Tourmaline, or Schorl
- Class 2. Earths.
- Order 1. Homogeneous.
- Gen. 4. Argilla.
- Spec. electrical.
- Tourmaline. Kirw. 1. 271.
- Le Schorl. Broch. 1. 226.
- Schwarzer Schorl. Emmerl. 1. 95.
- Tourmaline. Haüy, 3. 31.
- Borax electricus. Linn. Syst. Nat. ed. 12. t. 3. 96.
This substance I have received from my friends Dr. Penneck and Mr. Dunkin of Penzance, found in that neighbourhood; and I have gathered some curious curvilinear varieties of it at the Logan Rock. I have also been favoured with some aggregated kinds, by the Rev. Mr. Neck, from Devonshire.
The specimen here figured was received from the former gentleman, and is somewhat remarkable for the largeness of the crystals*, though they are somewhat confused; but more so still, from one end passing by fine straight fibres into the Quartz which accompanies it, giving it an appearance of a termination. The crystallized end is shown; at the upper part of the geometrical figure on the left hand of the plate, and the other end on the lower part of the same figure in fibres.
The other figure is one out of a gangue of Mica from the neighbourhood of Aberdeen. These crystals are not very regular, but are remarkable for holding small Garnets, imbedded in them;—see the lower figure.
Tourmaline is by some authors said to be distinct from Schorl, whilst others consider these substances only as varieties of each other. In some instances they appear to be distinct j especially when sufficiently large; but of this more shall be said hereafter.
Tourmaline generally presents straight prisms†, mostly blackish or dark-coloured, sometimes showing a greenish or brownish colour at the transparent edges or flaws. My specimens from the Logan Rock are greenish, but those imbedded in Mica are brownish. Some foreign specimens are remarkable for varying their colour according to the direction in which they are viewed. Tourmaline is well known for its electrical properties. The crystals have mostly a rich gloss, are smoothish, with more or less numerous striæ placed longitudinally on the prism, and horizontally on the pyramids, excepting when they are approaching to fibrous, as below observed.
- * The crystals are found more perfect at the Brazils, in Spain, Switzerland, &c.
- † Sir John St. Aubin is said to have in his possession a crystal without a prism—a great rarity.