The ‘average atomic weight’ of the rare earth metals is 99, from which it appears that these consist chiefly of yttrium, with a smaller quantity of the metals of higher atomic weight. It was discovered in 1898 by Benedicts, accompanying fluocerite (_q.v._) in a quartz quarry at Oesterby in Dalekarlia. ~Thortveitite.~[45]--A silicate of yttria earths, chiefly scandia, of the formula R₂O₃,2SiO₂. Scandia forms about 37 per cent. of the whole (R. J. Meyer); yttria with small quantities of the other yttria earths forms the bulk of the remainder of the bases, the ceria group being almost completely absent. Ferric oxide (with traces of manganic oxide and alumina) forms about 3 per cent. Thorium is present only in traces, and radioactivity is barely perceptible. [45] J. Schetelig, _Centr. Min._ 1911, 721. Thortveitite is the first mineral to be discovered in which the content of scandia is greater than 2 per cent.; in 1908 Crookes[46] examined a very large number of yttria minerals for scandia, and finally chose for extraction of the earth Wiikite (_q.v._) which has a scandia content of 1·2 per cent.[47] [46] _Phil. Trans._ 1908, A, ~209~, 15. [47] According to Eberhard, some varieties of Wiikite have a much lower scandia content. Thortveitite is orthorhombic; _a_ : _b_ : _c_ = 0·7456 : 1 : 1·4912; commonly combinations of pyramids _o_ {111} and _s_ {211} with prism _m_ {110}, in radial aggregates of crystals elongated parallel to the _c_ axis. Cleavage parallel to _m_, fair. Twin plane _m_ (110), twinning very common. Refraction strong; birefringence strong, negative. Acute bisectrix perpendicular to (001), plane of the optic axes (010). Hardness, 6-7; sp. gr. 3·571. Extremely brittle; lustre brilliant, vitreous to adamantine. Colour, greyish-green, white to reddish-grey on alteration; in transmitted light yellowish-green, after ignition, reddish; the change being probably due to presence of oxides of iron. It is fusible with difficulty, and only partially attacked by hydrochloric acid. It was found by Thortveit, in 1910, in a pegmatite vein in granite, at Iveland, Sätersdalen, S. Norway, accompanied by euxenite, monazite, beryl, and the usual vein-materials (quartz, felspar, etc.). It was analysed and recognised as a new mineral by Schetelig (_loc. cit._). * * * * * The following minerals, of which particulars will be found in the alphabetical list, also belong to this class: _Bagrationite_, _Bodenite_, and _Muromontite_, varieties of allanite with differences in composition and physical properties. _Yttrialite_, a weathered variety of gadolinite. _Elpidite_, _Erdmannite_ and _Cainosite_, more complex silicates. _Rowlandite_, a comparatively simple silicate of the yttrium metals. _Yttrogarnet_, a variety of garnet containing yttrium metals. (_b_) SILICATES OF THORIUM AND ZIRCONIUM