Pyrochlore and betafite were investigated from several types of Canadian occurrences. Nine new analyses for 20 constituents and 11 analyses for 8 constituents are presented. A general formula A 16 -xB 16 (O,OH) 48 (F,OH) 8 is proposed for the pyrochlore-betafite series with x representing vacant sites in the unit cell. Differential thermal and thermogravimetric curves indicate 2 states of water. Betafite and thorian pyrochlore are metamict, but recrystallization can begin well below the exothermic reaction indicated on D.T.A. curves. The cell edge of ignited minerals tends to decrease as Ti and Fe increase. Molecular weights derived from density-cell edge data correspond qualitatively to those calculated from analyses. Observed X-ray intensities agree with calculated values, but the role of Fe is uncertain. Frequency diagrams suggest a natural division of pyrochlore and betafite at 15% U.

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