-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
grains, 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 be distinguished from hornblendic gabbros, which
contain more basic felspars, rarely quartz and occasion