Stillwellite, (Ce,La)BSiO5, from Langesundfiord is described. Composition of the rare earths is determined by X-ray fluorescence. X-ray powder data and other physical data are compared with natural stillwellites and with synthetic CeBSiO5, LaBSiO5, PrBSiO5, and NdBSiO5.
Contributions to the mineralogy of Norway. No. 34. Stillwellite in the Langesundfjord nepheline syenite pegmatite dykes.