AUGITE. The minerals to which this name has been applied present us with some of the most interesting and at the same time most difficult investigations that can fall under the notice of the mine ralogist and chemist, and have frequently occupied the attention of the most eminent men in both sciences. Nor are these bodies unworthy of such attention ; for not only would a thorough knowledge of their constitution, and the relation which they bear to other mine rals, particularly to the genus Hornblende, tend much to the perfection of the mineralogical system ; but, owing to their frequent occurrence in nature, and from their forming one of the principal ingredients in many porphyritic and trap rocks, such as the Syenite, Diallage, and Schorl-Rocks, Greenatone, &c., they form a class of bodies of the highest importance to the geologist A due regard to the circum stances which are favourable to the formation of one or other of the species, to the exclusion of the rest, would be likely to afford a safe guide in many geological inquiries into the character and formation of rocks of igneous origin. Werner was the first to divide a large class of minerals occurring commonly in basalt, lavas', and other volcanic rocks, into two species, to which he applied the names of Augite and Hornblende. This division was founded on the difference existing between the crystallised forms and structure, which, according to the experience up to that time, were never associated with each s 31 is meant the edge formed by the intersection of the faces s and a, s 31 31 and 31, &c.
other. The same division was shortly after adopted by Haiiy, who applied to them the names of Pyrosene and Amphibole, and gave the measurements, determining the oblique rhombic prisms, with their most general modifications characteristic of either species, which however we have modified by the later measurements of Rose, Mitscherlich, and Kupffer.
Professor Mobs, however, together with Professor Jameson of Edinburgh, has used the term Augite to denote the eighth genus of their respective systems, which consists of the four species designated as follows : First species. The Oblique-Edged Augite, corresponding with the
Augite of Werner, and Pyroxene of Haiiy.
Second species. The Straight-Edged Augite, correspondiug to Hornblende and Amphibole.
Third species. Prismatoidal Augite, containing as sub-species the minerals Epidote or Zoisite.
Fourth species. Prismatic Augite; Tabular Spar or Wollastonite.
Berzelius, on the contrary, viewing the subject in a chemical point of view, has been induced to use the term Augite or Pyroxene, Horn blende or Amphibole, in the same signification as employed by Werner and Batty. According to him, the Augites are composed of one equi valent of the bisilicate of lime united with one equivalent of the bisilicate of magnesia.
There are several varieties of this genus formed by the removal of the magnesia or lime, which are replaced either by ono or both of the isomorphous substances—the protoxide of iron, and protoxide of manganese. Of these the following are the principal : 1. Diopside, which may be considered as the type of the Augite Genus, is readily recognised by the form of its crystal given in fig. 1, and by the direction of its four cleavage planes, the most perfect corresponding with the faces M, those in the direction of r and 1 being less easily obtained ; and by its pale-green or grayish-white colour, and vitreous lustre. Its hardness is 5'5, and its specific gravity is 3'299. Alone before the blowpipe it melts into a colourless semi transparent glass ; with borax, very readily into a transparent glass. It consists chiefly of silica, lime, and magnesia, as will be seen by the following analysis of a variety from Tammare by Bonsdorff :— Silica . . . . 5483 Protoxide of Iron . 0'99 Lime . . . . 2416 Alumina . . . . 0.28 Magnesia • . . 18'55 Loss by heating . . 0'32 • Several varieties, little differing from the above, are called Rackalite and Fassalte, names indicative of their locality.