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Annealing is commonly used in the recrystallization of metamict minerals in an attempt to reconstruct the original structure. Annealing at 750 °C of Nb-rich chevkinite-(Ce) from the Biraya rare-metal deposit, Russia, resulted in the structural transformation  C 2/ m  →  P 2 1 / a , which defines chevkinite stability in different environments. This transformation seems to be a rapid version of a naturally occurring process that possibly involves twinning of the crystals. Nb-rich chevkinite-(Ce) occurs naturally as two polymorphs, one with the  C 2/ m  space group and the other with  P 2 1 / a . The latter is the stable form under ambient conditions. There are some distinct differences in the values of the structural parameters, such as the average  M -O distances or site scattering values of particular sites for both space groups, which can be associated with the redistribution of some lighter cations, mainly Mg 2+ , within the crystal lattice. The use of complementary experimental techniques (electron probe microanalysis, X‑ray diffraction, and photoelectron spectroscopy) has delivered information on the structure and transformation of a very complex, highly zoned and partially metamict solid solution. It should be useful in determining the structure of any mineral where cation disorder is present.

american mineralogist

Cation ordering, valence states, and symmetry breaking in the crystal-chemically complex mineral chevkinite-(Ce): Recrystallization, transformation, and metamict states in chevkinite