~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,Th/2)--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. ~Thalénite.~[44]--A silicate of yttria earths with water and small quantities of alumina, ferric oxide, carbon dioxide and alkalies. The ratio of rare earths to silica gives the formula R₂O₃,2SiO₂, or R₂Si₂O₇; if the water be included, the formula becomes H₂R₄Si₄O₁₅. The presence of both water and carbon dioxide indicates, however, that the mineral has been somewhat altered, and the simpler formula R₂Si₂O₇, (cf. Thortveitite, below) probably expresses the composition of the original mineral. It contains considerable quantities of nitrogen and helium, though uranium and thorium appear to be absent. [44] Benedicts, Abstract in _Zeitsch. Kryst. Min._ 1900, ~32~, 614. Monoclinic; _a_ : _b_ : _c_ = 1·154 : 1 : 0·602. β = 80° 12´. Common forms are the pinakoids {100} and {010}, hemi-prism {110}, hemi-pyramids {111} and {111̅}, and others, and the hemi-dome {021}. Angles, (100) ∧ (010) = 91° 0´; (100) ∧ (110) = 48° 9´; (100) : (111) = 59° 4´. Double refraction weak. No cleavage. Brittle. Hardness 6¹⁄₂. Colour, bright flesh-red; translucent, with greasy lustre; sp. gr. 4·227, increasing to 4·29 after ignition. A yellow variety has sp. gr. 4·11-4·16, and is transparent.