Here's an example of how color is controlled by outer-shell electrons. We're all familiar with olivine as a green mineral. That's because we're familiar with garden-variety olivine that is rich in Mg2+ but contains some Fe2+. However, pure forsterite, with only the hard cation Mg2+, is white to gray. Other the other hand, pure fayallite, the only the intermediate cation Fe2+, is black. Another olivine-group mineral, tephroite, has the intermediate cation Mn2+ and likewise has color, in this case the pink typical of Mn2+-bearing minerals. The point is that olivine-group minerals provide an excellent example of the generalization observed in the previous slide.

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Railsback's main page about the Earth Scientist's Periodic Table of the Elements and Their Ions
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UGA Geology Department web page

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