| Abstrakt: |
The very rare mineral korobitsynite, first described from the Alluaiv and Karnasurt Mountains, Lovozero alkaline massif, Kola Peninsula, Russia, has been found recently at the Ariskop phonolite quarry, 25 kilometres S of Windhoek in central Namibia. The unit cell parameters of the Aris korobitsynite, obtained on a Bruker AXT four-circle-diffractometer, are a = 7.34(1), b = 14.12(1) and c = 7.11(3)Å and are in good agreement with the data given for the Kola material. The empirical formula of the korobitsynite from Aris, calculated from the microprobe analysis, assuming (Si,Al) = 4 and (O,OH) = 2, is: (Na2.32K0.01Ca0.01)S2.34(Ti1.21Nb0.75Zr0.01Ta0.01)S1.98(Si3.99Al0.01)4O12(O1.02OH0.98)2·3.75H2O. The korobitsynite from the Aris phonolites forms colourless and transparent, prismatic crystals up to 1mm in length, as individuals or in sprays scattered on a matrix of platy albite and analcime crusts. The crystals show {110} moderately dominating the prismatic habit, with minor {010} and {001} in addition. The korobitsynite from Aris is biaxial positive with 2Vmeas. = 28(0.5)°, na = 1.6468(5), nß = 1.6528(5) and n? = 1.7593(5), and has a = b, ß = c, ? = a. The optical properties of the Aris korobitsynite given are in excellent agreement with those of korobitsynite from the type locality. |
