Crocoite on Goethite (#61)
|Australasia||Dundas, Tasmania||2 ½" x 2 ¼" x 1 ½"|
Crocoite, PbCrO4, owes its color to chromium (Cr), but not to chromium alone. In the Learn More sections for specimens #23 and #35 (aquamarine and azurite/malachite, respectively), charge transfer was discussed as a cause of color. Charge transfer involves the transfer of an electron from one atom to another - for example from Fe2+ to a nearby Ti4+, to yield Fe3+ and Ti3+. The energy needed to effect the transfer comes from a photon that knocks the electron from one atom to the other. This eliminates ("uses up") that particular energy from the light spectrum. Because the energy of light corresponds to the wavelength (through the equation E=hc/λ, where energy is E, λ is wavelength, and h and c are constants), the wavelength corresponding to the energy used in the electron transfer is eliminated from the spectrum. The color you see is what is left.
We often th ink of charge transfer as primarily involving just metal atoms, but it need not be so. In crocoite, the Cr atom is surrounded closely by four oxygen atoms. In this arrangement, it is possible for electrons to be moved from the oxygens to the chromium, and there are several "excited states" into which the electrons can be transferred. Thus, there is a somewhat broad range of energies that can affect the electron transitions. The range of wavelengths involved corresponds to the blue end of the spectrum. With the blue wavelengths gone, the resulting color is orange or orange-red.
The chemical formula of crocoite is identical with "chrome yellow," an artificial product used as a paint pigment.