Abstract

Alkali-feldspar occurs in the Crowsnest Volcanics [Cretaceous] as phenocrysts and as free euhedra in the pyroclastics and their reworked derivatives. Composition varies from 60% to 95% Or-molecule a 2V from 50 degrees [perpendicular(010)] to 35 degrees [Parallel(010)]. Individual crystals show a range of 2V values, but only rarely contain more than one K-rich phase. Optics indicate that most of the K-feldspar is intermediate between high and low sanidine. A little is intermediate between low sanidine and orthoclase. The more Na-rich K-feldspars contain flecks to large areas of pink cloudy albite with chess-board twinning whose optical and X-ray properties indicate a structural state intermediate between high and low albite. This is thought to be a replacement which has inherited the Al-distribution of the K-feldspar, thus producing the intermediate optics. As both original and replaced material coexist in many instances, this material may be suitable for testing the ionic theory of feldspar formation proposed by Ferguson and co-workers.

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