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THE GEOLOGICAL SURVEY OF INDIA.
GYROLITE AND OKENITE FROM BOMBAY. BY W. A. K. CHRISTIE, B.SC., PH.D., M.INST.M.M., Chemist, Geological Survey of India. (With Plate 13.)
HE minerals discussed in this paper, although not represented until recently in the collections of the Geological Survey, have both been recorded before from India.1 Doubts have recently been thrown on the authenticity of many of the specimens of okenite in European collections 2 and the distinctive nature of the much rarer mineral gyrolite and its near relatives as mineral species has often been called in question. It may therefore be worth while to describe the new acquisitions of these non-aluminous zeolites.
In an unnamed collection of minerals from the Deccan Trap of Bombay Island, acquired in 1922 by purchase from Mr. J. Ribeiro, there is a small specimen of gyrolite from Nowroji Hill 4 (19° 57′ 72° 53′). It occurs as a mammillary aggregate, the roughly hemispherical parts of which vary in diameter from 0.5 to 1 cm. The surface has a delicately chiselled appearance reminiscent of the manner in which hair is represented in sculpture. The nodules on
1 Gyrolite from Poona : M. F. Heddle, Mineral Mag., VIII, 199, (1889); F. Cornu, Mineral. Mitth., XXV, 515, (1906) and Sitz. K. Akad. Wiss. Wien, CXVI, Abt. 1, 1235, (1907). Cornu gives a partial analysis. Okenite from Poona: S. Haughton, Journ. Geol. Soc. Dublin, II, 114, (1868), with an analysis.
20. B Boggild, Danske Vidensk. Selskab. Math. -fys Meddeleser, IV, part 8, 1, (1922).
3 Cf. F. Cornu, loc cit.
Cf. map, Rec., Geol. Surv. Ind., LIV, Plate 4, (1922).
fracture show a series of irregularly overlapping plates, producing a radiate structure. The mineral has a perfect platy cleavage, thin laminæ being flexible but not elastic. The hardness is about 3. The specific gravity is 2-388-2-390 (acetylene tetrabromide and xylol, C.). The lustre the yellowish white, mammillary surface is dull, on the cleavage faces pearly and shining. The cleavage flakes are transparent to translucent.
The mineral is uniaxial; cleavage plates sometimes, however, show the emergence of two axes with a very small axial angle. It is optically negative. For sodium light w 1·549 001, ɛ= 1-536002 (embedding method with mixtures of cinnamon and clove oils, controlled with an Abbe refractometer).
Its composition is as follows:
H2O below 107°C
H2O above 107°C
It contains no fluorine. The composition is in reasonable agreement with the original formula of T. Anderson,1 2 CaO, 3 SiO2, 3 H2O, with SiO2 52-12 per cent., CaO 32-31 per cent. and H2O 15.57 per cent. ; it is somewhat nearer to that of F. W. Clarke, 2 Ca (SigO)3H 10 with SiO2 53.51 per cent., CaO 33-17 per cent. and H20 13.32 per cent.
O. B. Boggild has determined gyrolite as tri-rhombohedral, although it appears that the material on which alone measurements were possible is held by F. Cornu and A. Himmelbauer 4 to be a distinct mineral, reyerite. My attempts to ascertain the symmetry from percussion and etched figures on cleavage plates were unsuccessful.
1 Phil. Mag., 4th Ser., I, 113, (1851).
2 Bull. U. S. Geol. Surv, DLXXXVIII, 108, (1914).
3 Author's abstract in Zeits. f. Kryst., XLVIII, 534, (1911), from Meddelelser om Gronland, 34, 91 et. seq., (1908).
Mineral. Mitth., XXV, 519, (1906) and A. Himmelbauer in C. Doelter, Handbuch der Mineralchemie, II, 471, (Dreɛden, 1914).
The double refraction is considerably higher than the only value previously recorded,1 0055 (w=1·5645, ɛ=1.5590), but there again the determination was apparently on reyerite of Cornu and Himmelbauer, who give for that mineral w=1.564 and an analysis resembling that of Boggild's material. Himmelbauer gives values of
for gyrolite varying from 1-540 to 1-548.
The gyrolite occurs with calcite, apophyllite and okenite. The calcite gives one the impression of having been the first to crystallize, gyrolite nodules being attached to plane crystal faces. Thin sections from other parts of the specimen, however, show this apparently abnormal sequence 2 to be illusory; there calcite is seen to be enveloping and replacing gyrolite. Projecting from the gyrolite nodules and later than them are many crystals of apophyllite ( 1-536001, positive), showing (100), (001) and (111), some of them double-ended, attached by a prism face. Likewise later than the gyrolite and perched on the top of it are three little tufts of fibrous okenite, between 2 and 3 mm. in diameter and very similar to that described below. The fine projecting spicules have straight extinction, positive elongation and about 1.541. In one instance okenite spicules are seen penetrating and included in a crystal of apophyllite. The order of crystallization was probably gyrolite, okenite and apophyllite, calcite.
The beautiful specimens of this mineral here described were. collected by Dr. C. S. Fox in 1921 in a quarry in the Deccan Trap at the north-eastern foot of Golangi Hill 3 (19° 0′; 72° 54′) on Bombay Island. The okenite occurs with other zeolites in large geodes in somewhat soft, green basalt, probably intrusive at a horizon of intertrappean, carbonaceous shales.
It occurs as an aggregate of fibrous, radiate nodules up to 3 cm. in diameter, whose mammillary surfaces have a delicate, furry covering of prismatic crystals of the same mineral, the furry spicules being up to 1.5 mm. in length and varying in thickness from 0.001 mm. to 0.02 mm. The delicate fibres forming the bulk of the nodules show a general radiate arrangement, although the fibres
1 Boggild, loc. cit.
Cf. C. N. Fenner, Ann. N. Y. Acad. Sci., XX, 175, (1910) and T. L. Walker and A. L. Parsons, Univ. Toronto Studies, Geol. Ser., No. 14, 29, (1922).
3 Cf. map, Rec. Geol. Surv. Ind., LIV, Pl. 4, (1922).