Webster's English Dictionary

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orthoclase

or.tho.clase \'o.r-th*-.kla-s, -.kla-z\ n [G orthoklas, fr. orth- + Gk 
   klasis breaking, fr. klan]to break - more at HALT : a mineral KAlSi3O8 
   consisting of a monoclinic polymorph of c ommon potassium feldspar often 
   with sodium in place of some of the potassium