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Vol. 5 (2002 year), No. 1

Voloshin A.V., Pakhomovsky Ya.A. and Sorokhtina N.V.
Composition of the gadolinite group minerals from the amazonitic randpegmatites of the Kola Peninsula

The minerals of gadolinite group are characteristic and widespread minerals within the amazonitic randpegmatites and hydrothermalites (metasomatites) of the alkali-granite formation. The group consists of three mineral species: gadolinite-(Y), hingganite-(Y) and hingganite-(Yb). The gadolinite is abundant in undifferentiated amazonitic randpegmatites and metasomatites and both hingganites are peculiar to differentiated pegmatites. Yttrium is a predominant element within the structure of Y-minerals of the gadolinite group. According to the REE composition the gadolinite group minerals are divided on two lines: 1) gadolinite-(Y) and 2) hingganite-(Y) and hingganite-(Yb). The proportion between the "light" (La-Nd), "medium" (Sm-Ho) and "heavy" REE (Er-Lu) within the gadolinite structure from different parts of the randpegmatites and metasomatites in general is close to 1:1:1. Hingganites do not contain "light" REE concentrations; they are enriched in "heavy" REE (Er-Lu), and consequently are sharply distinct from gadolinite through the REE distribution. During the crystallization of mineral phases, selective crystal-chemical separation of "heavy" REE and yttrium takes place in differentiated pegmatites. As a result of fractionation, "light" (La-Nd) and "medium" (Sm-Ho) REE are concentrated within monazite and bastnesite, and "heavy" REE and Y have participated in formation of hingganite and other Y-REE minerals.

(in Russian, стр.10, fig. 10, tables. 4, ref 18, MS Word 95, MS Word 95 5780 Kb)