Perettiite-(Y), Y3+2Mn2+4Fe2+[Si2B8O24], a new mineral from Momeik, Myanmar

The new mineral perettiite-(Y), end-member formula Y23+Mn42+Fe2+[Si2B8O24], from pegmatites in the region of Momeik, north of Mogok, Myanmar, occurs as yellow needles enclosed in phenakite crystals. The mineral was named after the mineralogist and gemologist Dr. Adolf Peretti. The empirical formula using LA-ICP-MS data (based on 24 O pfu) is Y(2.06)Ln(0.53)Zr(0.02)Th(0.01)Mn(3.24)Ca(0.38)Fe(0.71)Mg(0.07)Al(0.11)Li(0.22)Si(1.95)B(7.44)Be(0.31)O(24). Calculation of the H2O content using IR absorption data yields a maximum hydroxyl/water content equivalent to 0.1 wt.% H2O. The strongest lines in the powder pattern [d in angstrom (I-meas.) (hkl)] are 4.63 (52) (010), 4.08 (28) (301, 103), 3.74 (20) (210), 3.05 (100) (113, 311, 303), 2.64 (67) (410, 014), 2.54 (60) (313), 2.12 (23) (600, 006), 1.87 (33) (420, 024), 1.84 (52) (415, 323), 1.57 (20) (026, 620), 1.44 (25) (133, 331). The perettiite-(Y) crystals show a tetragonal X-ray diffraction pattern but the structure could only be solved as a 50/50 pseudo-merohedral orthorhombic twin with the a and c axes having the same length. The structure, refined to R1 = 0.017 (space group Pmna with a = 12.8252(5), b = 4.6187(2), c = 12.8252(5) angstrom, V = 759.71(5) angstrom(3), Z = 2), is characterized by two eight-coordinated sites: one dominated by Y and lanthanoids (Ln) and the other by Mn2+ (with additional Ca2+ and Y3+). An octahedral site is occupied by (Fe2+, Mg) with additional Li+. These cation sites form an interlayer between two tetrahedral sheets of Si2B8O24 composition. Boron shows minor replacement by beryllium. Arrangement of 4-, 5- and 8-membered rings within (010) form the borosilicate tetrahedral sheets. Perettiite-(Y) is brittle, with irregular fracture and (010) cleavage; the estimated Mohs hardness is similar to 7. Using the empirical formula, the density was estimated to be 4.533 g cm(-3). Perettiite-(Y) is biaxial, alpha = 1.82(1), gamma = 1.84(1) (589 nm).