[awr-thuh-kleys, -kleyz]
a common white or pink mineral of the feldspar group, KAlSi 3 O 8 , having two good cleavages at right angles, and found in silica-rich igneous rocks: used in the manufacture of porcelain.

1840–50; ortho- + -clase < Greek klásis cleavage, breaking

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World English Dictionary
orthoclase (ˈɔːθəʊˌkleɪs, -ˌkleɪz)
a white to pale yellow, red, or green mineral of the feldspar group, found in igneous, sedimentary, and metamorphic rocks. It is used in the manufacture of glass and ceramics. Composition: potassium aluminium silicate. Formula: KAlSi3O8. Crystal structure: monoclinic

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American Heritage
Science Dictionary
orthoclase   (ôr'thə-klās')  Pronunciation Key 
A white, pink, yellow, or brown type of potassium feldspar. It is dimorphous with microcline, being more stable at higher temperatures than it is. Also called: potash feldspar. Chemical formula: KAlSi3O8.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
These large crystals are minerals in the feldspar family, namely the alkali feldspars orthoclase and microcline.
The pink mineral is called orthoclase, a member of the mineral family feldspar.
There is also some variation in the amount of quartz present, and in the relative importance of the orthoclase and plagioclase.
Granite--Naturally igneous rock formed of crystallized quartz and orthoclase.
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