| Abstrakt: |
REE fractionation into cerium Ceg (La–Eu) and yttrium Yg (Gd–Lu) groups, as well as Lasg (La–Pr), Smsg (Nd–Eu), Gdsg (Gd–Dy), and Ybsg (Y, Ho–Lu) subgroups in xenotime and florencite crystals from the ore occurrence, Polar Urals, is considered. REE subgroups and other minor elements of the minerals form isomorphic complexes, among which are LaK [(La,Се,Pr)Al3(P,As,Si)2O8(OH)6] and SmK [(Nd,Sm,Eu,Gd,Sr,Ca)Al3(P,S,Si)2O8(OH)6] in florencite, and GdK [(Gd,Sm,Eu,Tb,Dy)(Р,Si)O4] and YbK [(Y,Nd,Ho,Er,Yb,Lu,Sc,U)(Р,Аs,Si)О4] in xenotime. The paper discusses the distribution of these complexes in zones of crystal growth and in the volume of crystals. The chemical compositions of simple forms of crystals are compared—namely, bipyramids and prisms of xenotime, as well as several rhombohedra of florencite. Mineral inclusions with the same compositions as xenotime and florencite hosting them are characterized and the reasons for their appearance are considered. Based on the sequence of the formation of minerals, evidence is provided for regular trends in lanthanide composition of phosphates upon their crystallization (increasing role off light lanthanides relative to heavy lanthanides). The mechanisms of crystal growth are investigated, including relative linear and volume growth rates of crystal faces, the formation of co-growth surfaces of different faces and individuals, and the nature of oscillatory and trend zoning. The existence of the tetrad effect in the lanthanide compositions of two minerals is verified. [ABSTRACT FROM AUTHOR] |
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