Abstract

Universal stage measurements of 105 plagioclase crystals from seven tuffs in the western United States show that twin laws with the (010) composition plane constitute 94.3 percent. The composition range of the plagioclase is from An10 to An37 with 54 percent in the An24-28 interval. Most of the plagioclase has high-optics: a few crystals display transitional optics.

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