Some typical occurrences

of the rock-forming feldspars

Potassium feldspars*

sanidine potassium-rich volcanic rocks and near-surface minor intrusions—e.g., rhyolites, trachytes, and high-temperature contact metamorphic rocks

orthoclase potassium-rich dike rocks—e.g., rhyolite and trachyte porphyries; granites, granodiorites, and syenites**; moderate- to high-grade metamorphic gneisses and schists; and sandstones

microcline granitic pegmatites, hydrothermal veins; granites, granodiorites, and syenites**; low- to moderate-grade metamorphic rocks; sandstones and conglomerates

Plagioclase feldspars

albite granites; granitic pegmatites; low-grade metamorphic gneisses and schists; sandstones

oligoclase granodiorites and monzonites; sandstones; moderate-grade metamorphic rocks

andesine diorites; andesites; moderate-grade meta-morphic rocks, especially amphibolites

labradorite gabbros and anorthosites***; diabases and basalts

bytownite gabbros and anorthosites***; diabases and basalts

anorthite gabbros; contact-metamorphosed impure limestones; and high-grade metamorphic rocks

*Including perthites. In addition, anorthoclase occurs only in a few rather abnormal syenites (e.g., larvikite), and adularia—transparent, colourless to white, commonly opalescent potassium feldspar with a pseudo rhombohedral habit—occurs in some low-temperature hydrothermal veins. **Typical syenites consist of nearly 90 percent alkali feldspar. ***Typical anorthosites consist of about 90 percent plagioclase feldspar.