Harmotome or Staurolite

British Staurolite has only been observed hitherto at Strontian in Scotland, a place famous for Carbonte of Strontian—see tab. 65. It is generally found on gangue of Carbonate of Lime, which is mostly crystallized.

Its common form is a quadrangular prism, with the lateral solid angles truncated, and forming a four-sided pyramid, alternating at each end; or, in other words, making an elongated dodecaëdron, similar to that of garnet, but not of the same angle. Two of the opposite edges of the pyramids are mostly truncated, in British specimens. The crystals are generally larger than in those of Andreasberg, and more nearly resemble those of Oberstein.

Staurolite is chiefly admired for often assuming a cruciform appearance, looking like five crystals, four being united round a fifth. It appears however to be a regularity in the aggregating of the sides, without a sufficiency to fill up the lateral edges—see the lower figure.

They have been called twin crystals, as if two had passed across each other.

The whole appearance is somewhat glassy, of a blueish pearly lustre, having a foliated fracture on the broader faces. In other respects it is somewhat conchoidal, and hard enough to scratch glass.

Fusible by the blowpipe into a frothy enamel. it does not form a jelly when combined with acids. But if thrown on charcoal it emits a yellow phosphoric light.

On analysis by Klaproth it was found to contain

Its primitive form is said to be an octaëdron, divisible in the direction of the diagonals of the mutual base of the pyramids, so as to form four irregular tetraëdrons, or separate four solid angles, leaving a rhomboidal dodecaëdron, which might perhaps with more propriety have been called the primitive, to save confusion.