or·tho·clase 
(ôrthə-klās′, -klāz′)
(ôrthə-klās′, -klāz′)n.
A white to yellowish red monoclinic mineral of the potassium feldspar group that forms from medium- to low-temperature magmas.
[Greek ortho-, ortho- + Greek klasis, a breaking (from klān, to break).]