DIORITE (from the Gr. S&oi4eiv to distinguish, from hA through, Epos, a boundary)  , in petrology, the name given by Hauy to a family of rocks of granitic texture, composed of plagioclase felspar and hornblende . As they are richer in the dark Viii . IOcoloured ferromagnesian minerals they are usually grey or dark grey, and have a higher specific gravity than granite . They also rarely show visible quartz . But there are diorites of many kinds, as the name applies rather to a family of rocks than to a single species . Some contain biotite, others augite or hypersthene; many have a small amount of quartz . Orthoclase is rarely entirely absent, and when it is fairly common the rock becomes a tonalite; in this way a transition is furnished between diorites and granites . It is rare to find the pure types of " hornblende-diorite," " augite-diorite," &c., but in most cases the rocks contain two or more ferromagnesian silicates, and such combinations as " hornblende-biotite-diorite " are commonest in nature . The felspar of the diorites ranges in composition from oligoclase to labradorite, and is often remarkably zonal, the external layers being more alkaline than the internal . Small fluid enclosures and black: gains, probably iron oxides, often occur in it in great numbers . Weathering produces epidote, calcite, sericite and kaolin . The biotite is always brown or yellow; the hornblende usually green, but sometimes brown or yellowish brown in those diorites which have affinities to lamprophyres .

The augite is nearly always green but sometimes has a reddish tinge;

bronzite and hypersthene have their usual green and brown shades . Apatite, iron oxides and zircon are almost invariably present; sphene, garnet and orthite are occasionally observed; calcite, chlorite, muscovite, kaolin, epidote and bastite are secondary . The structure is not essentially different from that of granite . The ferromagnesian minerals crystallize comparatively early and have some idiomorphism; the felspar usually follows and only in part shows good crystalline outlines . Orthoclase and quartz, if present, are last to separate out, and fill the spaces between the other minerals; often they interpenetrate to form micropegmatite . In many diorites the plagioclase felspar has crystallized before the hornblende, which consequently has less perfect outlines and forms irregular plates which enclose sharply formed individuals of felspar . This produces the ophitic structure (very common also in the dolerites) . More rarely biotite and augite exhibit the same relations to the plagioclase . Orbicular structure also occasionally appears in these rocks; in fact the orbicular diorite of Corsica (also called " Napoleonite " or " Corsite ") was for a long time the best-known example of this structure . The rock seems composed of spheroids, about an inch in diameter, surrounded by a smaller amount of dark-coloured dioritic matrix . The spheroids have a radiate structure and often show concentric dark and pale shells . These consist of hornblende (dark green) and basic plagioclase felspar, labradorite and bytownite (grey or nearly white) .

Occasionally diorites have a parallel banded or foliated structure, but these must not be confounded with the epidiorites, which are metamorphic rocks and also have a conspicuous foliation . Diorites must also he distinguished from hornblendic gabbros, which contain more basic felspars, rarely quartz and occasionally

olivine; but the boundary lines between diorites and gabbros are admittedly somewhat vague, e.g. some authors would call rocks gabbro which others would regard as augite-diorite . The hornblendites differ from the diorites in containing little felspar, and consist principally of hornblende . Among varietal designations given to rocks of the diorite family are " banatite " for an augitediorite with or without quartz (from the Schemnitz district), " granodiorite " for a quartz-hornblende-diorite (essentially the same as tonalite) from California, &c., " adamellite " for the quartz-mica-diorite or tonalite of Monte Adamello (Alps), " ornite " for a hornblende-diorite rich in felspar, from Sweden . (J . S .