Oscillation zoning and mechanism of crystal growth in a paragenic intergrowth of "florencite" and "xenotime"

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Marco E. Ciriotti
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Oscillation zoning and mechanism of crystal growth in a paragenic intergrowth of "florencite" and "xenotime"

Messaggio da Marco E. Ciriotti » sab 10 apr, 2021 15:00

Referenza:
▪ Repina, S.A. & Muftakhov, V.A. (2020): Nature of oscillation zoning and mechanism of crystal growth in a paragenic intergrowth of florencite and xenotime. Zapiski RMO (Proceedings of the Russian Mineralogical Society), 149(5), 29-58.

Abstract:
Features of rare-earth element 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 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 Laк [(La,Се,Pr)Al3(P,As,Si)2O8(OH)6] and Smк [(Nd,Sm,Eu,Gd,Sr,Ca)Al3(P,S,Si)2O8(OH)6] in "florencite", Gdк [(Gd,Sm,Eu,Tb,Dy)(Р,Si)O4] and Ybк [(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.
Marco E. Ciriotti

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