It occurs with mica and pitchblende in pegmatite veins in granite, at Morogoro, in the Uluguru Mountains, German East Africa.
Yttrotantalite.
—This is a tantalo-columbate similar in composition to Samarskite, and isomorphous with it; though, as the name implies, the acidic oxide is chiefly tantalum pentoxide, the percentage of columbic anhydride being much lower than in the latter mineral. It is a pyro-salt of the formula R´´R´´´₂(Cb,Ta)₄O₁₄ + 4H₂O,[65] where R´´ = (Fe,Ca) and R´´´ = rare earth (chiefly yttrium) metals (Rammelsberg). Strutt found thorium and radium in it. The manner in which the water is combined in this, as in many other minerals, is at present undetermined.
[65] Dana gives R´´R´´´₂(Cb,Ta)₄O₁₅ + 4H₂O; this appears to be an error.
Crystal system—orthorhombic; a : b : c = 0·5411 : 1 : 1·1330. Common forms—pinakoids b {010} and c {001}, prisms m {110}, o {210}, p {120}, domes s {201} and β {011}. Habit, prismatic with m and b prominent, or tabular parallel to b. Colour yellow to black, white after strong ignition.
It is found at Ytterby in Sweden, and in South Norway.
Fergusonite, Tyrite, or Bragite.
—A columbate and tantalate of the rare earth metals, with uranium, iron, calcium, etc. The general formula is that of an ortho-compound, R₂O₃,(Cb,Ta)₂O₅ or R(Cb,Ta)O₄, where R = metals of the rare earths, chiefly of the yttrium group. Brögger includes the other constituents in the more complex formula (Th,U)(Si,Sn)O₄ + 12R(Cb,Ta)O₄; but the simpler formula agrees quite well with specimens from the most widely separated localities, and is usually adopted. The mineral is radio-active and contains helium.
Tetragonal, polar (with tetrad axis of symmetry only) c = 1·4643. (001) ∧ (101) = 55° 40´. Common forms—Basal pinakoid c {001}, tetragonal prism g {320}, pyramids s {111}, z {321}. Brittle. Hardness 5 to 6; sp. gr. 5·84, decreasing on hydration. Lustre dull, brilliantly vitreous on broken surfaces. Colour brownish-black. Translucent to opaque.
Fergusonite was discovered by Hartwell. It occurs with samarskite, and often with gadolinite and allanite, in Norway and Sweden, the Carolinas, Texas, the Urals, W. Australia, etc.