Beryllium-silicon disorder and rare earth crystal chemistry in gadolinite from the White Cloud pegmatite, Colorado, USA

Gadolinite, REE2FeBe2Si2O10, is a monoclinic orthosilicate member of the gadolinite supergroup of minerals and occurs in beryllium and rare earth element (REE) bearing granites, pegmatites, and some metamorphic rocks. Gadolinite from the White Cloud pegmatite, South Platte Pegmatite district, Colorado, USA, has been investigated and shows unusually variable REE compositions and distinct Be-Si disorder. Crystal structure and chemistry of two petrographically distinct gadolinite samples from this locality have been studied by electron microprobe chemical analysis, laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS), single-crystal X-ray diffraction (XRD), and micro-Raman spectroscopy. Within these samples, the gadolinite was found to range from gadolinite-(Y) to gadolinite-(Ce). Regions of nearly full occupancy of Fe at the M site, and partial substitution of Si for Be at the Q tetrahedral site, as well as substitution of Be for Si at the T site were observed, with up to 15% vacancy at the Fe site and up to 15% disorder between Be and Si at distinct tetrahedral sites elsewhere. The layered nature of the crystal structure allows for large variation of the radius of the cation at the A site which contains the REE. This study shows that Be may substitute for Si and that Be may be more abundant in geochemical systems than previously assumed.