Computer-refined unit-cell parameters and optical angle measurement of several series of alkali feldspars of intermediate structural state (prepared by alkali-exchange techniques), together with similar data obtained by Orville for the limiting series high sanidine-high albite and maximum microcline-low albite, now provide a comprehensive method to study alkali feldspars from all geologic environments. All samples of equivalent structural state prepared by alkali exchange from the same starting material („alkali-exchange series”) show a continuous variation in unit-cell parameters and optic axial angle with composition, but slightly different alkali-exchange series are obtained for samples of intermediate structural states when the starting material is K-feldspar than when the starting material is Na-feldspar. The evidence from the physical properties measured indicates that, although it is theoretically permissible to have an infinite number of intermediate structural states between the limiting series, in nature certain structural states have a relatively high frequency of occurrence while certain others have not yet been observed. Nonetheless, previous classifications based only on small numbers of feldspar series should be modified.

A revised terminology and format, based on cell parameters, for description of alkali feldspar is given for all natural alkali feldspars. Determinative curves of b vs c (and contoured for a) permit both the estimation of the structural state and the detection of anomalous unit cells, that is, if a(observed)a(estimated from b and c) >0.02Å. Composition may be determined directly from a if a is not crystallographically „anomalous,” or from unitcell volume if a is „anomalous.” An appendix describes in detail the measurement and indexing of the X-ray powder diffraction patterns of an alkali feldspar preparatory to computer refinement of unit-cell parameters.

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