Thalenite from Arizona.

Thalenite occurs as a minor constituent of a single small pegmatite within an extensive area of granite a few miles S of Kingman, Arizona. Partly crystalline and partly metamict, this thalenite has composition Y3(Si3O10)(OH), with extensive substitution of Y by REE, especially Dy, Er and Yb. Upon heating, even at moderate T, both the crystalline and the metamict thalenite are converted to a phase with a structure corresponding with that of thortveitite, Sc2Si2O7.-J.A.Z.