~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. On heating it glows suddenly between 500° and 600°C.,[66] losing all its helium, and with decrease in density (5·619 to 5·375). At the same time it gives out a considerable amount of heat--8·09 C.[67] for 1 gm. (see p. 38). [66] Ramsay and Travers, _Zeitsch. physikal. Chem._ 1898, ~25~, 568. [67] The heat of combustion of a gram of hydrogen is 342 K. ~Sipylite.~--Essentially a columbate of rare earth metals, with oxides of tantalum, tungsten, zirconium, uranium, iron and calcium, and some water. Mallet, the discoverer, gives the formula as R₂O₃,Cb₂O₅, the basic oxides including, besides the rare earths, Cb₂O₅ with Ta₂O₅ and WO₃, and some water. An alternative formula, making it a complex pyro-salt, is also given, but from its great similarity in form and angles to fergusonite, the first formula is preferred. Strutt finds that it contains not only uranium, radium and helium, but also thorium in considerable quantity (ThO₂ = 4·9 per cent.), a fact which had been overlooked by Mallet. The rare earths contain a high proportion of erbia. It is tetragonal, _c_ = 1·4767, (001) ∧ (101) = 55° 54´. The crystals are octahedral, with the form _p_ {111}; _p_ ∧ _p_´ = 79° 15´, _p_ ∧ _p_´´ = 128° 50´. Cleavage distinct ∥ _p_. It is usually granular and amorphous. Colour brownish-black to brownish-red, lustre resinous. Brittle. Hardness 6; sp. gr. 4·89. Translucent. Its behaviour on heating has been already mentioned (see p. 39); it is infusible. Boiling hydrochloric acid partially dissolves it; the solution gives the turmeric test for zirconium, and on diluting and adding metallic tin a sapphire-blue colour is developed, due to the columbium present. Boiling concentrated sulphuric acid decomposes it slowly.