DIORITE, the name given by flatly to a family of rocks of granitic texture, composed of plagioclase felspar and hornblende. As they are richer in the dark coloured 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 common the rock becomes a monzonite. It is rare to find the pure types of "hornblende-diorite," "augite-diorite," etc., but in most cases the rocks contain two or more ferromagnesian silicates, and such combinations as "hornblende-biotite-diorite" are com monest 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 grains, probably iron oxides, often occur in it in great num bers. 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 hyper sthene have their usual green and brown shades. Apatite, iron oxides and zircon are almost invariably present ; sphere 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 min erals 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 in terpenetrate to form micropegmatite. In many diorites the plagio clase felspar has crystallized before the hornblende, which con sequently 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. Or bicular structure also occasionally appears in these rocks; in fact the orbicular diorite of Corsica (also called "napoleonite" or "cor site") (q.v.) was for a long time the best-known example of this structure. Occasionally diorites have a parallel banded or foliated structure, but these must not be in. any manner confounded with the epidiorites, which are metamorphic rocks without con spicuous foliation.
Diorites must also be 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 horn blendites 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 augite diorite with or without quartz (from the Schemnitz district), "granodiorite" for a quartz-hornblende-diorite (essentially the same as tonalite) from California, etc., "ornoite" for a horn blende-diorite rich in felspar, from Sweden. (J. S. F.)