Hellandite.
—Hellandite[43] is a mixed silicate of rare earths with lime, magnesia, alumina, ferric and manganic oxides, with considerable quantities of water. The formula approximates to 3H₂O,2R´´O,3R´´´´₂O₃,4SiO₂, where R´´ = (Ca,Mg,Th2)—Thorium being able to replace two atoms of calcium or magnesium—and R´´´ = (Al,Fe´´´,Mn´´´ and rare earth metals). This may be written as a basic orthosilicate, R´´₂[R´´´´(OH)]₆(SiO₄)₄, a basic salt of the acid H₁₆Si₄O₁₆ (= 4H₄SiO₄). This composition puts it in the class containing topaz and some rarer silicates.
[43] Brögger, Zeitsch. Kryst. Min. 1906, 42, 417.
The mineral is crystalline, the crystals being well developed, but often dull and opaque by alteration (hydration).
Crystal system—Monoclinic, holosymmetric, a : b : c = 2·0646 : 1 : 2·507. β = 109° 45´. Habit usually prismatic, with {100}, {010}, and several prisms {hko}, terminated by various pyramid forms.
Angles (100) ∧ (001) = 70° 32´; (100) ∧ (110) = 62° 22´; (010) ∧ (110) = 27° 14´; (110) ∧ (11̅0) = 125° 0´.
Twinned on (001), twin plane (001), forming knee-shaped twins. Hardness varies from 51⁄2 in the least altered to 1 in the most altered specimens; sp. gr. 3·70 in least altered specimens, decreasing with hydration. Colour of fresh crystals, reddish-brown; on alteration they become brownish-black, yellow, or even white.
The mineral dissolves easily in hydrochloric acid, with evolution of chlorine; it is less soluble in nitric and sulphuric acids. It readily fuses to a yellow mass.
It was first discovered by Brögger at Lindvikskollan, in 1903, and later, in larger quantities, at Kragerö in Norway. It occurs in pegmatite veins in granite.