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coupled substitution The substitution of two or more elements in a crystalline compound, which maintains electrical neutrality. In forming a solid solution series, ionic size is more important than ionic charge, as this can be compensated for elsewhere. For instance, when a plagioclase feldspar solid solution series forms, in passing from albite (NaAlSi3O8) to anorthite (CaAl2Si2O8), Al3+ replaces Si4+. This leaves a negative charge that is balanced by the (coupled) substitution of Ca2+ for Na+.