~Allanite.~--Allanite, or Orthite, as it is often called, is a mineral of the epidote family, containing rare earths. The general formula for Epidote is H₂O,4R´´O,3R´´´´₂O₃,6SiO₂, where R´´ is a divalent and R´´´ a trivalent metal, or vicarious series of metals. In the case of Allanite, R´´ = (Fe´´,Ca), R´´´ = (Al,Fe´´´,E), where E stands for metals of the cerium and yttrium groups (Engström’s formula). Groth formulates it as a basic salt, R´´´₃(OH)R´´₂Si₃O₁₂, of the acid H₁₂Si₃O₁₂ (= 3H₄SiO₄). Crystals are fairly common, but the mineral usually occurs massive or in rounded grains. Crystals--Monoclinic, holosymmetric; _a_ : _b_ : _c_ = 1·5509 : 1 : 1·7691, β = 64° 59´. Common forms--Ortho- and basal pinakoids _a_ {100} and _c_ {001}; _m_ {110} and other prisms, _e_ {101} and other hemi-ortho-prisms, _o_ {011}, _d_ {111} and other hemi-pyramids. Angles, (100) ∧ (110) = 54° 34´; (001) ∧ (101) = 63° 24´; (001) ∧ (011) = 58° 3´. Tabular, parallel to _a_, or long and slender by elongation parallel to axis _b_. Birefringence weak, variable. Refraction strong. Colour brown to brownish-black; almost opaque. In flakes very strongly pleochroic, the colours for light parallel to the three vibration directions ~c~, ~b~ and ~a~ being brownish-yellow, reddish-brown, and greenish-brown respectively. Brittle. Hardness 5¹⁄₂-6; sp. gr. 3·5-4·2. On heating, allanite becomes amorphous and isotropic with increase of specific gravity (cf. Gadolinite). Before the blowpipe it loses water, and melts to a black magnetic glass, many varieties phosphorescing strongly (_vide supra_). With hydrochloric acid it gelatinises, unless previously heated strongly, in which case it is not attacked. Analyses show that the rare earth content varies considerably (vicariously as regards ferric iron and aluminium), ceria earths varying from 3·6 to 51·1 per cent. and yttria earths from traces up to 4·7 per cent.[37] Thoria is usually present, 0 to 3·5 per cent. In 1909 Fromme[38] found small quantities of beryllia in the mineral, and in 1911 Meyer[39] found amounts of scandium oxide up to 1 per cent. It contains traces of uranium, and is weakly radioactive. Ramsay, Collie and Travers found no helium (1895), but in 1905 Strutt found radium in it, so that the presence of helium seems _a priori_ probable. [37] _Vide_ Schilling, pp. 70-75 for analyses of this mineral. [38] Fromme, _Tsch. Min. Mitt._ 1909, ~28~. [39] Meyer, _Sitzungsber. königl. Akad. Wiss. Berlin_, 1911, 379. Many varieties of the mineral are known, differing in habit, colour, water content, specific gravity, etc., and the percentage composition varies very much by reason of vicarious replacement of the bases. Goldschmidt[40] has found ‘Epidote-orthites’ which are isomorphous mixtures of orthite with an iron epidote; he concludes that most orthites are probably similar solid solutions, and in this way accounts to a large extent for the varying composition.