Crystallographic orientation relationship between bastnaesite, fluocerite and cerianite observed in a crystal from the pikes peak pegmatites

Abstract

AbstractThe crystallographic orientation relationship of a fluocerite (Ce,La)F3crystal overgrown with bastnaesite (Ce,La)[CO3]F from the Pikes Peak area in Colorado, USA was investigated by transmission electron microscopy. The epitaxial overgrowth of bastnaesite on fluocerite was confirmed by evaluating the electron diffraction patterns of both minerals. During the examination, a third mineral phase, which was unexpected in this phase assemblage, was detected and identified as cerianite, CeO2, by EDS analysis in conjunction with electron diffraction. Its orientation relationship to the two main mineral phases can be described as a syntactical growth. The La-concentration of bastnaesite changes with distance from the cerianite interface, as confirmed by EDS. The reduction in Ce-content in close proximity to the interface of cerianite yielded a model rationalizing the local precipitation of cerianite. For all three mineral phases, a special orientation relationship was established.