11. Riebeckite-granites have close affinities to aplites, shown especially in the prevalence of alkali feldspars . 12. Crystal twinning is common in feldspars, especially polysynthetic twins in plagioclase and Carlsbad twins in alkali feldspars . 13. As shown in the QAPF diagram, alkali feldspar granite contains between 20 %-60 % quartz. 14. Granite is classified according to the QAPF diagram for coarse grained petrologic convention contains both plagioclase and alkali feldspars . 15. In the case of the alkali feldspar minerals, thin white albite layers will alternate between typically pink microcline. 16. The alkali feldspar may have unusual compositions unlike any terrestrial feldspar, and they are often Ba-rich. 17. When a granitoid is devoid or nearly devoid of plagioclase, the rock is referred to as alkali feldspar granite. 18. "' Nepheline syenite "'is a holocrystalline plutonic rock that consists largely of nepheline and alkali feldspar . 19. They consist of quartz, plagioclase, orthoclase or alkali feldspar , rare mafics ( pyroxene ), and rare zircon. 20. Perthite is an intergrowth of alkali feldspar of intermediate composition : the coarseness of perthitic intergrowths is related to cooling rate.
