an·or·tho·clase 
(ăn-ôrthə-klās′)
(ăn-ôrthə-klās′)n.
A white to colorless triclinic mineral of the alkali feldspar group that contains a higher proportion of sodium than potassium and is a common constituent in the matrices of slightly alkaline volcanic rocks.
[Greek an-, not; see A-1 + orthos, straight + klasis, a breaking (from its having two planes of cleavage intersecting at a slightly oblique angle); see ORTHOCLASE.]