A more highly specialised type of diorite [12,103] occurs in the Wadi Muqur. This is a very beautiful heavy rock (sp. gr. 2·87), with shining prisms of hornblende, often three centimetres or more in length, running through a mass of white felspars (see [Plate XXIII]). Microscopic examination reveals the presence of some quartz mixed with the plagioclastic felspars.
Where the alteration of diorite has been very intense, as in the country rock [12,124] of the old Romit mine, the hand specimen differs from that of the unaltered rock in that not only are the white constituents of duller aspect, but the dark mineral, instead of being black and shining, has a dull greenish-grey appearance. Under the microscope one sees no trace of the original felspar, its place being taken by calcite and a confused aggregate of small plates of kaolin and sericite, while the hornblende is all replaced by chlorite, with little strings of limonite. Granules of quartz, often of relatively large size, are scattered through the mass; some of these are clouded by minute enclosures, and evidently represent the accessory quartz of the original rock, while others are clearer and are probably of secondary formation.
Fine-grained varieties of diorite are found much more abundantly than the normal plutonic type, occurring as irregular masses mixed with schists and as dykes in schists and other rocks. These fine-grained diorites are essentially similar to the coarser-grained rocks, but are generally even more altered. The fine-grained diorite [11,517 A], which forms the top of Gebel Um Tenedba, for instance, consists of a mixture of clouded and altered plagioclase and augite altering to chlorite, with separated iron oxide along the cleavage planes of the hornblende. Another specimen of fine-grained diorite, from a dyke in Wadi Kreiga [12,154], is even more highly altered, the hornblende being almost entirely chloritised, with formation of abundant little granules of epidote.
The main rock of Gebel Beida [12,160] may also be classed as an altered fine-grained diorite. It is a dark fine-grained greenish-grey rock with white and greenish-yellow spots, usually about two millimetres diameter, scattered through it, and abundant strings of a greenish yellow mineral. The sp. gr. is 2·96. Under the microscope it is seen to be of similar nature to the rocks last described, the hornblende being largely chloritised. There is a considerable amount of accessory augite, and this, though clouded, has resisted alteration better than the hornblende. The whitish spots seen in the hand specimen are made up of a fine mosaic of quartz grains, with tiny flakes of a micaceous mineral (sericite?) probably all of secondary origin, while the greenish-yellow strings are other alteration products in the shape of epidote and calcite.
Augite-diorite.
Under the head of augite-diorite are classed holocrystalline rocks containing, in addition to the plagioclase and hornblende of ordinary diorites, notable quantities of augite. The augite-diorites thus form a link between the diorites proper and the diabases or plagioclase-augite rocks.
The presence of augite along with the hornblende is difficult to ascertain in the field or in hand specimens, and can as a rule only be detected by the microscopic examination of thin sections. But augite diorites are generally of somewhat darker aspect than normal diorites in the mass, owing to a less abundance of felspar, and are generally tougher under the hammer.
Fig. 20.—Augite-diorite, Wadi Um Hargal [11,535], × 47. h, hornblende; a, augite, with celephytic border c, of hornblende and iron oxide; f, felspar (labradorite); ap, apatite.
An augite-diorite [11,535] which occurs on the pass at the head of Wadi Um Hargal, near Gebel Kahfa, is a heavy (sp. gr. 2·87) grey rock of medium grain, very fresh and hard, in which can be seen lustrous black crystals mixed with a rather small quantity of white felspars. In thin section, hornblende, the most abundant constituent, is in fairly large allotriomorphic crystals, with well marked cleavage and strong pleochroism (bluish-green to pale yellow), containing abundance of irregular granules of iron oxides. The augite, which is present to about one-third the amount of the hornblende, is in crystals of similar size, and likewise showing well-marked cleavage, easily distinguished by their pale brown colour, absence of pleochroism, and higher extinction angles (about 40°); some of the crystals show irregular cracking and clouding by decomposition products, and are surrounded by celephytic zones of greenish matter of rather lower double refraction containing flakes and strings of iron oxide, probably representing a marginal alteration to hornblende. The felspar, though considerably decomposed, still shows plagioclastic twinning clearly, and appears from the extinction angles to be an acid type of labradorite. The rock contains considerable amount of magnetite scattered through it, often in fairly large irregular grains, also a few small grains of apatite, and one or two small granules of quartz.