The bulk compositions of 23 samples of zoned plagioclase from the Donegal Granite have been determined by chemical, optical and x-ray methods. The results of the indirect methods are compared with the results of the chemical analyses. The mean differences between the x-ray and optical values and the chemical analyses are 1.6% and 2.5% An respectively. On average the x-ray and optical results are low by 0.7% and 0.6% An respectively.

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