Pyrophysalite

Various have been the opinions respecting the identity of Topaz, Pycnite, and Pyrophysalite: Haüy considers the two latter as varieties of the former; other eminent mineralogists have considered them all distinct species; Bournon looks upon Pyrophysalite and Pycnite as varieties, but distinct from Topaz; unfortunately the analyses of Topaz have differed so much from each other, that a difference in the proportions of the constituent parts did not appear sufficient to constitute a distinguishing character, while their physical characters might justly he supposed to vary according to the more or less perfect union of the particles or the purity of the substance. The Pyrophysalite lies between Topaz and Pycnite approaching in form to the crystal of Topaz and in opacity and softness to Pycnite. From some recent analyses by Berzelius it appears that Topaz and Pyrophysalite agree very nearly in the proportion of their constituents, while Pycnite contains less alumine. and may therefore be considered as another species; but Pyrophysalite resembling Topaz in its analysis, in the form of its crystal, and in possessing the laminated structure peculiar to it, must be considered as a variety of the same species. They bear the same relation to each other as the Emerald or Beryl that accompanies the Pyrophysalite does to the Beryl of Siberia, found with the Topaz. The same circumstances have probably contributed to the opacity or imperfection, if 1 may so say, of the two minerals.

The back figure exhibits the irregular elongated form in which the Pyrophysalite generally penetrates the granite of Finbo; it is covered with Mica, partly foliated, and partly so compact as to resemble Serpentine. The granite contains a peculiar variety of Feldspar, composed of radiating laminæ . The front figure is from a fragment of a very large prism; in some parts it is semitransparent: I much regret that so large a crystal should have been broken. The upper figure is taken from the end of a crystal remarkable for the regularity of its form, and instructive, from the resemblance it bears to Topaz, which is particularly manifested in the modifications upon the prism. This valuable specimen was lent me by H. J.Brooke, Esq. Secretary to the Geological Society.

The following is the analysis of Pyrophysalite by Prof. Berzelius, compared with the Brazilian and Saxon Topazes and the Pycnite.^*

As these analyses agree very nearly with the atomic theory they must approach closely to perfection, and Pycnite^† must be a distinct species.