A typical olivine-free gabbro [11,509] forms the main rock of Gebel Dahanib. A specimen taken from the summit, where the rock is rather finer in grain than that of the rest of the mountain, is very hard and heavy (sp. gr. 3·15), and is formed of a mixture of dark dull-looking mineral with shining white to colourless felspars. Under the microscope it is seen to be a holocrystalline aggregate of fairly fresh labradorite and pyroxene, both in allotriomorphic forms, in about equal proportions. About half the pyroxene is in the form of diallage, the remainder being mostly ordinary augite; it is almost colourless in thin section, but some of the crystals show a slight pleochroism, colourless to pale pinkish-brown. The augite crystals are much cracked, and frequently show signs of alteration with formation of calcite, epidote and serpentinous matter. There are a few small irregular areas of very pale green hornblende mixed with the augite, of which they may possibly be products of alteration. One or two elongated crystals in the slide, barely distinguishable from the augite under ordinary light, show a fibrous structure and straight extinction with low double-refraction colours; these are probably bronzite. There are only a few very tiny grains of iron oxide, and olivine appears to be absent from the slide examined.
Fig. 29.—Gabbro, Um Eleiga [11,527 A], × 17. pl, plagioclase; pc, clouded plagioclase; a, augite; m, magnetite; s, serpentinous matter probably from alteration of augite and hornblende.
A fine grained gabbro free from olivine occurs associated with diorite round the old mines of Um Eleiga [11,527 A]. It is a speckled black-and-white rock which would at first sight be taken for a fine-grained diorite rather than a gabbro. Its sp. gr. is 2·93. On microscopic examination the rock is found to be a holocrystalline aggregate of plagioclase, augite, altered hornblende, and magnetite, with granitic structure. The plagioclase (labradorite) is considerably altered and clouded by kaolinic matter, especially near the centres of the crystals, but still shows its characteristic twinning clearly. The augite, which is sometimes in the form of diallage, is fairly abundant, mostly in irregular grains, though occasionally inclining to prismatic forms; it is nearly colourless, but much cracked and slightly clouded. A clouded pale green to brown mineral, which sends off long tongues into cracks in the surrounding felspars, is also fairly abundant. This mineral polarises in yellows and greys as a confused serpentine-like aggregate of minute fibres, but contains clear and nearly colourless areas representing the original mineral from which it is derived; these clear areas, in which prismatic cleavage is usually well marked, are sometimes augite, but in many cases they polarise in lower colours (greys and yellows) than the augite, with low extinction angles, and are probably hornblende. Magnetite is very abundant in quite large irregular grains, often surrounded by a thin shell of hornblende or of the clouded alteration product just mentioned. There are a few small six-sided prisms of apatite, mostly included in the felspars.
Fig. 30.—Hypersthene-gabbro, Hadal Aweib Meisah [12,126], × 4. f, felspar (labradorite); a, augite; hy, hypersthene (the augite and hypersthene have the same appearance in ordinary light); m, magnetite; hb, hornblende, forming celyphitic borders round the augite and magnetite.
Another variety of fine-grained gabbro, likewise free from olivine, but containing hypersthene and some hornblende [12,126], forms the upper portion of Hadal Aweib Meisah, and a very similar rock [11,521] occurs in the hills five kilometres south-south-west of Marwot Elemikan. The rock is very hard, of a grey colour on fracture, weathering to blocks which have a rusty-brown skin. Its sp. gr. is 2·98. It is highly magnetic, and causes great disturbance of the compass needle in its neighbourhood. The hand specimen shows a mixture of shining white felspars with duller black minerals (see [Plate XXIV]). On microscopic study, the rock is found to be a holocrystalline aggregate of plagioclase, augite, hypersthene, hornblende and magnetite. The plagioclase, which forms about two-thirds of the rock, is a very clear and fresh labradorite of a rather acid type. The augite is usually in more or less rounded grains, often aggregated into irregular strings and mixed with hypersthene and magnetite. Only in a few cases does the augite show diallagic lamellation. It is nearly colourless, with a slight greenish or pinkish-brown tinge and faint pleochroism. The prismatic cleavages are usually distinct, and in addition the crystals are irregularly cracked. Twinning is fairly frequent, as also are inclusions of magnetite in the augite. The hypersthene occurs in grains similar to those of the augite, with which it is mixed, and from which at first sight it is not easily discriminated; but it can be picked out by its more marked pleochroism, straight extinction, and lower double refraction. Magnetite is abundant in large irregular grains, and in smaller granules included in the augite. Hornblende occurs in subordinate amount to the other minerals, and is principally seen as a celyphitic zone round the magnetite and augite crystals, especially between these crystals and the felspars. It is usually fairly clear, strongly pleochroic (deep greenish-brown to very pale yellowish-brown), and where surrounding two or three grains of other minerals the whole zone extinguishes at once, showing it to be a single crystal.
| Ball.—Geography & Geology of South-Eastern Egypt. | PLATE XXIV. |
BASIC AND ULTRA-BASIC IGNEOUS ROCKS.
