-clase

Minerals showing characteristic cleavage.

Greek klasis, breaking.

Orthoclase (Greek orthos, straight) is a common rock-forming feldspar mineral that has two characteristic cleavages at right angles; plagioclase (Greek plagios, slanting) is a white form of feldspar, also common in rocks, having two cleavages at an oblique angle; periclase (Greek peri, utterly) is a colourless mineral occurring chiefly in marble and limestone, so named because it cleaves perfectly; euclase (Greek eu, well) is a brittle mineral sometimes valued as a gemstone.