orthoclase
noun
or·tho·clase ˈȯr-thə-ˌklās 
-ˌklāz
: a monoclinic mineral of the feldspar group consisting of a silicate of potassium and aluminum
Word History
Etymology
German Orthoklas, from orth- + Greek klasis breaking, from klan to break — more at clast
First Known Use
1849, in the meaning defined above