Zonation
Variations in optical properties across a crystal correlate with
changes in mineral chemistry, which in turn reflect changes in magma composition
as it evolves. If a mineral becomes unstable, say to an influx of new
magma, it may contain resorption features, such as those shown in the two
photomicrographs below:
Alternating zones in a crystal suggest rhythmic changes in composition,
such as a crystal moving between different regions of a magma chamber:
Zoned plagioclase crystals from Sample 84, Hornblende Andesite.
