A crystal-chemical investigation of alpine gadolinite

Gadolinite-(Y) specimens from various localities in the Alps have been examined by electron microprobe and single-crystal X-ray diffraction. In general, dysprosium is the most abundant rare-earth, although a few samples contain approximately equal amounts of Dy and Yb, and in one instance, Gd predominates. In contrast to many non-Alpine occurrences, most of these specimens show only limited amounts of the lighter REE. There is an almost constant presence of calcium (up to 4 wt% CaO, and possibly twice that amount for more questionable samples); iron is often markedly deficient with respect to the theoretical formula, and in at least one case (Glogstafelberg), the material should more properly be called hingganite-(Y) (4.0 wt% FeO). In some specimens, a significant substitution of B for Be (up to about 4.2 wt% B2O3) can be deduced from crystal-structure data, on the basis of linear interpolation of the measured Be-O bond lengths with respect to other gadolinite-group minerals. This substitution is more extensive for specimens high in Ca and low in Fe, and which therefore grade toward datolite. No evidence for replacement of Si by B has been found. Minor amounts of thorium (up to 0.4 wt% ThO2) commonly are present, and uranium (0.3 wt% UO2) was found in one specimen. As for xenotime and monazite, the behavior of Y is not uniquely determined by the ionic radius, some specimens being especially enriched in this element with respect to the middle-heavy rare earths (up to 41.5 Wt% Y2O3).