Both minerals are orthorhombic, but they are not isomorphous. The mistake was due to the fact that whilst the crystallographic data were determined from the upper crystals of columbite, the crystals of samarskite were used for analysis. ~Plumboniobite.~[63]--This is a recently discovered mineral closely related to samarskite and yttrotantalite (_q.v._). It is essentially a columbate[64] of yttrium metals, lead and uranium, with water, ferrous oxide, titanium dioxide, stannic oxide, alumina, lime, and cuprous oxide. The formula given is R´´₂Cb₂O₇,R´´´´₄(Cb₂O₇)₃, where R´´ = (Fe,Pb,Ca,UO), and R´´´ = Al and yttria metals, with isomorphous (?) metatitanate. The mineral is radio-active, and gives considerable quantities of gas on being heated with sulphuric acid (carbon dioxide 0·19, helium and nitrogen 0·22 per cent.). The yttria earths are rich in the oxides of gadolinium and samarium, and the mineral should prove a valuable source of these elements. It is remarkable that the ceria earths are almost entirely absent. [63] Hauser u. Finch, _Ber._ 1909, ~42~, 2270; Hauser, _ibid._, 1910, ~43~, 417. [64] It is to be understood that small quantities of columbium are replaced by tantalum. The mineral is massive, with some indication of crystalline structure. It is dark brown to black, transparent in flakes, and under the microscope is seen to be isotropic, with doubly-refracting inclusions, undoubtedly of a secondary nature. Hardness 5 to 5¹⁄₂; sp. gr. 4·80 to 4·81. Unlike samarskite, it does not glow on ignition. 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.