Nature of oscillation zoning and mechanism of crystal growth in a paragenic intergrowth of florencite and xenotime

Repina S. A., Muftakhov V. A.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2020. V. 149. N 5. P. 29-58

https://doi.org/10.31857/S0869605520050068

Language: Russian

Abstract

Features of rare-earth element fractionation into cerium Ce_g (La–Eu) and yttrium Y_g (Gd–Lu) groups as well as La_sg (La–Pr), Sm_sg (Nd–Eu), Gd_sg (Gd–Dy), and Yb_sg (Y, Ho–Lu) subgroups in xenotime and florencite crystals from the Au-REE occurrence, Maldynyrd Range, Near-Polar Urals are considered. Rare-earth element subgroups and other minor elements of the minerals form isomorphic complexes, among which are La_к [(La,Се,Pr)Al₃(P,As,Si)₂O₈(OH)₆] and Sm_к [(Nd,Sm,Eu,Gd,Sr,Ca)Al₃(P,S,Si)₂O₈(OH)₆] in florencite, Gd_к [(Gd,Sm,Eu,Tb,Dy)(Р,Si)O₄] and Yb_к [(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. Chemical compositions of simple forms of crystals are compared, namely bipyramids and prisms of xenotime, and several rhombohedra of florencite. Mineral inclusions with the same compositions as xenotime and florencite enclosing them are outlined and reasons of their appearance are analyzed. Based on the determination of the sequence of formation of minerals, evidences are provided for regular trends in lanthanide composition of phosphates during their crystallization (increasing a part of 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 lanthanide compositions of two minerals is verified.

Keywords: xenotime, florensite, oscillatory zoning, fractionation of REE, tetrad-effect, relative face growth rates, Near-Polar Urals