Der Ordnungs-Unordnungs-Übergang in getemperten Mikroklinen und Albiten

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Abstract
Heat treatment of alkali feldspars with ordered Al,Si distribution in the temperature range near the melting point produces a transformation to a state of disordered Al,Si distribution. This transformation has been studied with several natural albites and microclines and their Na, K-exchange products.The rate of the transformation varied greatly, sometimes even within a single specimen. The lattice constants of all specimens investigated changed directly towards those of the disordered state. No influence of the temperature on the way of the transformation could be observed. As the comparison of cell dimensions of Na-exchanged heat-treated microclines and those of heat-treated albites showed, the way of the transition was also the same in K and Na feldspars.According to the results of Stewart and Ribbe (1969) the observed variation of cell dimensions is due to a simultaneous migration of Al atoms from the T1(0) site to all other T sites. The same order-disorder transition has been deduced by Stewart and Ribbe for synthetic Na feldspars. Lattice constants of natural authigenic albites, taken from the literature, differ considerably from those of the heat-treated synthetic albites. Perhaps authigenic albites represent a different type of partly-ordered Al,Si distributions.