It is infusible, but at a red heat it loses much water, and becomes very brittle, with increase of specific gravity; no glowing is observed. It is attacked by boiling concentrated sulphuric acid, and by fused potassium bisulphate; also by hydrofluoric acid (40 per cent.), with separation of the insoluble rare earth fluorides. No good crystals have been found, and no crystallographic data are known; examined by polarised light it appears isotropic, but this may be due to alteration. Colour, yellowish- to greenish-brown. Streak, yellowish-white. Hardness 5¹⁄₂; sp. gr. 4·179, increasing to 4·678 after ignition (cf. p. 38). The mineral was found in a granite-pegmatite at Risör, South Norway. ~Wiikite.~[75]--A mineral of very complex composition, for which no definite formula can be assigned. Its chemical nature may be understood from the following analytical data: Columbic and tantalic anhydrides = 16·0; Dioxides of titanium and zirconium = 23·4; Silica = 17·0; Ceria = 2·5; Yttria = 7·6; Scandia = 1·2; Thoria = 5·5; Ferrous oxide = 15·5; Uranic oxide = 3·6; water (and gas) = 5·8 per cent. [75] Crookes, _Phil. Trans._ 1908, A, ~209~, 15. Traces of lime, magnesia, stannic oxide and sulphur are also present. The mineral is infusible; on heating, helium, sulphuretted hydrogen and water vapour are given off, and a white sublimate is formed. The evolution of gas is almost explosive, the mineral breaking with a curious fracture. It is black and perfectly amorphous, showing no trace of crystalline structure or action on polarised light. Hardness, 6; sp. gr. 4·85. Wiikite is partially attacked by acids, readily by fused potassium bisulphate. It is radioactive. The mineral was found with monazite in a felspar quarry at Impilaks, Lake Ladoga, Finland. It is important as the source of scandium used by Sir William Crookes in his investigations of that element; some specimens of the mineral contain over 1 per cent. of the oxide (see p. 44). * * * * * The following related minerals, of which descriptions are given in the alphabetical list, are to be included here: _Arrhenite_, _Chalcolamprite_, _Endeiolite_ and _Wöhlerite_, are complex tantalo-columbates containing silica. _Hainite_ contains both silicon and titanium. _Dysanalyte_ is a titano-columbate believed by Hauser[76] to be merely an impure form of perovskite (see p. 14). [76] Vide _Zeitsch. anorg. Chem._ 1908, ~60~, 237. _Ilmenorutile_ and _Strüverite_ are closely allied minerals believed by Prior[77] and Schaller[78] to be isomorphous mixtures of rutile with Tapiolite or Mossite (ferrous tantalo-columbates). [77] _Min. Mag._ 1908, ~15~, 78. [78] _Abstr. Chem. Soc._ 1912, ~102~, ii. 773. _Pyrochlore_ is a complex titano-columbate containing elements of the cerium or yttrium groups. _Blomstrandite_ is an hydrated titano-columbate of rare earth elements, with calcium and uranium; it must not be confused with blomstrandine.