8.97(OH)0.03], or ideally (Ce,La)Al2B3O9. Dmeas 3.45(5) g·cm-3, Dcaic 3.47 g·cm-3. The mineral is hexagonal, P̄6̄2m, with α = 4.610(1), c = 9.358(7)Å, Z = 1. The most intense lines of the powder pattern (Gandolfi camera) are [as d in Å - I/Io (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.