Plagioclase-sanidine intergrowths that resemble perthite have been grown directly from ternary feldspar melts. Synthetic feldspar starting materials initially either saturated or undersaturated with water were cooled from a liquid and crystallized. The cooling histories were either continuous at some linear cooling rate or step-cooled at 50°C intervals with isothermal crystallization plateaus. The intergrowths occur as thin, parallel lamellae that resemble microperthite, or in patches that resemble patch perthite. The plagioclase in the intergrowths is An25-15 Or5-20 and the sanidine is Or75-100 An2-7. Both are often zoned normally within the stated ranges. The compositions, the presence of zoning, and the short run times indicate that the intergrowths grew directly from the melt and did not develop by exsolution. The lamellae most likely result from the breakdown of a planar plagioclase interface into a cellular interface comprising parallel projections. Not all lamellae can be explained this way, nor can the patches whose growth mechanism is not understood. Phenocrysts in volcanic rocks that show no evidence of an extended high-temperature, subsolidus history and have microperthitic or patch-perthitic intergrowths are likely candidates for natural occurrences.

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