Nature (journal)/Volume 1/Number 9/Mineralogy

This rare mineral has been studied by different crystallographers with apparently contradictory results. Haüy, Phillips, Lévy, Scheerer, and Waage have included it in the clino-rhombic system; Miller, Nordenskiöld, and Von Lang regard it as ortho-rhombic. The question could not be definitively settled by angular measurements, inasmuch as the primitive prism is a limiting form, bearing upon the corresponding elements of its anterior and posterior portions modifications whose incidences only differ by a few minutes. The author showed in 1860 that some species of gadolinite are mono-refractive, some bi-refractive, and some are mixtures of those two kinds; but it was not until the summer of last year that he was able to accumulate sufficient material for an exhaustive investigation. It now appears (1) that the Hitteröe crystals measured by Waage and the author, and analysed by Scheerer, have an energetic bi-axial refraction on two optic axes; the orientation of these axes, that of their bisectrix and their inclined dispersion, prove that the primitive form is an oblique rhomboidal prism, whose plane of symmetry is the same as that of the axes: this variety contains 10 to 12 per cent. of glucina. (2) The most homogeneous of the Ytterby crystals, measured by Von Lang and analysed by Berlin, are mono-refractive; they exhibit a certain number of peculiar modifications, in addition to those shown in the Hitteröe crystals, of which they are the pseudomorphs; and they do not contain glucina. (3) The heterogeneous specimens are forms in transition from the first to the second variety; they contain from 2 to 6 per cent. of glucina. These three kinds of gadolinite differ entirely in their symbolic chemical relations. The bi-refractive kind has the formula Ṙ₃S̈i; the mono-refractive is a sort of peridote, Ṙ₂S̈i; and the transition forms give an undecided result, the ratio between the oxygen of the silica and that of the bases varying from 3:4 to 4:5. These differences of constitution probably originate in local circumstances. The Hitteröe mineral stems associated with malacon and polycrase, in a granite vein composed of quartz, orthose, and oligoclase (with a little mica), and crossing the "gabbro" of which the greater part of the island of Hitteröe is formed: but that of Ytterby is chiefly accompanied by yttrotantalite and fergusonite, and imbedded in a red lamellar orthose, divided by large plates of black mica. [Ann. Ch. et Phys. (4) xviii. 305.]