Royite, a New Variety of Quartz, from the Jharia Coal-Field by N

ROYITE, A NEW VARIETY OF QUARTZ, FROM THE JHARIA COAL-FIELD BY N. L. SHARMA (Indian School of Mines, Dhanbad) Received July 10, 1940 Introduction THE mineral occurs in more or less reticulated or radiating blades (Fig. 1), deposited along the joint planes of the laminated sandstones and shales of the eastern portion of the Jharia Coal-field. It was first discovered by the author along the joint plane of a bed of ferruginous shaly sandstone, behind Rao Bahadur D. D. Thacker's Labourers' School at Pure Jharia Colliery. Subsequently, a specimen of the same mineral was collected by the author from the debris of the IX seam incline of Messrs. Dana and Premji in Tisra area. Here the mineral occurs along the joint plane of a carbonaceous and micaceous shale. Later on, Mr. G. C. Chattopadhyay found a specimen of this mineral in the debris from the shaft of Ghansadih Colliery in Godhur area. The mineral at this locality is deposited along the joint plane of carbonaceous shale. Recently, the author (with his geology students) has noticed the same mineral in the debris from the incline of VI and VII seam, Basaria Colliery, on the western bank of the Bansjor stream. At this locality, the mineral occurs as beautiful radiating blades along the joint planes of carbonaceous or micaceous shales and ferruginous sandstone. Thus the mineral has been found all over the eastern portion of the Jharia Coal-field, though always in very limited quantity. Chemically, the mineral is composed mostly of silica. Physically, however, it is so unlike ordinary quartz that it is difficult to identify it correctly, unless one knows its chemical composition. It has been, therefore, thought advisable to give a new name to this variety. Prof. S. K. Roy, during his last visit to Zurich, showed the Tisra area specimen of the mineral to Prof. P. Niggli who also agreed that it was a new mineral; but at that time, the chemical analysis of the mineral was not available. Physical Characters The mineral is brownish-black or black in colour with no streak. In thin fragments it usually shows greyish colour. The mineral occurs in long 215 216 N. L. Sharma but very thin prismatic crystals, varying from I inch to 12 inches in length, inch to I inch in breadth, and a inch in thickness. The mineral blades from Basaria Colliery are, however, thicker than those from other localities, and vary from 8 inch to 3 inch in thickness. The crystals are sometimes tapering at one end. The hardness of the mineral is 7, and its specific gravity is 2.65. The mineral shows good prismatic cleavage. It has vitreous luster, shining on cleavage faces, due to which the extremely thin black blades of the mineral, when deposited along the joint pl.nes of black carbonaceous shale, can be easily distinguished from the latter. The Basaria specimens, however, show on cleavage surfaces a characteristic schiller lustre. The prismatic crystals of the mineral show transverse cleavage cracks, which make the crystals break in rhomb-like form. The Basaria crystals, due to their being thicker than those from other localities, break in small six- or four-sided prismatic columns. None of the cleavage faces of these cloumns gave ideal shining surface for goniometric measurements, but the value obtained is very nearly 60° for each of the six interfacial angles of the vertical zone. Thus the mineral appears to be hexagonal. Small pyramidal crystals of ordinary quartz in Pure Jharia and Basaria specimens, and white encrustations of colloidal and cryptocrystalline silica in Ghansadih and Tisra specimens, are found deposited in the interstitial spaces between the much bigger black crystals of this interesting mineral. At Basaria big prismatic rock-crystals of ordinary quartz have been found with nodular limonite in the debris from the neighbouring incline of V seam. The new mineral and the ordinary crystals of quartz usually show no sign of interaction with the rock on which they are found ; but in one of the Basaria specimens, a veinlet appears to have branched out from the main vein of the new mineral formed along the joint plane. No ordinary quartz is associated with this mineral in the veinlet. Microscopic Characters Under the microscope, the longitudinal sections of the mineral are long and rectangular, sometimes hexagonal (Figs. 3 and 4). They have often somewhat irregular margin with the shaly matrix. They always show one set of cleavage lines, but in addition to this, one or two other, rather in- distinct, sets of cleavage lines are often seen in many sections. Dirty brownish clayey and ferruginous matter has been deposited in small patches along the cleavage planes and along certain transverse cracks traversing the section. In Basaria