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Encyclopedia Britannica



DIORITE (from the Gr. S&oi4eiv to distinguish, from hA through, Epos, a boundary)

This article appears in Volume V08, Page 289 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: DIO-DRO
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. F.)


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