Pyroxenites.
Heavy dark green rocks, of medium to coarse grain, and consisting almost entirely of a schillerized-looking mineral, form the hill-masses of Gebels Um Ein and Qrein Salama, and similar rocks are found in connexion with gabbro at Gebel Um Gunud and elsewhere. From the diallagic appearance of these rocks in the hand specimen, they were classed in the field as pyroxenites or diallage rocks. But an examination of the slides cut from the specimens reveals the main constituent to be hornblende, and the rocks must therefore be placed in the division of amphibolites. It is, however, highly probable that these rocks were originally pyroxenites, the hornblende having originated mainly from the alteration of augite; the change from augite to hornblende in the gabbros has already ([p. 302]) been remarked, and in these ultra-basic forms the same process appears to have gone on.
Some parts of the great ultra-basic mass of Gebel Gerf consist of bronzite-rock. Specimens from the least altered portions consist almost entirely of bronzite, sometimes with a little olivine; the rocks are, however, generally found passing into serpentine, such as forms the main portion of the mountain, and on microscopic examination even the freshest portions always exhibit more or less serpentinisation. It will accordingly be preferable to treat of these rocks under the heading of serpentines.
Amphibolites.
Rocks consisting almost entirely of hornblende occur in considerable masses in the neighbourhood of Gebel Um Gunud, and form the conspicuous hills Gebel Um Ein and Qrein Salama; they also occur in a small patch about three kilometres east of Erf el Fahid. In the field, especially with the coarser-grained varieties, there is often considerable difficulty in identifying the main constituent of the rock as hornblende, owing to the schillerized appearance of the mineral being more suggestive of diallage; the mass east of Erf el Fahid, for instance, was classed in the field as a basic gabbro or diallage-rock, instead of a very basic diorite or hornblende-rock, such as the microscopic examination proves it to be. These rocks are always very heavy and extremely tough; they weather commonly into great rusty-looking rounded blocks, and a sledge hammer is required to get a specimen.
Fig. 43.—Amphibolite, from hills near Gebel Um Gunud [11,511], × 30. Almost the whole of the figure is occupied by hornblende, which is seen clouded and altering to granular epidote at e. Part of a large grain of calcite, bordered and streaked with iron oxide, is seen at c, while q is a small crystal of interstitial quartz. The slide also contains a small proportion of plagioclase felspar, not shown in the figure.
The amphibolite of the hills about Gebel Um Gunud [11,511] is a very hard and heavy coarse grained greenish-black rock composed almost entirely of shining platey-looking crystals of hornblende, which often reach two centimetres in length and breadth. The sp. gr. is 3·08. The microscopic slide shows the rock to be granitic in structure and to contain, in addition to hornblende, small amounts of plagioclase, quartz, calcite, and iron oxides. The irregular-shaped hornblende crystals show strong pleochroism, from pale olive-brown to moderately deep green. The extinction angles are large, being frequently over 20°. Twinning of the crystals is fairly common. Many of the crystals are clouded, and the alteration of the mineral has resulted in the formation of much epidote. The plagioclase is only present in very small quantity, strongly clouded by decomposition, but still showing twinning clearly. The quartz is clear, and is likewise present in very small proportion, mostly interstitial. Calcite is fairly plentiful in large rounded and irregular grains, rather turbid-looking, of a brownish or greenish tinge, outlined and streaked with strong lines of iron oxide, in a way which at times suggests that the calcite may possibly be the result of alteration of a lime olivine.
A specimen of amphibolite [11,528] obtained from a hill near the junction of Wadi Abu Marwa with Wadi Naait, about seven kilometres north-east of Gebel Um Gunud, is a dark green rock of rather fine grain, made up of shining plates and small fibrous crystals of hornblende and chlorite. Its sp. gr. is 2·97. Under the microscope the hornblende is in rather ragged-looking irregular crystals of various sizes, all interlocked with each other, rarely showing any approximation to idiomorphism except in the smaller crystals, which are commonly limited by prismatic faces; these smaller crystals are frequently included in the larger ones. The pleochroism is fairly strong, a pale olive brown, c moderately deep green. Prismatic cleavage is well marked. Extinction angles are often large, ranging up to 24°. A few of the crystals are slightly bent, and many show more or less clouding owing to partial alteration to chlorite, but on the whole the rock is fairly fresh. Except for a few specks of magnetite, and the alteration product chlorite, the rock contains nothing but hornblende.
The hornblende-rock or amphibolite of Gebel Um Ein [12,130] is of medium grain, almost entirely made up of shining crystals of hornblende with a little chloritic matter. The sp. gr. is 3·03. The microscopic slide shows large individuals of hornblende inclining to idiomorphism, set in a sort of ground mass of smaller crystals. The hornblende shows the same strong pleochroism and high extinction angles as in the two rocks just described; it is frequently clouded and speckled with iron oxides, and often full of lighter-coloured patches which extinguish with the rest of the crystal; in many cases these lighter patches contain a central granule of iron oxide, and they are doubtless due to a bleaching by segregation of the iron. Between crossed nicols many of the crystals are full of small fibres extinguishing differently from the rest of the crystal; some of these fibres polarise in low colours and are probably chlorite or serpentine, while others show brilliant tints and are possibly actinolite.