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the original hypersthene. Round this is a finely granular zone mainly composed of green and blue hornblende from which fingers of hornblende stretch out into the surrounding albite aureole. This in its turn seems to be eating its way into a large potash felspar.
Beautiful developments of myrmekite (Pl. 21, fig. 4) are nearly always associated with biotite, and in one case biotite has actually replaced the albite of a myrmekite. Where biotite occurs as a corona round hypersthene, and is itself surrounded by a corona of albite, the albite seems to have developed direct from a potash felspar without intermediate myrmekite.
Felted masses of biotite, hornblende, iron-ore and apatite often occur along irregular cracks (24,447) in the felspars, and do not seem to have replaced earlier ferromagnesians. In such cases biotite certainly developed directly at the expense of earlier formed felspars.
The biotite and hornblende do not appear to be original constituents of the rock. They are closely associated with the albite which has replaced so much of the potash felspar, and are probably the same age as it. There is little doubt that all these minerals are the results of reactions between late magmatic waters and the original minerals of the rock.
A great part of the Malakanagiri hypersthene-gneiss is made up of large crystals of perthite retaining traces of their proper crystal form in spite of the corrosion of their margins. In some slides (23,230 B) a little microcline is present, but on the whole it is rare.
Both the perthite and the microc'ine show the microperthitic structures described by Holland 1900, p. 140 in his charnockite memoir.
Corroding the perthite and in some cases completely replacing it, are numerous masses of albite, oligoclase, and quartz. Although some of these minerals have clearly developed at the expense of earlier potash felspars, some were probably also original constituents of the rock. There is, however, no means of distinguishing original from secondary where all display irregular boundaries.
The salic minerals in the hypersthene
Rocks from different areas vary greatly in the relative proportions of soda and potash felspar, but in typical specimens they seem to be present in about equal amount.
Quartz occurs in all these rocks, in rather large patches with rounded boundaries but of no particular shape. There is a tendency for it to be concentrated in irregular bands associated with patches of albite. In appearance it is water-clear and fresh. It shows
under crossed nicols a brush polarization similar to that seen in vein quartz. It also occurs as minute rounded granules along the edges of the larger crystals in rocks where mortar structure is well developed, and as small vermicular masses in myrmekite.
The larger patches of quartz are full of minute inclusions. The commonest of these resemble tiny bubbles, and are arranged about cracks in the mineral. I cannot say whether they really are bubbles or not. In some slides thin black bars are arranged along the crystallographic directions of the quartz just as Holland (1900, pp. 135-140) describes in the Madras charnockite. In other slides of similar rocks no bars were seen.
How far the quartz is an original constituent of these rocks is uncertain, but many of the larger pieces have been cracked like the potash felspar, and are probably the same age as it.
Felspars grading from albite to andesine are extremely abundant. They can readily be distinguished from the potash felspars, as their refractive index is always greater than that of Canada balsam, but they are not so easy to separate from quartz, as they have a similar birefringence, and are often not twinned.
In a great many cases albite can be seen in process of development from a potash felspar with myrmekite as an intermediate stage. This peculiar intergrowth of quartz and albite develops like a wart on the surface of the albite and extends deep into the substance of the perthite (Pl. 22, fig. 1). It is usually in optical continuity with the albite with which it merges insensibly. Its junction with the potash felspar is very different. The line of junction here is distinct. From the junction itself starts a host of minute streams of quartz flowing towards the core of the wart. On their way these streams run together, and form vermincular masses and blebs. Judging from the birefringence and refraction of these blebs they are undoubtedly composed of quartz. The matrix in which they develop is almost always albite, but instances where they have grown in oligoclase are known (24,428).
In cases where the plane of the micro-section has chopped off the head of such a wart, an inclusion of myrmekite showing a very beautiful network of fine quartz veins is seen in the middle of a perthite crystal. In one section of this kind the inclusion was in optical continuity with an adjacent albite, from which a second wart of myrmekite was protruding into the same perthite crystal. Although the most conspicuous myrmekite growths are like warts, it is
common to see a linear development of myrmekite between crystals of albite and potash felspar. In such cases the corrosion of the myrmekite is never deep.
The blebs of quartz which develop in the myrmekite always disappear before they reach the main albite crystal, for that mineral never contains many quartz inclusions. Probably most of the myrmekitic quartz finds its way into the fine granular aggregate which separates the larger felspar crystals.
In many cases albite developing from potash felspar is cloudy. This cloudiness seems to be due to an intergrowth of fine flakes of sericite with the albite (23,230 B).
All the felspars in the Malakanagiri gneisses are cracked, whether they be early formed ones like the potash felspars, or late developments like the soda ones. It is noteworthy that the system of cracks in the perthite is severed and obliterated where myrmekite warts have crossed it, and that the albite developing from the myrmekite has developed a different system of cracks. This indicates, I think, that the rock was continuously under stress from the time when it was consolidated till the albitization ceased.
