Discreditation of tombarthite-(Y)

Abstract

ABSTRACTTombarthite-(Y) is discredited as a mineral species. No type material was available, but material used for the original description has been located and neotype material defined. The main reason for the erroneous description of tombarthite-(Y) is the result of chemical analyses being carried out on heated material, which removed elements such as C and F. New semi-quantitative chemical analyses show that at least F is present in the fresh material, but absent after a heating scheme identical to that of the original description. Modern powder X-ray diffraction methods (XRD) confirm that the material identified as tombarthite-(Y) is a mixture of metamict and crystalline phases. Consequently, what was known as tombarthite-(Y) is not a mixture of the same minerals in equal amounts in different samples, but mixtures of various minerals depending on the sample. The main minerals identified are thalénite-(Y), xenotime-(Y) and kainosite-(Y). The discreditation of tombarthite-(Y) relies on new analyses of a large number of samples from the collection of the Natural History Museum (NHM) in Oslo and has been approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (proposal 16-K).