[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.
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.