Zoning
Zoning is a gradual or stepwise change in a crystal's chemistry from its core to its edge.
A crystal is zoned when its composition changes from its centre outwards. This happens because the melt or fluid around it changes as it grows. In olivine, for example, the core of a crystal may be magnesium-rich and the rim more iron-rich, because the crystal took magnesium out of the melt early on.
Zoning is a valuable record. In unequilibrated chondrites, chondrule olivines and pyroxenes are often strongly zoned, showing they cooled too quickly for their chemistry to even out and that the rock was never strongly heated afterwards. As heating increases through petrologic types 4 to 6, diffusion smooths out the zoning until all grains share nearly the same composition.
In igneous meteorites such as shergottites, zoning in pyroxene and olivine records the evolution of the magma and sometimes the mixing of different melts. Zoning is mapped with electron microprobes, which produce false-colour images of elements across a crystal.
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