[58] _Zeitsch. anorg. Chem._ 1912, ~74~, 79. Crystal system--rhombohedral; in forms and angles very close to hæmatite, but the two differ in symmetry (hæmatite has _t_, 3δ, _c_, 3π; ilmenite has only _t_, _c_). _c_ = 1·38458; (111) ∧ (100) = 57° 58¹⁄₂´; habit, tabular, thick; or in thin laminæ. Usually in embedded grains or rolled crystals in sand. Hardness 5 to 6; sp. gr. 4·5 to 5·0, increasing with percentage of ferric oxide. Iron black, opaque; streak black to brownish-red. Lustre sub-metallic. Slightly magnetic. The mineral is infusible; when powdered, it dissolves slowly in boiling hydrochloric acid, the filtered yellow solution giving the characteristic blue colouration of titanium salts on addition of tinfoil. In fused potassium hydrogen sulphate it dissolves readily. The variation in composition can be judged from the following limits: TiO₂ Fe₂O₃ FeO 3·5 93·6 3·3 per cent. 52·8 1·2 46·5 „ Ilmenite is a widely distributed mineral. In crystals it occurs chiefly at Kragerö and Arendal in Norway, at Miask in the Ilmen mountains, in Dauphiné, the St. Gothard, etc.; in the massive form at Bay St. Paul, Quebec, and other localities in America; and in sands at Menaccan in Cornwall, Iserwiese in Bohemia, Puy de Dôme, dép. Haute Loire, France, and in Brazil, Australia, and New Zealand. The mineral was discovered at Menaccan in Cornwall by McGregor, about 1790. He described it as containing iron and a new oxide; the unknown oxide was obtained in 1795 from rutile by Klaproth, who gave the name Titanium to the new metal it contained. * * * * * Short descriptions of the following titanates are also given (see list): _Davidite_ and _Knopite_; these are complex titanates containing elements of the cerium and yttrium groups. _Arizonite_ and _Pseudobrookite_--ferric titanates. _Perovskite_, calcium titanate, and its variety _Hydrotitanite_. _Pyrophanite_, a manganese titanate isomorphous with ilmenite, and _Senaite_, a species intermediate in composition between these two. _Geikielite_, the magnesium analogue of ilmenite, with the variety _Picroilmenite_, which is rich in iron. _Uhligite_, a titanate of zirconium, calcium and aluminium. _Derbylite_, _Lewisite_ and _Mauzeliite_, an interesting series of titano-antimonates. _Warwickite_, a boro-titanate. CHAPTER IV THE TANTALO-COLUMBATES (_a_) TANTALO-COLUMBATES CONTAINING NO TITANIUM DIOXIDE ~Samarskite~, Yttro-ilmenite or Eytlandite (Urano-tantalite).--Samarskite is a tantalo-columbate[59] of the rare earth metals, with iron, calcium, and uranium. [59] In this and all similar minerals, columbium (niobium) and tantalum are to be regarded as vicarious; they replace each other in all proportions. It seldom happens that a pure columbate is found free from tantalum, or _vice versa_; one or other may predominate, but the two are almost always found together.