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Potassium Feldspar (Microcline)

Gallery of Feldspars

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The polished "granite" (actually a quartz syenite) of a park bench displays large grains of the alkali feldspar mineral microcline. (more below)
Don't take this for granite
Photo (c) 2007 Andrew Alden, licensed to About.com (fair use policy)
Alkali feldspar has the general formula (K,Na)AlSi3O8, but varies in crystal structure depending on the temperature it crystallized at. Microcline is the stable form below about 400° C. Orthoclase and sanidine are stable above 500° C and 900° C, respectively. Being in a plutonic rock that cooled very slowly to yield these large mineral grains, it's safe to assume that this is microcline.

This mineral is often called potassium feldspar or K-feldspar, because by definition potassium always exceeds sodium in its formula. The formula is a blend ranging from all sodium (albite) to all potassium (microcline), but albite is also one endpoint in the plagioclase series so we classify albite as a plagioclase.

In the field, workers generally just write down "K-spar" and leave it at that until they can get to the laboratory. Alkali feldspar is generally white, buff or reddish and is not transparent, nor does it show the striations of plagioclase. A green feldspar is always microcline, the variety called amazonite.

Learn more about the geology of the feldspars

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