Etymology
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plagioclase (n.)

"triclinic feldspar," 1868, coined in German 1847 by German mineralogist Johann Friedrich August Breithaupt (1791-1873) from plagio- "slanting" + Greek klasis "a fracture," from stem of klan "to break" (see clastic). So called because the two prominent cleavage directions are oblique to each other. Related: Plagioclastic.

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plagio- 

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."

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*plak- (1)

also *plāk-, Proto-Indo-European root meaning "to be flat;" extension of root *pele- (2) "flat; to spread."

It forms all or part of: flag (n.2) "flat stone for paving;" flagstone; flake (n.) "thin flat piece,; flaw; floe; fluke (n.3) "flatfish;" placenta; plagal; plagiarism; plagio-; planchet; plank.

It is the hypothetical source of/evidence for its existence is provided by: Greek plakoeis "flat," plax "level surface, anything flat;" Lettish plakt "to become flat;" Old Norse flaga "layer of earth," Norwegian flag "open sea," Old English floh "piece of stone, fragment," Old High German fluoh "cliff."

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