The predominant rock of the mountain mass of Gebel Gerf [12,128] is a serpentine apparently derived from a wehrlite or olivine-diallage rock. In the hand specimen, it is a rather heavy black rock (sp. gr. 2·75) with small pea-green spots and a few small bronzy and glassy looking crystals here and there. The microscopic slide (see [Fig. 49]) shows the olivine to be largely altered to serpentine in the ordinary manner with separation of magnetite, but some granules of the original mineral remain, especially aggregated, together with augite granules, round the diallage crystals. The diallage is tolerably fresh, and easily recognisable by its fine striation and oblique extinction. Besides the diallage, there is some ordinary augite, not always very easily distinguishable from the unaltered olivine owing to its cleavage not being very marked. Some of the diallage crystals show undulose extinction as the result of strain.
Fig. 50.—Serpentinisation of bronzite, as seen in a serpentine derived from bronzite-rock, Gebel Gerf [12,119], under a high power between crossed nicols. The clearer portions of the figure are unaltered portions of a single large crystal of bronzite; the serpentine is seen forming mainly along the cleavages of the original mineral, with subsidiary cross-fibres.
At many points the serpentine of Gebel Gerf contains a great deal of bronzite, crystals of this mineral of typical aspect and measuring up to five millimetres diameter being visible in the hand specimen scattered liberally through the duller serpentine. The sp. gr. of this portion of the mass is 2·81. The slide [12,119] shows practically nothing but bronzite altering to serpentine, so that locally the parent rock has been a pyroxenite rather than a peridotite. The serpentinisation of the bronzite has proceeded mainly along the fibres of the original mineral, but there are numerous cross-fibres (see [Fig. 50]), and here, as in the hornblende of the rock of Gebel Abu Dahr, the change to serpentine appears to have been accompanied by the production of tremolite. In another slide [12,127] the main constituent appears to be still bronzite, but here it is of a pronounced olive-brown colour even in thin section, and is accompanied by some nearly colourless diallage; some of the olive-brown fibrous crystals show extinction-angles of a few degrees, and it is possible that these are hornblende.
Though the greater part of the Gerf serpentine has been derived from rocks free, or practically free, from felspar, it is possible that these were mixed with other rocks, such as gabbros and diabases, in which basic plagioclase formed an essential constituent. One of the specimens brought back from the mountain [12,115], having a sp. gr. of 2·67, is a somewhat wedge-shaped block, about ten centimetres square and four centimetres thick, covered entirely with the shining black to greenish glaze which is characteristic of shattered serpentine, except where chipped with the hammer to test its coherence and to look for remains of crystals in it. The fractured surface is mostly dull and nearly black, with little strings and spots of greenish matter and scattered shining specks. But although the block so thoroughly resembles a serpentine, the slide cut from it proves the rock to be a fine-grained diabase, consisting essentially of augite and plagioclase, with a good deal of iron oxide. The plagioclase is tolerably fresh, in lath-shaped forms still showing twinning very clearly, while the augite is very clouded and is changing to hornblende and chlorite or serpentine. In the field nothing was noted which would separate this rock from the rest of the mass, which it resembles almost perfectly in appearance, and whether the diabase forms a dyke or sheet, or the specimen represents a fragment of diabase caught up by the magma, is not certain. A dyke of this material, shattered and serpentinised on all the fractured surfaces, would be indistinguishable from the surrounding serpentine. It is even possible, though not likely, that the diabase is not a separate intrusion, but merely a variation of the same plutonic mass which has given origin to the main bulk of the serpentine.
Other variations in the serpentine which point to parts of the Gerf mass having originally been of a gabbroid type, occur in the mountains round Bir Korbiai and Bir Meneiga. A specimen from near Bir Korbiai [12,125], is a greyish-black fine-grained rock full of little white and greenish specks and strings, with pale citron-green serpentinous matter covering the slickensided surfaces of the fragments into which the mass readily separates. The sp. gr. is 2·67. The microscopic slide shows serpentine with pronounced knitted structure in places, as though derived from augite, and considerable patches of calcite which may represent an original lime-felspar. The calcite has, however, been largely redistributed along cracks, where it is often mixed with extremely coarse fibres or plates of serpentine.
The specimen from the mountains round Bir Meneiga [12,104] is a hard dull slatey-grey rock with black streaks and spots; it is strongly magnetic and shows a tendency to schistose structure. The sp. gr. is 2·54. On examination with a lens the grey matter has in places something of a resemblance to saussuritised felspar. The slide cut from this specimen contains no original minerals. The bulk of it seems to be composed of very minute fibres or plates, generally showing a distinctly parallel arrangement along the planes of schistosity, but exhibiting knitted structure in places, and thus probably consisting in part of kaolinic matter derived from altered felspar, and in part of serpentinous matter derived from alteration of augite; while scattered through the slide are abundant patches and strings of calcite and magnetite. The rock is too intensely crushed and altered for one to be certain of its origin, but it appears to have been originally a diabase.
Fig. 51.—Serpentine derived from alteration of harzburgite, Gebel Korabkansi [12,140], × 17. s, serpentine, mostly formed from olivine; o, unaltered olivine; b, altering bronzite; c, chromite or magnetite, enveloping olivine.
The serpentine of Gebel Korabkansi, to the west of Gebel Gerf, is an altered olivine-bronzite rock (harzburgite) of moderately coarse grain. A slide [12,140] cut from the least altered portion, where the sp. gr. of the rock is 2·72, shows olivine to have been the most abundant constituent; it is largely serpentinised in the typical manner, but abundant kernels remain unaltered. Mixed with the olivine crystals are others of a fibrous mineral, altered partly to serpentine and partly to a confused mass of tremolite fibres; in the few cases where any of the original material remains, this fibrous mineral shows straight extinction, and is probably bronzite. The alteration of the pyroxene has here proceeded more rapidly than that of the olivine. The pyroxenic mineral is easily picked out in the slide by ordinary light, owing to the fact that the olivine has altered to greenish-yellow serpentine, while the mixed material produced from the pyroxene is of a dirty-white aspect. Another slide from Gebel Korabkansi [12,112] exhibits in places mesh-structure with clear kernels which extinguish very obliquely to the meshes, showing augite or diallage also to have been a constituent of the parent rock; and there is a single ragged-looking crystal of dark green fibrous hornblende, altering to nearly colourless serpentine. The sp. gr. of the specimen from which this latter slide was cut is 2·63. Both the slides from Gebel Korabkansi are rather poor in iron oxides, but there are a few irregular crystals of chromite, generally enveloping olivine.