The localization of the trivalent rare-earth-element ions (REE3+) in synthetic polycrystalline anorthite is investigated by optical spectroscopy, using Eu3+ as a representative local probe. This ion is found either trapped in defects such as possibly twin and cleavage planes or substituted for Ca2+ in the structural sites. But the substitution of Eu3+ for Ca2+ is probably accompanied by deformation of the aluminosilicate framework, which results in the development of numerous additional sites that form three subsets, each related to an original site. Such a distribution may explain the low partition coeffcients of REEs between plagioclases and magmatic liquids and the fact that the REE partition coefficients are independent of atomic number.

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