Asisite, a silicon-bearing lead oxychloride from the Kombat Mine, South West Africa (Namibia)

Abstract

Asisite, ideally Pb7Si08CI2, is a new species from the Kombat mine in South West Africa (Namibia). Its unit cell is tetragonal, 14/mmm, with a = 3.897(2), c = 22.81(2) A, and Z = 1. It occurs as transparent yellow to yellow-green platy crystals having an adamantine luster, a perfect {001} cleavage, and a calculated density of 8.041 glcm3. Optically, asisite is uniaxial negative with Wealc = 2.39 and E~ale = 2.32 at 590 nm and r < v for the refractive indices and absorption. In reflected light, it possesses characteristic greenish-yellow internal reflections.Luminancevaluesin air (C luminant)are 17.1 % (0) and 16.2%(E') and in oil (ND= 1.515) 5.27% (0) and 4.63% (E'). The strongest powder-diffraction lines are [dobs(A),Iobs,hkl]2.957(100) (015), 2.752(60) (110), 1.627(60) (0.0.14,125), 1.602(50) (208,0.1.13) 3.46(40) (013), and 1.757(40) (1.1.10).The crystal structure, which has been refined to an unweighted residual of 0.075 for all 147 reflections, consists of layers of the tetragonal (red) PbO structure alternating with single sheets of CI- ions, all arranged in layers normal to c. The Si could not be located experimentally, and its structural role is uncertain. Asisite is a naturally occurring member of a large family of nonstoichiometric Ph-transition element and post-transition element oxyhalides.