Fig. 58.—Beryl and quartz, from a lenticle in mica-schist near Sikait [10,580], × 30. b, beryl in idiomorphic hexagonal crystals; q, quartz, allotriomorphic.
Emeralds (Beryl).—At Zabara and Sikait the mica-schists contain crystals of beryl (silicate of beryllium and aluminium, Be3Al2Si6O18), the clear variety of which forms the gem emerald. The beryls are mostly found in lenticular bands of quartz which occur in the mica-schist, but sometimes they can be seen in the schist itself. The crystals are mostly well developed hexagonal prisms of a pale emerald-green colour, with characteristic vertical striation. The coloured figure on [Plate XXV] will give a good idea of the usual appearance of the mineral. In microscopic slides (see [Fig. 58]) the beryls are conspicuous only by their clear cut hexagonal outlines; they are quite colourless, with low polarisation colours about the same as those of quartz. Both at Zabara and Sikait there are numerous ruins and ancient mines where emeralds have been sought; most of them are irregular shafts and tunnels, twisting about as the old miners followed the varying directions of the bands of schists. It is commonly believed that gem emeralds were at one time extracted from these mines, and it seems incredible that the mining should have been carried on to so great an extent as is shown by the ruins and old workings, unless stones of considerable value were obtained. The Zabara mines were re-opened by Cailliaud in the time of Mohammad Ali Pasha (1817), but the stones extracted were of little value, being clouded and full of flaws. A similar result followed a more recent (1904-5) vigorous attempt by Mr. James, acting on behalf of Mr. Edwin Streeter, of London, to work the emerald mines of Sikait; plenty of beryls were found, but none clear enough to be of any great value, and the enterprise was abandoned, Mr. James concluding that either the ancient miners had worked out all the bands containing stones of any value, or else, what is perhaps more likely, the ancients were satisfied with a duller stone for a gem than our modern jewellers. The dull forms of beryl are in our own day of very little value, being principally used as a source for beryllium salts in chemical laboratories.
Fig. 59.—Tourmaline crystals in graphitic talc-schist, Sikait mines [9,908], × 17. t, tourmaline crystals, irregularly cracked and clouded; g, talc-schist, heavily clouded by graphite.
Tourmaline.—Besides beryls, the mica and talc schists of Sikait contain in places abundance of black tourmaline in well-developed crystals. At some spots this mineral is so plentiful as to form practically small patches of tourmaline-rock [10,395]. In thin section [9,874 and 9,908] the tourmaline crystals, which are much clouded and irregularly cracked, show beautiful pleochroism (colourless to deep orange), and very high double refraction colours in prismatic sections. Like the beryl, however, tourmaline is only of value as a gem when it is clear and transparent, and all the crystals so far obtained are dull and opaque.
Calcite, in rhomb-shaped crystals of a brown colour due to presence of included iron oxides [10,382] likewise occurs in places in the mica-schists of Sikait.
Chlorite-schists.
Though many of the decomposed hornblende-schists contain more or less chlorite, I have only in two localities come across rocks in situ which contain so large a proportion of this mineral as to deserve the name of chlorite-schists. The first is in the hills of Um el Huetat (latitude 25°), where typical chlorite-schists are mixed with mica, talc, and hornblende schists. The second locality is between Gebels Ras Shait and Nugrus, where the rock [10,388] is remarkable not only in its peculiar appearance but also by its strongly magnetic character. It is a thoroughly schistose rock of a rather pale greyish-green colour and rather silky appearance, with rusty looking spots. The sp. gr. is 2·77. The microscopic slide shows the stone to consist essentially of an aggregate of elongated plates and fibres of low double refraction, which from the hand specimen seem to be chlorite, but in the slide look more like antigorite. Magnetite grains are liberally scattered through the chloritic mass. The rusty spots visible in the hand specimen are translucent foxy red in the slide, in irregular broken forms with well-marked cleavage and nearly straight extinction. They are somewhat doubtfully regarded as deeply iron-stained hornblende. Mixed with the foxy red material are aggregates of granules of a highly refracting but isotropic mineral of deep bottle-green colour (? spinel), and chloritic wisps. There are also some clear colourless grains, resembling apatite in appearance except that they sometimes show well-marked vertical cleavage and high extinction angles; these are possibly a colourless augite, but the double-refraction colours are far lower than is usual with this mineral.
Typical chlorite-schists occur in the Wadi Salib Abiad, and near Gebels Ribdab and Muqsim, in the extreme south-west portion of the region. Hearing, from some wandering Arabs while at Gebel Abu Dahr in February 1907, that prospectors were at work in the Wadi Salib Abiad, I sent a guide to find out who they were and what they were doing; the guide reported that on his arrival they had gone away, but there were some old workings in a green rock of which he brought a sample. I did not get an opportunity of visiting the locality personally, but the specimen brought back by the guide [11,523] is a beautiful apple-green chlorite-schist, with some brownish calcareous-looking bands. The microscopic slide shows some little quartz and talc besides the chlorite, and there are scattered minute highly-refracting rounded grains of a feebly translucent mineral of a reddish-brown colour, probably rutile. A similar rock from the eastern side of Gebel Muqsim has been reported by Mr. Charteris Stewart, who also records a normal chlorite-schist as occurring on the north side of Gebel Ribdab.