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Modal analyses of granitoids by quantitative X-ray diffraction



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American Mineralogist Volume 72 pages 433437 1987 Modal analysesof granitoids by quantitative X ray diffraction Plpu D Ml NrA Glnv A Cooxr Department of Geology and Planetary Science University of Pittsburgh Pittsburgh Pennsylvania 15260 U S A Ansrnlcr Modal proportions of quartz plagioslsse perthite and or orthoclaseare determined in artificial mixtures presumed to simulate granitoids and in actual granitoids by X ray diffraction This procedwe utilizes the ubiquitous presenceof quartz in all samples as an internal standard Selectedpeak ratios along with the total volume fraction of these minerals and the predetermined constants can be used to calculate the volume fraction of various minerals The percentage relative error for this technique excluding counting statistics has a standard deviation of 7o o X ray diffraction analysis is capable of routine rapid modal analysis provided the preparation and analysis of the sample is carefully controlled With minor modification this method can be adapted to other rock types INrnooucrroN quantiry volume percent major minerals in granitoids l distinguish betweenplagioclaseand Modal analysesof granitoids are important for the fol The method can 2 distinguish betweenorthoclaseand feldspar and alkali lowing reasons l for classificationand comparison with give modal proportions of quartz microcline and thereby other rocks 2 for preliminary estimatesof pressureand perthite orthoclase and plagioclasein granitoids The temperature of crystallization utilizing Q An Ab Or phase principles of the technique can be used to quantify any relations and petrography 3 for preliminary characterrock by following a similar procedure modified to ization of tectonic environment of the rocks Bowden et type of needs suit one s al 1984 The standard procedure for modal analysesis point counting of minerals in thin section Chayes 1956 Blsrc pRrNcrplEs AND cALCULATToNS The point counting method has severaldrawbacks l it The basicequationutilized is Klug



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