Last week we discussed Color Center Crystal's and what gives them their color. This week we will discuss Charge Transfer Minerals. They have more intense, brighter color that is often more stable than The Color Center Crystals. Wulfenite is a good example of this. Not all Wulfenite is brightly colored, in fact some is colorless, while some is an intense red-orange color. these red-orange Wulfenite have been proven to be Charge Transfer Minerals.
The most famous Wulfenites from Red Cloud mine in Arizona have an incredible intense red color. Research has shown that chromium, valence 6, is transfer charged electrons back and forth with oxygen, valence 2, to cause the color. The bright red-orange color of Wulfenite found in the San Francisco mine Sonora, Mexico, have not been tested but it would be a reasonable assumption that they contain the same two stunning minerals.
One interesting form of quartz is rose quartz. It is a Charge Transfer Mineral, but it's color can hardly be considered rich. One study has shown that the pink of rose quartz can be due to pink clay, probably montmorillonite clay, diffused in the material.
One of the gems that can take on a panoply of colors is sapphire. One reason sapphires are highly regarded is their intense color, which is often due to some sort of transfer action. Sapphire has the same chemical makeup as ruby; both these corundum gems starts out as a colorless aluminum oxide. Ruby's color is caused by the transition metal element chromium, valence 3. Sapphire on the other hand is a Charge Transfer Mineral that can play host to any one of several trace elements.
Rich violet sapphire may have the most complicated cause of color. This Charge Transfer Mineral requires traces of iron, valence 3, and titanium, valence 4, to shift electrons to divalent oxygen, but this action can only take place when chromium, valence 3, is present.
Blue sapphire is very much like violet sapphire, as the iron-oxygen-titanium transfer takes place to form the color; but in this case iron, valence 3 and iron, valence 2, are participating.
Yellow Sapphire is a bit simpler, as oxygen, valence 2, shifts electrons with iron, valence 3. Keep in mind that all this electron transfer business going on requires energy.The light that enters a sapphire crystal supplies that energy by giving up a part of its color spectrum, depending on which trace elements are present and functioning!
The most desired of sapphires is the orange-pink variety called padparadscha a Sinhalese word for "lotus blossom". Unlike all the other sapphires, this orange-pink gem only needs chromium, valence 3, to function as a color center. It can also gain color when chromium, valence 3, works with magnesium, valence 2, to replace two aluminum, valence 3, atoms. This one is not Charge Transfer, but simply substitution, and the color is stable.
