oligoclase
noun
ol·i·go·clase ˈä-li-gō-ˌklās 
ˈō-,
-ˌklāz;
ə-ˈli-gə- : a mineral of the plagioclase series
Word History
Etymology
German Oligoklas, from olig- olig- + Greek klasis breaking, from klan to break — more at clast
First Known Use
1832, in the meaning defined above