Oligoclase is always present as well as albite. It can be distinguished from that mineral by its good twinning, and relatively high refraction. In some cases this is distinctly higher than quartz. Its birefringence is also notably lower than that of albite. In cases where it has been possible to observe the extinction angle of the oligoclase it has been about 10°.
As with albite some of the oligoclase has developed at the expense of potash felspar with myrmekite as an intermediate stage (24,428). As this is relatively rare it is probable that most of the oligoclase is an original constitutent of the rock.
Iron-ore occurs as large irregular masses with rounded boundaries. It is usually associated with the femic minerals which it replaces locally, and from which it is doubtless partly derived. It also occurs alone or with biotite along cracks and between the crystals of other minerals (24,447). Here it has most probably been deposited from solution.
Associated with the iron-ore and biotite, and also to a small extent disseminated throughout the rock, occur numerous large rounded grains of apatite. In places it is so common that it seems to be an intergrowth with the iron-ore. In some specimens minute
i diomorphic apatite crystals are included in the felspars. These seem to be quite distinct from the rounded ones, and have probably been formed during the consolidation of the original magina.
Zircon is present in all the hypersthene-gneisses but is much less than apatite. Sometimes it occurs as very large crystals among the ferromagnesian minerals.
As already stated, the amount of each minerals present in the rock varies much from place to place. The variation is, however, continuous, so that it is never possible to draw a boundary between the more basic and acid types. The rock analysed (24,448) (49/571) may be taken as representative of the most basic hypersthene gneisses. Roughly it is composed of quartz, ferromagnesians, and felspars in the proportions 10: 25: 65. About 40 per cent of the felspar in this rock is of the alkali variety. The rock may be described as a granulitic quartz-monzonite.
this rock to one with practically no ferroAlkali felspars in the more acid types are greatly in excess of the soda-lime ones. An intermediate type, where ferromagnesians are present in small amount, might be described as a granulitic quartz-bearing hypersthene-syenite. With the virtual disappearance of the ferromagnesian minerals the rock becomes a gneissic soda-granite. On the whole the more acid the rock the more it has been affected by subsequent soda solutions.
All gradations from magnesians are found.
Associated with the hypersthene gneisses, and frequently developed along their margin, are numerous masses of medium-grained grey gneiss. As far as could be judged, there is a gradual passage from the rather coarse hypersthene-bearing rock to the much finer grained grey gneiss.
In the hand specimen (46/293) the rock is fresh and even-grained. It is composed of a pale felspathic matrix in which are scattered numerous dark grains of quartz.
The microscopic texture is somewhat granulitic, but the grains. frequently interlock, and are rarely rounded. Occasionally large porphyritic masses of potash felspar are present. True mortar structures do not occur in these rocks, but a fine granular aggregate
of quartz and felspar frequently separates the larger grains in the rock. Brush polarization, distorted twins, and cracks in the larger mineral grains are all common.
Myrmekitic intergrowths between the potash and the soda felspars are everywhere seen. They eat conical holes into the potash felspar and doubtless weaken the border zone much as pholases boring in hard rocks weaken and ultimately destroy them. By the break-up of this zone the greater part of the granular aggregate separating the larger crystals is formed.
I attribute the much finer grain of these rocks, compared with the hypersthene-granite-gneisses, to the weakening of the large porphyritic felspars in the process of albitization. The resultant weakness of the larger crystals has greatly accelerated their reduction to finer particles by the regional crushing forces.
These rocks are almost devoid of ferromagnesian minerals. The only one which is at all common is biotite, and this is always present in minute quantities. When present it is generally partly replaced by iron-ore (24,428). In one specimen (24,446) I noted a cruciform twin believed to be zoisite, surrounded by a mass of biotite and muscovite. As the epidote minerals do not ordinarily occur in these rocks this is rather remarkable. Probably the whole mass of biotite and zoisite was a xenolith.
The gneissic soda-granites are almost entirely composed of felspar with a little quartz. Potash and soda felspars are present in varying proportions, but in an average specimen they are both present in equal amounts. Both perthite and microcline are common, but the former predominates. All the potash felspars show the microperthitic structures characteristic of charnockite.
As in the hypersthene-gneisses the potash felspar seems to have crystallised early, and to have been later replaced extensively by soda felspar. The perthite is always much corroded and one frequently sees large crystals of it split into several smaller ones by myrmekitic growths (23,230 A). Where the replacement is almost complete one sometimes sees a large crystal of albite pseudomorphic after microcline and containing large irregular relics of that mineral in its interior (23,230 B). Sometimes large potash felspars are so eaten away that they present an amoeba-like appearance (Pl. 21, fig. 2).