Minute-sized crystals of HFSE minerals in evolved, fluorite-bearing granite, an example from the Sörvik granite, central Sweden

Abstract

Scanning Electron Microscopy and Energy Dispersive Analysis are used to study minute-sized crystals of high-field-strength-element minerals in an evolved batch of fluorine-rich granite. The batch separated due to buoyancy from the bulk magma before fluid separation was reached. Fluorine acted as the main complexing agent in the magma, facilitating the transport of many elements. Enhanced concentrations of elements such as Nb, Y, rare earth elements, Th and possibly Be gave the granite an accessory mineral composition similar to that of NYF pegmatite. Biotite was chloritized and often no remaining biotite can be identified. Frequent are two minerals from the aeschynite—euxenite—polycrase group. One is blocky and formed together with chlorite. It was replaced by an acicular variety, which continued to crystallize into the late hydrothermal stage. Bastna¨site and synchysite are also common in the granite. Both started crystallizing together with chlorite, but only synchysite can be followed into the late hydrothermal stage. Hingganite and gadolinite occur as inclusions in fluorite and chlorite. Galgenbergite formed as fracture-fillings. Some other rare minerals have merely been identified as accidental inclusions, but their identification is less certain. They include brenkite, fluocerite, fergusonite and possibly håleniusite. Chemical heterogeneities in the minerals point to incomplete equilibration.