Compositional variations and alteration of gadolinite-group minerals from the Heftetjern granitic pegmatite, Southern Norwayтезисы доклада Тезисы

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[1] Ermolaeva V. N., Varlamov D. A., Chukanov N. V. Compositional variations and alteration of gadolinite-group minerals from the heftetjern granitic pegmatite, southern norway // Proceedings of the XXXV international conference "Magmatism of the Earth and related strategic metal deposits. — Институт геохимии и аналитической химии РАН Moscow, 2018. — P. 84–86. Recently a new member of the gadolinite group, the OH-dominant analogue of gadolinite-(Y) with the idealized formula (Y,Ca)2(Fe,)Be2Si2O8(OH,O)2 (“hydroxylgadolinite-(Y)”) has been discovered in this association (Chukanov et al., 2017). Crystals of gadolinite-group minerals studied in this work occur in close association with albite in the Heftetjern granitic pegmatite situated in Telemark, Southern Norway. Two samples have been investigated. One of them (Sample 1) is rather uniform in composition which corresponds to typical gadolinite-(Y), with the exception of a thin peripheral zone and a thin zone along a crack which are composed of Fe-deficient gadolinite-(Y) and hingganite-(Y). Irregular shape of the borders between gadolinite-(Y) and hingganite-(Y) (Fig. 2) indicates that the zonality does not have a growth origin, but is due to partial substitution of gadolinite-(Y) with secondary phases. The typical empirical formulae of these minerals in Sample 1 are: (Y1.47Dy0.11Nd0.07Er0.06Ce0.06Yb0.06Ho0.05Tm0.04Sm0.03Gd0.03Pr0.02Eu0.02Tb0.01Ca0.01Lu0.01)2.05Fe0.84Be2Si2.00O8 [O1.83(OH)0.17]2 (gadolinite-(Y)) and (Y1.03Ca0.70Dy0.08Gd0.05Er0.05Sm0.03Yb0.03Tb0.03Nd0.02Ho0.02La0.01Tm0.01)2.07Fe0.40Be2Si2.00O8[(OH)1.72O0.28]2 (hingganite-(Y)).Another one (Sample 2) presumably belongs to a later association and shows distinct signs of profound late alterations. Its inner zone principally consists of REE,Ca-deficient “hydroxylgadolinite-(Y)” with the typical composition (Y0.94Ce0.14Nd0.13Dy0.10Yb0.09La0.05Sm0.04Er0.04Ca0.03Pr0.03Ho0.03Lu0.03Gd0.02Tb0.02Eu0.01Th0.01)1.71 (Fe0.63Al0.13)0.76Be2(Si1.96P0.04)2.00O8[(OH)1.12O0.82]2, and contains subordinate areas of gadolinite-(Y) and hingganite-(Y). The typical empirical formulae of these minerals in Sample 2 are: (Y1.27Ce0.13Yb0.08Ca0.07Nd0.05Er0.05Dy0.05La0.03Sm0.03Ho0.03Tb0.02Pr0.02Gd0.01Eu0.01Tm0.01Lu0.01)1.87(Fe0.43Al0.21)0.64Be2(Si1.96P0.04)2.00O8[O1.07(OH)0.93]2 (Al-bearing gadolinite-(Y)) and (Y1.13Ce0.17Nd0.15Dy0.09Sm0.06Ca0.06Pr0.05Gd0.05Yb0.05Er0.03La0.02Ho0.02Tb0.02)1.84Fe0.43Be2Si2O8[(OH)1.5O0.5]2 (hingganite-(Y)). As a conclusion, four stages of evolution of Y-bearing minerals can be distinguished: a) crystallization of gadolinite-(Y), b) formation of intermediate gadolinite-(Y) – hingganite-(Y) members and “hydroxylgadolinite-(Y)”, c) substitution of the earlier phases by Ca,REE-deficient hingganite-(Y) and d) crystallization of late hydrous Y-bearing silicates in interstitial space and cavities. The assumption on a late-stage origin of “hydroxylgadolinite-(Y)” is in a good agreement with its non-metamict state, which is confirmed by its perfect crystal structure (Chukanov et al., 2017) and perfect cleavage.

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