Thalenite-(Y) from Golden Horn Batholith, Washington State, and comparison with new analyses from the White Cloud Pegmatite, Pikes Peak Batholith, Colorado, USA

By: Markus B. Raschke, Evan J.D. Anderson, Jason Van Fosson, Julien Allaz, Joseph R. Smyth, Radekkoda, Philip M. Persson, and Randy Becker

Mineralogical Magazine (2017) DOI: 10.1180/minmag.2017.081.044

Thalenite-(Y) (ideally Y3Si3O10F) is a heavy-rare-earth-rich silicate phase occurring in granite pegmatites that may help to illustrate REE chemistry and behavior in natural systems. Crystal structure and mineral chemistry of thalenite-(Y) was analyzed by electron microprobe analysis, X-ray diffraction, and micro-Raman spectroscopy comparing thalenite-(Y) from a new locality in the peralkaline granite of the Golden Horn Batholith, Okanogan County, Washington State, USA, in comparison with new analyses of thalenite-(Y) from the White Cloud pegmatite in the Pikes Peak batholith, Colorado, USA. The Golden Horn thalenite-(Y) occurs as late stage sub-millimeter euhedral bladed transparent crystals in small miarolitic cavities in an arfvedsonite-bearing biotite granite. It exhibits growth zoning with distinct HREE vs. LREE-enriched zones. The White Cloud thalenite-(Y) occurs in two distinct anhedral and botryoidal crystal habits of mostly homogenous composition. In addition, minor secondary thal?nite-(Y) is recognized by its distinct Yb-rich composition (up to 0.8 apfu Yb). Single-crystal X-ray diffraction analysis and structure refinement reveals Y-site ordering with preferential HREE occupation of Y2 vs. Y1 and Y3 REE sites. Chondrite normalization shows continuous enrichment of HREE in White Cloud thalenite-(Y), in contrast to Golden Horn thalenite-(Y) with a slight depletion of the heaviest REE (Tm, Yb and Lu). The results suggest a hydrothermal origin of the Golden Horn miarolitic thalenite-(Y), compared to a combination of both primary magmatic followed by hydrothermal processes responsible for the multiple generations and a range of spatial scales in White Cloud thalenite-(Y).