Sulphate of Barytes

This figure leads from nearly the primitive thickish rhomboidal prism to the flatter plated crystals, which are piled by the side of each other upon one of their most acute edges, (see tab. 70,) consequently giving it a very different appearance from the right-angled plates more usually seen, as in tab. 72, the edges of which are parallel to the diagonals. The crystals upon this specimen are mostly truncated upon the acute solid angles, the truncations forming little triangular facets; and they are all so deeply truncated at the obtuse solid angles, that the truncations meet. Tab. 70, bottom figure, shows one of these faces; and Tab. 95 exhibits both.

This specimen came from Dufton. It is convenient here, as it partly leads to those flatted primitive forms that stand on the acute edges and are often very thin, without any decrement on the angles, but occasionally passing by minute degrees towards an elliptical form, produced as it were by being placed by the side of each other when forming, but slipping or falling a little out of the parallel with those above or below. This sometimes seems the cause of the curved-like appearance which happens in Sulphate of Barytes like that of Pearl-spar, tab. 19; more particularly when the latter is a flatter variety. The weight and pearly lustre will, however, most generally distinguish them. The next figure partakes of this appearance.