Feldspars are the dominant rock-forming minerals on Earth, appearing in nearly every igneous and metamorphic rock a Rockie will encounter. Pink, white, grey, or cream, they break along two cleavage planes at roughly 90 degrees — that right-angle break is your field key. Weathered surfaces go chalky; fresh surfaces are glassy. Learn to spot feldspar and you automatically understand the rock holding everything else together.
Feldspar Identification Sheet
The mineral that makes up 60% of the Earth's crust — and still trips up new collectors.
Streak
White
CleavageTwo directions at approximately 90°; flat, mirror-like cleavage faces are the key field feature
Crystal habit
Prismatic
Blocky prismatic crystals; perthitic textures visible in hand specimen in granites
Common lookalikes
Quartz (harder at 7, no cleavage, glassy throughout); calcite (softer, rhombohedral cleavage, acid reaction). When in doubt: if it has two flat shiny faces that meet near 90°, it's feldspar.