Mica-schists.
Mica-schists, composed mainly of golden-brown lustrous laminæ of biotite with more or less quartz, occur near the base of Gebel Zabara and at Gebel Sikait [10,626], as well as in small quantity at one or two other points. They are always associated with gneiss, and appear to form irregular bands, alternating and mixed with talc and other schists. The laminæ of mica can seldom be separated in any large size, breaking up at a touch into small scales; they are often highly contorted.
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.