specimens, minute biotite flakes have been observed along certain cracks. Royite, A New l/ariety of Qisaytz, from the Jhania Coal-Field 217 The two refractive indices of the mineral agree in value with those of quartz. The mineral has weak birefringence, the sections showing grey and yellowish polarisation colours of the first order. The mineral shows lamellar structure. The various lamella; extinguish in slightly different positions, and usually have more or less straight margin parallel to c-axis. In some of the fragments from Basaria, simple twinning is observed. The elongation of the mineral is slow, and the extinction angle, Z A C, being undulose, varies from 0° to 5 0 , especially in Pure Jharia specimens. In convergent light, the prismatic sections show two hyperbola: which flash in the field of view as the stage is rotated. Some sections of the mineral show elongated hexagonal outline and three sets of cleavage lines (Figs. 2 and 3). The cleavages are inclined to each other at angles varying from 56° to 66° in different sections. In some of these sections, the three sets of cleavage lines are very nearly inclined at 60° and 120° with each other. Such sections are probably more transverse than longitudinal, and they, therefore, show prismatic cleavage lines. These hexagonal sections show grey polarisation colour of the first order under cross- ed nicols, and give a uniaxial positive figure in convergent light. In other hexagonal sections, one of the three cleavages is prismatic, whereas the other two may be taken to be rhombohedral, as occasionally a few sections of the mineral show cleavage lines making an angle of about 38° or 67° with the prismatic cleavage, thus corresponding to interfacial angles, m r (1010 : 1011) and m z (1010 : 0111) respectively. A few crushed fragments of the mineral, mounted on a slide and examined in convergent light confirm that the mineral is optically uniaxial positive. Chemical Composition Only in the Pure Jharia and the Basaria specimens, are the mineral blades sufficiently thick to be detached from the rock on which they are deposited. In the former case, however, it was not possible to make the mineral completely free from the ferruginous and clayey matter which occurs as a thin coating over the crystals and also filling the cracks in those blades. This impurity cannot be removed even on boiling the crystal fragments with dilute hydrochloric acid for sometime. A rough chemical analysis of the Pure Jharia specimen, undertaken by Dr. S. C. Niyogy of the University College of Science and Technology, Calcutta, showed that the mineral contained 85.5% Si0 2 and amongst the bases Fe, Mg and Al were the most important. It was first thought that the mineral might be a silicate of these metals, but such a high proportion of SiO 2 is quite abnormal for any such silicate. 218 N. L. Sharma The mineral specimens recently discovered in Basaria area, are much purer than the Pure Jharia specimens, their blades are also thicker and, therefore, more easily detachable from the rock than those from the other locality. Dr. Niyogy has kindly carried out a complete chemical analysis of the mineral fragments from Basaria area with the following result : Per cent. Si0 2 .. 95.78 Spectroscopic analysis shows MgO .. 2.04 the presence of Co, Ba, AI 203 .. 1.80 and Sr, but the quantities Fe 203 .. 0.14 are too small for estima- CaO .. 0.37 tion. MnO .. 0.03 Ti0 2 .. Trace 100.16 Discussion If we assume the mineral to be a silicate, and take Mg and Al for the main bases (as shown in the above analysis), the chemical formula of the mineral works out to be approximately 3 MgO. Al 2 03 . 90 Si0 2 . No silicate of this composition is known to occur. Moreover, it has been observed under the microscope that there is plenty of foreign brownish clayey, limonitic and biotitic matter deposited along the cracks in this mineral. In Pure Jharia specimen, the proportion of Fe 203 was found to be 5.5%, whereas in Basaria specimen the amount of Fe 203 is negligible. In the former case, the mineral specimen analysed is associated with ferruginous shale, whereas the Basaria specimen analysed is associated with carbonaceous shale. Fe 2O 3 may thus be regarded definitely as an impurity in this mineral, derived from the wall-rock. The presence of 1.51 % of MgO, 1.12% of Fe 203 and 0.58% of CaO has been observed in Asmanite, a variety of Si0 2 (vide Dana's System of Mineralogy, sixth edition, p. 193). Asmanite is mixed with bronzite, and this may account for these impurities. Similarly, the deposition of limonite and microscopic flakes of lamellar biotite along some of the cracks of the present mineral, may account for the small percentages of MgO, A190 3 and Fe 2O 3 found in it.

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