Plagioclase transect from rim to core across part of a phenocryst in AT-1610. The electron microprobe traversed along the red line in the image to the left. The composition along that line is shown in the graph to the right. The mol % anorthite essentially tracks the amount of calcium oxide in the crystal. Higher calcium concentrations reflect more water in a magma that probably also has lower silicon (like basalt or basaltic andesite). The lower calcium plagioclase usually crystallizes in magma that has more silicon (dacite and rhyolite), and also possibly less water. This crystal is compositionally zoned, with lower calcium concentrations towards the rim, and a highly calcic core. The core (~An80-85) has a coarsely sieved texture, meaning that it contains glass pockets inside. Data collected in the Advanced Instrumentation Laboratory, UAF.
Image courtesy of the AVO/UAF-GI.
Please cite the photographer and the Alaska Volcano Observatory / University of Alaska Fairbanks, Geophysical Institute when using this image.
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