Erythrite (#58)

Lustrous opaque purple lathes forming aesthetic near spherical clusters.
Africa Bou Azzer, Morocco 1 ½" x 1 ½" x 1"

 

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Like many minerals, the name of this one is of Greek derivation - erythros means red in Greek. Erythrite is Co3(AsO4)2 • 8H20, where Co is cobalt and As is arsenic. The color is due to Co, and ranges from crimson to peach red to pale rose to pink.

Cobalt does not always cause reddish color in minerals. You may have heard of a color called cobalt blue, caused, as you might suspect, by cobalt. So why is cobalt responsible for two completely different colors? The key is how many oxygen atoms surround the Co atom. In the Learn More for specimen #23, we considered color arising from "crystal field effects." Cobalt in erythrite is surrounded by 6 oxygen atoms. In minerals that are blue because of Co, it is surrounded by 4 oxygen atoms:

Cobalt has 7 d electrons in the 5 d orbitals, 3 orbitals with 1 electron each, and 2 with 2 (paired} electrons each. The different  arrangements of oxygen atoms repel these orbitals differently and in different amounts. Thus the photons that promote electrons to higher energies in one case are of different wavelengths than those that promote electrons in the other case. Different wavelengths are thus lost from the visible spectrum when Co is surrounded by 6 oxygen atoms than by when surrounded by 4.

 

 

Cobalt has 7 d electrons in the 5 d orbitals, 3 orbitals with 1 electron each, and 2 with 2 (paired) electrons each. The different arrangements of oxygen atoms repel these orbitals differently and in different amounts. Thus the photons that promote electrons to higher energies in one case are of different wavelengths than those that promote electrons in the other case. Different wavelengths are thus lost from the visible spectrum when Co is surrounded by 6 oxygen atoms than by when surrounded by 4.