The crystal structures of zircon, scheelite, and monazite are very closely related.
All three have chains of alternating polyhedra and planes of closest packed or pseudoclosest
packed cations. Using these similarities the unit cells of these structures can be
placed in analogous orientations. This in turn leads to a better understanding of the
geometrical aspects of the reconstructive phase transformations that occur among the
structure types as functions of temperature and pressure. In essence the phase
transformations require the cation planes of one structure to the cation planes of another.
Phase transformations also occur via compositional pathways.
Crystal structure parameters were modeled for compounds with the Zlfcon,
scheelite and monazite structure types using multiple regression techniques. Data
consisted of structure refinements of 26 zircon-, 13 scheelite-, and 13 monazite-type
compounds. These compounds include but are not limited to the lanthanide vanadates
and phosphates, the alkali earth molybdates and tungstates, and KTc04. The structural
parameters studied included bond lengths, bond angles, polyhedral volumes, unit cell
edge lengths, tetrahedral quadratic elongations and atomic coordinates of individual
atoms; they were modeled as a function of the Shannon radii of the cations and the
product of the M and T cation charges. Correlation coefficients for these regressions
exceeded 0.9 for nearly all parameters studied except for the y coordinate of M, the z
coordinate of 0 1, and T -0 I-M2 angle of the monazite compounds.
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