Mineralogy and chemistry of a pegmatitic feldspar from Hafafit, Egypt

This pegmatitic feldspar is a perthite composed of microcline and low albite roughly in the ratio of 2:1. The microcline is of high triclinicity (0.84). Chemically this alkali feldspar contains 10.58% K2 0, 3.37% Na2 0 and 0.25% CaO. These are equivalent to 62,6% microcline, 28,5% albite and 1.2% anorthite. The plagioclase intergrown with microcline is albite with 4% An. The feldspar in question shows enrichment in Rb and Y with depletion in Sr, Ba and Zr, these characterize strongly fractionated rocks. The enclosing pegmatite is shown to be most probably one of the latest residues of a fractionally crystallizing magma directly formed at low temperature range. The present pegmatite is to be contrasted, on mineralogical and geochemical grounds, with the garnetiferous pegmatite of W. Gemal area. Thus two distinctive types of pegmatites are recognized in the south Eastern Desert of Egypt