- Euclasius. Gmel. Syst. Nat. ed. 13. 3. 442.
- Euclase. Journal des Mines, 28. 258. Haüy, 2. 531. Tabl. 32.
This substance was first brought from Peru by Dombey, by whom alone the few European cabinets that have possessed it long, have been supplied; lately it has, however, been brought from near Casson* in the Minas Gerais, in South America, as Mr. Mawe informs me, who will in his Journal, about to be published, give some particulars concerning it. The upper specimen, from this place, was borrowed from the cabinet of Edmund Rundell, Esq. and is the deepest coloured specimen I have seen. The next specimen, one of Dombey’s, is in the British Museum, and was in the Greville collection. The lowest is in the collection of Mrs. Mawe; it is from Casson.
The crystals are rhomboidal prisms of about 65°. terminated by obtuse pyramids, on one side of which are generally two faces, and on the other generally from two to six, or even more; on the obtuse edges of the prisms are generally three planes, of which the centre one is a primitive, another primitive face is found upon the acute edge, and parallel to which the crystals are easily divided into lamina. In the direction of the edge that divides the larger terminal faces of the lower figure, are observable many small flaws; if we consider these to be parallel to a third primitive face, we shall have an upright tetraëdral prism, with an oblique-angled base of 130°. 9′.† for the nucleus, as seen in the geometrical figure, and not simply a rectangular prism with an imaginary base, as Haüy states.
In the geometrical figures, I have added Haüy’s letters of reference to such faces as from rough measurements they seem to correspond with; his measurements are as follow; the incidence of f upon f 106° 18′, r upon r 156° 10′, the edge k upon M 130° 9′, the edge z upon M 141° 40′, s upon s 114° 18′, the acute angle of the prism 65° 42′, s upon T 122° 51′. The other faces of the prism do not occur in Haüy’s figure; their measures are difficult to obtain, from their edges being ill defined. The mean of various trials upon those close to M gives their incidence upon it 159° 40′, or upon s 168° 8′; but these measures do not exactly correspond with each other. The other face measures about 164° upon s.
This substance is so brittle in the direction of the laminæ as to be useless in Jewellery, although it will scratch Quartz. Spec. Grav. 3.0625.
The larger outlines represent the two sides of Mr. Rundell’s specimen, the smaller terminal faces upon which, as will be seen by the letters of reference, correspond with the larger ones on the other specimens: the centre outline is from Mrs. Mawe’s specimen.
- * Where it is found among topazes.
- † Supposing Haüy’s measures to be correct, this face is parallel to the acute edge of the prism.