The diorite family is a group of intermediate plutonic rocks in which soda lime feldspar largely predominates over alkali -feldspar. It is a common rock type. The color of diorites is dark-gray or greenish, running into almost black in some varieties. It results from the color of the hornblende and the proportion of this to feldspar. The different varieties are due to the color, coarseness of grain, mineral make up, etc. The texture of the rock is the granular one. The porphyritic texture, while not unknown, is far less common than in granite. Sometimes the black hornblende prisms are distinct enough to produce an impression of porphyritic texture which is dispelled as soon as one compares the average size of the crystal grains.
According to the rule we have adopted, the ratio of orthoclase to plagioclase must not exceed one-third to two thirds. The dominant feldspar is a soda-lime feldspar ranging from oligoclase to labradorite; but some orthoclase is notably present in all diorites. While any kind of feldspar may be present, in the great majority of cases, as learned from microscopic studies, it is a variety containing considerable lime. This latter point however can rarely be determined on the hand specimens because the rock is not often coarse grained enough to permit the recognition of twinning striations on their cleavage surfaces. Usually more or less iron oxide in fine grains can be seen, and very often considerable biotite is present in shining flakes, with sometimes a bronzy luster. The hornblende is usually black, sometimes dark green, and, while often in bladed or prismatic forms, it is also often in short thick crystals or grains and sometimes in small masses of them and of biotite separated by the light-colored feldspar. With regard to silica, the limits are those of the intermediate group, namely 52 to 66 per cent. With more than 66 per cent, the diorites pass into the granodiorites, and with less than 52 per cent., into the gabbros.
It is not uncommon for the more acid diorites to contain a certain amount of quartz, and this can sometimes be identified with the lens. It is customary to distinguish these rocks as quartz -diorites or tonalites. The original tonalite type is a quartz-diorite containing abundant dark mica, together with hornblende. The tonalites are allied to the banatites, the difference between them consisting in the relative proportion of orthoclase and plagioclase. The most common ferromagnesian constituent in the diorites is green orbrown hornblende, in crystals or allotriomorphic grains; but brown biotite, pale green or colorless augite (salite or malacolite) and rhombic pyroxenes also occur.
Among accessory minerals
include the iron ores (magnetite,
ilmenite,
pyrites),
apatite,
sphene and
zircon are the most frequent. Secondary minerals are
chlorite,
epidote,
limonite and
calcite. Typical diorites have a granitic structure, but with
a texture less coarse than in the granites. Frequently the grains are
allotriornorphic and equidimensional. Occasionally, however, there is a
tendency towards porphyritic structure, and then the plagioclase, and more
rarely the hornblende is idiomorphic. Orthoclase and quartz occur
interstitially, with a tendency towards micrographic intergrowth.
The chemical composition varies considerably with the relative amounts of feldspar and hornblende, with the particular varieties of these two which are present, and is also somewhat influenced by the accessory minerals which may occur. The following table illustrates this and it shows also how the increase of lime, iron and magnesia over the proportions of these oxides in granites and syenites, causes the increase in the amount of hornblende.
Miarolitic cavities occur as in granite; they are often masked by being filled with calcite. Pegmatite dikes also occur and the minerals are somewhat different from those in the granites. Fluidal or somewhat parallel arrangements of the component minerals are not uncommonly seen, and these produce tendencies to gneiss like structure. Diorites are also frequently cut by complementary dikes, of much the same general appearance as those in granites, or these are found in their immediate neighborhood in dikes and sheets. Thus they may be traversed by light-colored aplites and felsites and by dark, heavy, basaltic traps. Their jointing is like that described for granites.
Spheroidal structure, similar to that described when dealing with the granites, is well developed in the ball-diorite or napoleonite of Corsica, where the spheroids consist of hornblende and feldspar, and show a well- developed radial and concentric arrangement.
A usual classification of the diorites based on the ferromagnesian minerals is: hornblendediorite, mica-diorite, augite-diorite and hypersthene or enstatite-diorite.
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