Feldspar and Petuntse
- Class 2. Earth.
- Order 1. Homogeneous.
- Gen. 4. Silex.
- Spec. Petuntse.
- Div. 1. Crystallized.
Feldspar is a very common substance, chiefly found in aggregates of various descriptions. It is observable in the granites which come from Aberdeenshire to pave the principal highways in this metropolis, and remarkable in the Moor Stone of Devonshire on London and Westminter bridges, where the imbedded crystals are very bold and distinctly seen, especially after rain. The present specimen comes from near Monymusk in Aberdeenshire, by favour of my kind friend Mr. James Reid. The crystals, being nearly independent, allow us to see their determined form distinct from the gangue, which is more confused Feldspar with Quartz and dark crystallized Mica, forming a Granite. The little middle figure was easily detached, and makes a short six-sided prism. I have put a geometrical figure of it at the bottom of the plate on the left hand, showing the form of the fractured rhomb, and the lower side of a prism with a triangular face, formed of a bevilling from the edge of the sharpest angle of the side of the truncated end, as in the little middle crystal, which also shows the parallel fractures or flaws. The right hand modification is rather more common; viz . a six-sided column with two terminal faces, one primitive, or parallel to the fracture of the crystal, as in those before spoken of; and two directly opposite, forming at each end of the prism one primitive face and one opposite truncation, alternating with those at the opposite end.
These are of the usual colour, viz. a lightish red*. They are almost too hard to be scraped with a knife, but Feldspar varies much in hardness; the crystals in the Moor Stone on Westminster bridge stand above the rest of the stone, are consequently of a harder nature, and do not wear so fast. In other instances it is found decomposing, soft, and nearly powdery†.
The primitive faces, or sides as it were, of those figured at the apex and base of the crystals, fracture smoothly and with facility. The other four break irregularly or roughish. The former generally show some sparkling illinitions, which are very apparent in some specimens, and serve to distinguish which of the terminal faces is the primitive one.
- * White or transparent Feldspar, being found at Adula, is called Adularia Moon Stone, Feldspath nacré.—Haüy, &c.
- † This is often called Kaolin, and is frequently found in China manufactories.