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crystal zoning

crystal zoning A texture developed in solid-solution minerals and characterized optically by changes in the colour or extinction angle of the mineral from the core to the rim. This optical zoning is a reflection of chemical zoning in the mineral. For example, a plagioclase can be zoned from a Ca-rich core to an Na-rich rim. Zoning results from the mineral's inability to maintain chemical equilibrium with a magma during rapid cooling; the zonation represents a frozen picture of the continuous reaction series for that mineral. Zoning can be of three types, the first two applying mostly to plagioclase feldspars. (a) Normal zoning is where the mineral is zoned from a high-temperature core composition to a low-temperature rim composition. (b) Reverse zoning is where a mineral is zoned from a low-temperature core composition to a high-temperature rim composition. (c) Oscillatory zoning is where the mineral chemistry continuously oscillates between high- and low-temperature compositions going from the core to the rim. Compare CORONA.

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