Pyroxene or Augite

Augite is very variable in its appearance, arising partly from the different forms of its crystals, and partly from its various colours; and hence it has been divided into several species, and new varieties have had names given to them as soon as found.

The most common kind is of a deep black colour, with a conchoidal fracture; it abounds in perfect crystals dispersed through Basalt, Lava, and Trap rocks, where-ever those rocks occur (see figs. 3, 4, and 5, tab. 156); but though it has been long found in Scotland and Ireland, I have not met with specimens sufficiently illustrative to figure in British Mineralogy. These crystals are often mackled, either by reversing one against the side of another, as in fig. 5 of tab. 156, or by two or more crossing each other obliquely. A rarer and more magnificent variety is of a greenish black colour; it occurs in groups of crystals attached by their bases to the mixture of Hornblende and Augite, in which they are situated, and which forms part of the primitive Greenstone, or Hornblende rock, of Arendahl, in Norway (see figs. 1 and 2, tab. 256. A brighter green variety, which approaches much in colour to Epidote, is found handsomely crystallized upon granular Augite (Coccolite) at Traversella, in Piedmont. These crystals are much modified; the primitive planes of the prism being almost lost; the larger planes upon most of the crystals being secondary; many of the crystals are mackled, the mackling plane being in the diagonal of the prism; in some of the specimens white Carbonate of Lime relieves the green Augite very beautifully (see fig. 1, tab. 157.) Augite of a dull green colour also abounds along with Epidote, Hornblene, Meionite, &c. in the masses of primitive Limestone, thrown out with the Lava by Vesuvius. (See tab. 154, f. 22.)

Another dull green variety of Augite has lately been brought to England from Fassa, in the Tyrol, with the names Pyrgom and Fassaït, (see fig. 2 of t. 157); it is of a yellower colour than that from Traversella, but its crystals have the same general form as the smaller ones from that place; the very acute pyramids of these crystals serve to diguise them, but the black ones found in Basalt have sometimes facets much inclined towards the prisms that would, if enlarged, form similar pyramids (see fig. 4. tab. 157. The nearly colourless variety, formerly called Diopside by Haüy, ineluding Mussite and Alalite, is the most distinct of all; but Haüy is satisfied of its identity: the Alalite is represented at fig. 3 of tab. 157; it is accompanied by orange-coloured Garnets of great beauty, and green Talc (Chlorite.) The Mussite is of a greener colour, less regularly crystallized, and often somewhat radiated in the mass. They are both found in the alp called la Mussa, near Ala, in Piedmont. A scopiform variety that was confounded with Lievrite, of which it was only the matrix, has been figured upon tab. 29 of this work; it has been considered by some as a variety of Epidote; but it is now determined that it is Augite.

The primitive form of Augite is an oblique prism, with a slightly rhomboidal base, whose angles measure 92°18′ and 87°42′; the base is inclined upon the obtuse edge at 106°6′, consequently the crystals found in Basalt may be known from Basaltic Hornblende, which they resemble by the near approach of the primitive planes of the prisms to a right angle; (they are the narrower faces upon the crystals figured). Augite is difficultly fusible; its specific gravity varies from 3.226 to 3.310, Haüy. its hardness rather exceeds that of glass.