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 particularly sui table for
detailed studies, as they have very simple chemistry and develop some of
the most complex microstructures known. The microstructures are often
slow to form but are easily preserved, so that they are potentially
extremely informative about the geological history of the rocks in which
they occur. Furthermore, their study involves physical and chemical
methods of increasing sophistication so that the results obtained are
not always immediately understandable to research workers outside the
field of modern mineralogy. Progress in knowledge about feldspars is
probably slower in penetrating the fields of petrology and geochemistry
than that on other mineral groups. For these reasons among others, i was
particularly appropriate to hold a third NATO ASI on feldspars
approximately ten years after the last one.