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زمینشناسی و معدن::
میدان پیوستگی
As one example, the Cr34 ion that gives the red colour to corundum A1203 in ruby can have only discrete energy levels, and energy is absorbed or emitted during transitions between these levels (the positions of the levels are dependent on the 'ligand field' and the transitions are controlled by 'selection rules').
There are some additional colour alterations derived from transition metal impurity valence changes, either in ligand field (cause 5) or charge transfer (cause 7) environments, and some based on organic compounds (cause 6); all of these are discussed in Section 23, together with some general comments on photochemical changes.
Transitions involving ligand field effects
If instead of Al5+ there are Fes+ ions present, as in some forms of citrine quartz, then the usual pale yellow colour of FeH in a ligand field is seen.
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