DISCOVERY OF CALCIOBETAFITE AT THE MCLEAR PEGMATITE, ST. LAWRENCE COUNTY, NY

The McLear pegmatite, located in St. Lawrence County near Dekalb Junction, is a small granitic intrusion that was mined for feldspar from 1907 to 1933 (Jensen, 1978). The pegmatite occurs in a series of metamorphosed limestones and quartzites, all of which experienced peak metamorphism during the Grenville orogeny. The pegmatite itself has been dated to 1042 +31/-7 million years old (Lupulescu et al., 2008).

In September 2007 RWM found an unusual mineral specimen in the waste rock of the old McLear pegmatite quarry. Embedded in the white glassy matrix of quartz and feldspar crystals was a small (~ 1mm diameter), black, glassy crystal. Preliminary examination of the specimen in the laboratory indicated that it might be a crystal of the rare mineral calciobetafite, an unusual oxide compound containing Ca, Nb, Ti, U, and Th.

To confirm the identity of this mineral we collected additional rock samples from the same field site in the summer of 2008. Mineral samples were separated and prepared for chemical analysis using the college's scanning electron microscope (SEM) and energy dispersive X-ray system (EDS). Results confirmed the presence of calciobetafite (Ca,RE,Th,U)₂(Nb,Ta,Ti)₂O₇, as well as the existence of a number of other unusual minerals (e.g. uraninite - UO₂, thorite - ThSiO₄, and titanite - CaTiOSiO₄).

In addition to this being the first reported occurrence of calciobetafite in North America, it is also the first detailed examination of the mineralogy of an extremely unusual pegmatite since the 1920s. Ca, Ti, and Nb-bearing minerals are generally rare in granitic pegmatites; why this intrusion contains such a diverse assemblage of these minerals is not easily explained. Additional studies currently underway may shed light on the origin and evolution of this intriguing igneous rock.