The fundamental crystal chemical behavior of REEs in minerals is not well understood. Among those minerals whose atomic arrangements are unknown or poorly characterized are the most common REE-bearing minerals, including 1) bastnaesite, the most abundant REE-bearing ore mineral, and its associated REE fluorcarbonates; 2) the common REE (PO4) "dimorphs", moazite and xenotime, and 3) high-Si monazite, of particular interest because substituent Si may play an important role in controlling the REE chemistry of monazites. Preliminary to this submission an extensive effort has been undertaken to obtain high quality specimens of these minerals in order to undertake high-precision crystal structure analysis of the phases. Preliminary film studies attest to the high quality of the crystals and ultimately assure success in the crystal structure studies. The results of these studies will help to elucidate the crystal chemical behavior of the REEs in the most common REE-bearing minerals.
