Microheterogeneity of crystal growth zones as a result of REE fractionation.

Structural features of rhythmically zoned florencite and xenotime crystals from quartz veins of the Au-REE occurrences of the Near-Polar Urals are considered. The crystals were studied in the growth zones 0.01-0.15 mm wide. Rhythms consist of small structural elements: oriented intergrowths of several mineral species, layered-block distribution of Sr and Ca, syntaxic intergrowths of minerals of the same structural type, cocrystallization caverns, and late dissolution cavities with inclusions of argentite and pitchblende. The growth zones of crystals show successive change in lanthanides: from light to heavy REEs in florencite, and, vice versa, in xenotime, which is explained by the Ce and Y activity. The mechanism of element fractionation in the crystals is of dual nature. The rare-earth intergrowths contain four lanthanide groups: (La-Ce-Pr), (Nd-Sm-Eu-Gd), (Gd-Tb-Dy), (Ho-Er-Tm-Yb-Lu). This is caused by complexing ability of the elements, which is expressed in the tetrad effect of REE fractionation. Crystals, in turn, separate isomorphic complexes according to the properties of its facets and concentrations of elements in solutions. The structures of crystals and their growth elements are considered from the crystallochemical viewpoint. [ABSTRACT FROM AUTHOR]