ol·i·go·clase 
(ŏlĭ-gō-klās′, -klāz′, ō′lĭ-)
(ŏlĭ-gō-klās′, -klāz′, ō′lĭ-)n.
A white, gray, green or red variety of plagioclase feldspar that contains a higher proportion of sodium than calcium and is common in silica-rich igneous rocks.
[OLIGO- + Greek klasis, cleavage (from klān, to break; see PLAGIOCLASE FELDSPAR).]