NAMANSILITE, NAMN3-SOUTH-WALES, AUSTRALIA(SI2O6 PYROXENE, FROM THE HOSKINS MINE, NEW)

Namansilite, the sodium-manganese (Mn3+) pyroxene, analogous to aegiri ne, occurs as a rock-forming mineral in metamorphosed Mn-rich chemical sediments near Grenfell, New South Wales, Australia. The composition of clinopyroxene in these rocks varies in an apparent solid solution s eries from aegirine to namansilite, with less than 10 mole percent jad eite, johansennite, diopside and enstatite. Crystals reach 0.5 mm in l ength and are monocline, C2/m. A sample averaging 85 mole % namansilit e has a = 9.515(1) angstrom, b = 8.628(2) angstrom, c = 5.347(1) angst rom, and beta = 105.37(1)degree; at 80 mole % namansilite, a = 9.523(1 ) angstrom, b = 8.634(1) angstrom, c = 5.350(1) angstrom, beta = 105.4 7(1)degree. The density, measured by density gradient column of grains with numerous minute bubbles, is 3.565(1)g/cm3, calculated density = 3.595 g/cm3. The structural formula calculated for the average composi tion of the most Mn-rich pyroxene is Na1.01(Fe0.11Mn0.87)SIGMA0.98Si2. 01O6. Optical properties are alpha = 1.740(5) (589 nm); beta = gamma = 1.89(1) (620 nm). This pyroxene exhibits dispersion of the optic axes ; at wavelength 640 nm, OAP//(100), 2V(x) = 10-degrees, at 620 nm 2V(x ) = 0-degree at 600 nm, OAP//(010), 2V(x) = 10-degree. XC = 24-degrees . The mineral is markedly pleochroic, with alpha yellow, beta and gamm a ruby-red.