microperthite
Quick Summary
Microperthite is a type of alkali feldspar characterized by microscopic intergrowths of sodium-rich feldspar (albite) and potassium-rich feldspar (orthoclase or microcline). This separation occurs when a molten mixture of sodium and potassium feldspar cools slowly, leading to instabilities that cause the two phases to separate into distinct, tiny crystals. While perthite can sometimes be seen with the naked eye, microperthite's structure is only discernible under a microscope. This contrasts with cryptoperthite, where the crystals are so small they require X-ray diffraction to detect. The name "perthite" originates from a locality near Perth, Ontario, where it was initially believed to be a single mineral.12
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