The feldspars form the most abundant group of minerals in the crust of the Earth and Moon and also occur in many meteo rites. They playa fundamental role in all rock-forming processes at shallow depths, but are rare or absent from the upper mantle. Their detailed study is thus essential for the understan ding of such varied processes as magma genesis and differentia tion, metamorphism, al teration, erosion and sedimentation. This interest is show by the fact that two previous NATO Advanced Study Institutes on feldspars were held in Oslo in 1962 and in Manchester in 1972. The feldspars are...