_Yttrocrasite._ (Ca,Pb)O,(Th,U)O₂,3E₂O₃,16TiO₂,6H₂O. Yttr = 25·7; Cer = 2·9; ThO₂ = 8·7; TiO₂ = 49·7%. Orthorhombic; axial ratios unknown. G = 4·80. H = 5¹⁄₂-6. Black; lustrous. Burnet Co. Texas. ~Yttrofluorite.~ _n_CaF₂ + _m_YF₃ in isomorphous mixture? Yttr = 20-25; Cer = 1-2%. Cubic. G = 3·54-3·56. H = 4¹⁄₂. Closely resembles fluorspar, except in badness of cleavage. Northern Norway. _Yttrogarnet._ A variety of garnet with E and Zr. Yttr = 1-6·7; ZrO₂ = 0-3%. Cubic (cf. Garnet). Dark reddish brown (cf. Garnet). Stockö, Norway; Schreiberhau, Germany. _Yttrogummite._ UO₃, 3ThO₂, 3SiO₂, 6H₂O? E₂O₃ = 6·7; ThO₂ = 41·4%. Tetragonal; angles near Zircon. Usually massive. G = 4·43-4·54. H = 4-4¹⁄₂. Yellowish brown. Llano Co., Texas. _Yttrotantalite._ R´´R´´´₂(Cb,Ta)₄O₁₄ + 4H₂O; R´´ = Fe´´, Ca; R´´´ = E; Cer = 0-2·4; Yttr = 17·2-38·3%. Orthorhombic; isomorphous with Samarskite (_q.v._). G = 5·5-5·8. H = 5-6. Yellow to black. Ytterby, Sweden; South Norway. ~Zircon.~ ZrSiO₄, with Fe, Th, etc., in traces. ZrO₂ = 61·0-70·0%. Tetragonal; habit prismatic. G = 4·68-4·70. varying considerably. H = 7¹⁄₂. Colour very variable. Widely distributed as a rock mineral, in sands, etc. _Zirkelite._ (Ca,Fe)(Zr,Ti,Th)₂O₅, with E, U, Mg, etc. ZrO₂ = 48·9-52·9; ThO₂ = 0-7·3; TiO₂ = 14-15; E₂O₃ = 0-3%. Cubic; in twinned octahedra. G = 4·7. H = 5. Black; transparent in thin flakes. Jacupiranga, São Paulo, Brazil. CHAPTER II THE SILICATES (_a_) SILICATES OF THE YTTRIUM AND CERIUM METALS ~Cerite.~--Cerite is a silicate of the cerium metals, with small amounts of lime, ferrous oxide and water. Hintze gives the formula H₃(Ca,Fe)Ce₃Si₃O₁₃,[15] which Groth interprets as a basic metasilicate (Ca,Fe)[CeO]Ce₂(OH)₃(SiO₃)₃, _i.e._ a basic salt of the acid H₆Si₃O₉, a polymer of metasilicic acid, H₂SiO₃. [15] The symbol (Ca,Fe) here indicates that the iron and calcium occur in variable proportions, the variation however occurring in such a way that the equivalent of the two taken together is always the same, _i.e._ the iron can replace the calcium, or _vice versa_, atom by atom. The recognition of this possibility of ‘Vicarious Replacement’ between similar elements first brought order into the confused field of mineral chemistry, and allowed a systematic classification of minerals according to chemical composition to be made. Iron and calcium, or, according to the more convenient nomenclature of the mineralogists, lime and ferrous oxide, are here vicarious constituents. The symbol Ce here stands for elements of the cerium group, which are never found singly. Crystals are not very common, the mineral usually occurring granular or massive.