THE MICROSTRUCTURES OF PERTHITIC ALKALI FELDSPARS REVEALED BY HYDROFLUORIC-ACID ETCHING

Etching of alkali feldspar cleavage fragments with hydrofluoric acid v apor, followed by study of the surfaces by scanning electron microscop y (SEM), is a simple and rapid technique for characterizing the microt extures of crypto- and microperthites. This technique has a number of advantages over conventional transmission electron microscopy (TEM) in cluding ease of sample preparation and the large areas of crystals whi ch can be imaged. Alkali feldspars studied by the method can yield imp ortant information on the cooling history of igneous and metamorphic r ocks, fluid-feldspar interactions and the morphology and microstructur es of albite exsolution lamellae. Some of these applications are illus trated by examples of etched crypto- and microperthites from the Klokk en layered syenite, south Greenland and the Shap granite, north-west E ngland.