Fergusonite1 is nominally yttrium orthoniobate, YNbO4, or yttrium-lanthanon niobate-tantalate, (Y, Ln) (Nb, Ta)O4. The mineral is very impure (Dana, 1944), often radioactive and, in most cases, metamict (Barth, 1926). The metamict form becomes tetragonal when heated for prolonged periods of time between 400 and 800° (Barth, 1926). When heated to 1000° and cooled it is monoclinic (Komkov, 1959).

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