Feldspar and Petuntse Enlarge
May 1. 1807. Publish’d by Ja.s Sowerby London.
British Mineralogy
CCXII
Silex Petuntse

Feldspar and Petuntse

  • Class 2. Earth.
  • Order 1. Homogeneous.
  • Gen. 4. Silex.
  • Spec. Petuntse.
  • Div. 1. Crystallized.

This pretty specimen was sent from the same place as the last, and is of rather an unusual colour and appearance; having additional bevillings and truncations; viz. two on each of the more obtuse edges of the prism, forming four additional faces, and making in all ten faces to the prism and two small faces on the apex.

Feldspar differs in specific gravity from 2.272 to 2.7045. Under the blowpipe it melts into a whitish glass without addition. It also varies in analysis, containing

  • Silex from 43 to 70
  • Alumine 14 – 37
  • Lime, sometimes Oxide of Iron, and also Potash;
  • Barytes and Magnesia, according to Mr. Kirwan.
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