The Rare Earths: Their Occurrence, Chemistry, and Technology

by Stanley Isaac Levy

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.