Order-disorder relations in natural and heated rhomb porphyry plagioclases

Several plagioclase samples from core and rim of rhomb porphyry phenocrysts of the Oslo region were heated under dry conditions at 1050° C for 1, 10, 97, and 811 hours. X-ray powder photographs show that the structurally disordered to intermediate core and rim plagioclases crystallized under similar conditions. Between the slope of the (2 ₁₃₁–2 _1¯31) vs. heating time]-curve and the potassium content in solid solution of these plagioclases exists the relation: the lower the K content, the steeper is the slope.Dr. Gorica Harnik-optrajanova, EMPA, Swiss Federal Laboratory for Testing Materials and Research, Überlandstraße 129, CH-8600 Dübendori, Switzerland.