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Potassium feldspar comes in three different crystal structures that depend on the temperature it formed at. Orthoclase and sanidine are stable above 500° C and 900° C, respectively.Outside the geological community, only dedicated mineral collectors can tell these apart.
(Albite's specific gravity is 2.62, anorthite's is 2.74, and the others fall in between.) The really precise way is to use thin sections to determine the optical properties along the different crystallographic axes. An iridescent play of light can result from optical interference inside some feldspars.In labradorite, it often has a dazzling blue hue called labradorescence. Bytownite and anorthite are rather rare and unlikely to be seen.An unusual igneous rock consisting of only plagioclase is called anorthosite.For rockhounds without laboratories, it's enough to be able to tell the two main types of feldspar, plagioclase (PLADGE-yo-clays) feldspar and alkali feldspar.The one thing about plagioclase that's usually different is that its broken faces—its cleavage planes—almost always have fine parallel lines across them. Each plagioclase grain, in reality, is typically a stack of thin crystals, each with its molecules arranged in opposite directions.
Feldspars are hard minerals, all of them with a hardness of 6 on the Mohs scale.