| @AeroE, there technically 4 feldspars; plagioclase, orthoclase, sanidine & microcline. The last three are polymorphs, i.e. they have the same chemical composition, but are formed at different pressures & temperatures. Their presence in ingenous rocks is useful in narrowing the range of temperature & pressure (a proxy for depth)... as you can guess, I am an igneous petrologist/high temperature geochemist |