The blue colour then arises from a ion charge transfer effect, in which Fe++ and Ti++++ become Fe+++ and Ti+++. Above about 2% atomic substitution the red acquires a grey overcast and above 20% becomes brown/black.įor corundum to become blue, requires the atomic substitution of Al by Fe and Ti, both at proportional levels of less then 0.1%. The colour is caused by a ligand field effect. Below 1% Cr, the red fades to pink and eventually to colourless. the crystalline state contains a mix of Al2O3 and Cr2O3. The colour then to be observed varies according to quantity of impurity present and/or the mix of impurities - in combination with one ot more physical interactions at the atomic or molecular level.įor corundum to turn red, requires somewhere in the range of 1-3% of the Al atoms to have become substituted by Cr whilst the corundum was in the moltenstate. The mineral specie corundum, Al2O3, is colourless (allochromatic) in its pure state, If it has colour, the colour is due to the presence of some impurity. Currier Digital LibraryOpen discussion area. Techniques for CollectorsOpen discussion area. Minerals and MuseumsOpen discussion area. Mineralogical ClassificationOpen discussion area. Lost and Stolen SpecimensOpen discussion area. ╳Discussions □ Home □ Search □ Latest Groups EducationOpen discussion area.
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