plagioclase n.

before vowels plagi-, word-forming element meaning "slanting, oblique," from Greek plagios "oblique, slanting," from plagos "side," from PIE *plag- "flat, spread," variant form of root *plak- (1) "to be flat."

"consisting of broken pieces, breaking up into fragments," 1868 in reference to anatomical models, 1870 in geology, from Latinized form of Greek klastos "broken in pieces," from klan, klaein "to break," which is perhaps from PIE *kla-, variant of root *kel- "to strike" (see holt), but more likely of uncertain origin [Beekes].