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

Feldspar and Petuntse

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

Perhaps one of the most curious circumstances which happens in this substance, is that of its crystals macling, which in this specimen they most truly do, according to the literal sense of that word, as distinguished from the hemitrope*. The present specimen is of a peculiar kind, from the neighbourhood of Aberdeen, and may assist in explaining those of the same nature when they occur. It is part of a large crystal, which includes some Quartz and a few sparks of Mica, appearing at first a rude mis-shapen portion, to which I have added the outline for explanation.

The crystal is formed of two similar halves of different crystals, locked as it were into each other;—see the zigzag line No. 1.

The lower left hand figure represents a single crystal, in which the same faces are seen as are mentioned in tab. 212; the largest terminal face being the primitive one; but the small faces are greatly increased in size. If this crystal were divided in the middle in the direction of the dotted line, and one half turned round, it would not be in the least changed inform, answering nearly to Haüy’s remark, 3. 602.

The right hand figure is formed of two halves taken from the similar sides of two different crystals (or, which is exactly the same thing, of two pieces cast hi the same mould), each exactly corresponding with one half of the left hand figure. In this the primitive face is divided into two parts, and each part is brought to correspond with part of a secondary face, fig. 2, but may be easily distinguished by the fracture.

  • * A crystal one half of which is turned upon the other.—Haüy.
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