noun
orthoclase; common feldspar
A common rock-forming mineral in the feldspar group; used in geology and mineralogy.
正長石は花崗岩の主成分の一つです。
Orthoclase is one of the main components of granite.
長石 is the general term for feldspar; 正長石 specifically refers to orthoclase, a potassium-rich variety.
From 正 (ortho-) + 長石 (feldspar), a calque of the English mineral name 'orthoclase'.