Variations in the chemical compositions and lattice dimensions of (Ba,K,Na)-feldspars from Otjosondu, Namibia and their significance

Tech. Univ., Mineral.-Petrogr. Inst., Braunschweig, Federal Republic of Germany

Three groups: (1) Cn(BaAl ₂ Si ₂ O ₈ )-poor, Ab-rich ones containing unmixed albite phases, (2) Cn-rich varieties showing occasional unmixed celsian phases and (3) those with Cn contents intermediate between those of the first two groups. The compositions of the unmixed phases in the second group are Cn ₅₄ Or ₃₄ Ab ₁₂ and Cn ₉₅ Or ₅ . Almost all contain considerable amounts of Fe (super 3+) which probably replaces Al (super 3+) . As a result, their lattice dimensions are slightly larger than those of other (Ba,K)-feldspars.--Modified journal abstract.

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