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Inclusions and alteration products in complex silicate minerals from granitic pegmatites: A key to magmatic and post-magmatic processes

Complex silicates from granitic pegmatites are prone to various types of alteration during geological history. Two types of alteration usually prevail and are frequently interrelated: 1. Structure amorphization (metamictization) due to incorporation of radionuclides in the crystal structure, 2. Alteration by hydrothermal fluids that percolate through the rocks and gradually alter chemical composition of minerals. As a consequence various secondary mineral phases are left behind. Allanite, which crystallizes monoclinically (P21/m), is most frequently found as allanite-(Ce) and can be depicted by the chemical formula (CaCe)(Al2Fe2+)(Si2O7)(SiO4)O(OH). U and Th are partially incorporated in A2 position [1]. Gadolinite-(Y) has a chemical formula Y2Fe2+Be2Si2O10 with some U and Th substituted for REE. It is also monoclinic (P21/a) [2]. In this study three allanite and three gadolinite samples from the granitic pegmatites of South Norway were investigated for their structural properties and chemical homogeneity. All samples show different degree of structure amorphization as observed by X-ray diffraction experiments. An investigation using SEM equipped with EDS system revealed that all samples contain inclusions of other mineral phases. These could be addressed to: a) co-genetic mineral phases trapped in the mineral matrix, and b) mineral phases resulted from alteration of the original mineral matrix. Both types of mineral occurrences could be helpful in deciphering processes during and after mineral formation. Inclusions and alteration products could be mainly spotted along crackings and veinlets in the mineral grains. Some of them are euhedral, and this is especially valid for U and Th oxide phases, presumably the products of chemical alteration as observed in one of the allanite samples. An appropriate and successful characterization of mineral phases hosted by complex silicates that originate from granitic pegmatites is a key to understanding the composition and physical properties of the pegmatitic melts but also the processes of mineral-fluid interactions. This could provide an insight into postmagmatic processes in the pegmatitic environment.

allanite; gadolinite; pegmatite; inclusions; alteration products

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