<Tr> <Th> Diagnostic features </Th> <Td> Distinguishable from microcline by a lack in gridiron twinning . Distinguishable from sanidine by a larger 2V . </Td> </Tr> <Tr> <Th> Other characteristics </Th> <Td> Low negative relief; Alters to sericite or clay (commonly) </Td> </Tr> <Tr> <Th> References </Th> <Td> </Td> </Tr> <P> Orthoclase, or orthoclase feldspar (endmember formula K Al Si O), is an important tectosilicate mineral which forms igneous rock . The name is from the Ancient Greek for "straight fracture," because its two cleavage planes are at right angles to each other . It is a type of potassium feldspar, also known as K - feldspar . The gem known as moonstone (see below) is largely composed of orthoclase . </P>

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