DICKITE FROM JOB'S HILL ST MARY, JAMAICA

Transcription

1 Clay Minerals (197)8, 461. DICKITE FROM JOB'S HILL ST MARY, JAMAICA C. E. DAVIS, B. E. THOMPSON, W. A. TAYLOR Asr) D. A. HOLDRIDGE Seientzfic Research Council, Kingston, Jamaica (Received 2 February 197) ABSTRACT: Examination of rock samples from Job's Hill, St Mary, Jamaica, by chemical, X-ray, differential thermal, and infrared analyses showed predominant dickite with associated nacrite and kaolinite. The uniqueness of this deposit makes it possible that selected material could be used as a standard mineral. INTRODUCTION This material occurs mainly in veins near the junction of the Wagwater conglomerate and andesitic formation at Job's Hill, St Mary, approximately 2 m NNW of Kingston, Jamaica (Fig. 1). It was first brought to the attention of the Scientific Research Council by the Department of Mines (Government of Jamaica) in 1962, and tests were made both by staff of the Council and outside bodies. Short notes have been written by members of the Geological Survey Department on the geology of the formation with estimates of the amount of material available. However, so far as we have been able to ascertain nothing has been published on this remarkable and possibly unique deposit apart from a short reference in the Scientific Research Council's 3rd Annual Report. At present, the sole exploitation of the rock is for local use as a road metal. Because of its high refractoriness and white colour after firing it could be used in ceramic manufacture, but against this is the relatively small estimated amount of material (2, tons total and some 8, tons first grade). Although this amount of material makes it unlikely that it could be a substantial economic asset, the unusual nature of the rock warranted as full an investigation as possible. The acquisition of a Dupont 9 Differentiat Thermal Analyser and a Philips' Norelco X-ray Diffractometer, together with facilities for chemical and infrared analysis made this both rapid and adequate. Selected material might be acceptable as a standard mineral and as such would need to have adequate published data on its properties.

2 462 C. E. Davies et al.,5 OooOOO 15oo Wagwater facies ~'~ G ranodiorite Andesitic Voloanics [] Hornfels Eocene Upper Cretaceous %,~ Parochial -road i-j~., Bridle path f Faults i. Dickite outcrop FIG. 1. Map of Job's Hill area. A. Source of samples EXPERIMENTAL The samples were taken from the exposed northern vein by the bridle path [64645]. The 1962 sample is denoted No. 1 and a further sample in August 1968 is No. 2. This second sample comprised both an opaque and a translucent portion; each was examined separately. B. Five types of tests were carried out: Chemical Analysis using standard method for silica, ignition loss, and alumina; E.D.T.A. titration for calcium and magnesium; calorimetric methods for iron and titanium; and flame photometry for alkalis; Cation exchange capacity in which the mineral was calcium-saturated by treatment with NCaCIz, removing the excess Ca with water-methanol solvent and replacing

3 Dickite from Jamaica 463 the exchangeable Ca by treatment with N-NH4OAC. The calcium was determined by E.D.T.A. titration; Differential Thermal Analysis at 15 ~ C/rain. using the Dupont 9 Differential Thermal Analyzer and examining the derived trace; X-ray diffraction patterns using the Philips' Norelco diffractometer with an A.M.R. monochromator and a potentiometric recorder, using Cu Ka radiation, and a proportional counter. The mineral samples were smeared on glass slides and dried at 15 ~ C prior to X-ray analysis. They were then scanned between the angles 2 = 3 ~ and 2 = 35 ~ at the rate of l~ The positions and intensities of the characteristic peaks were measured; Infrared Analysis using the pressed disc technique. KBr and sample--previously dried over P2~ in vacuo--were mixed in the ratio 1 part sample to 2 parts KBr. Transparent discs 13 mm in diameter were made using 22, lb. total load. Analysis was carried out on a Perkin-Elmer 337 grating infrared spectrophotometer. RESULTS Table 1 indicates the chemical composition of the three samples studied. The calculated mineralogical composition is indicated in Table 2. The excess L.O.I. for sample No. 1 is consistent with the suggestion that excess alumina is present as diaspore or boehmite. Titania occurs as anatase in sample No. 1 but was not detected in sample No. 2. Quartz occurs in significant amount in sample No. 2 opaque. The C.E.C's of sample No. 1, sample No. 2 opaque, and sample No. 2 translucent are 1.82, 1-78 and 2-51 ml/1 g respectively. The values are indicative of a wellordered or well-crystallized kandite. Difference between the values for sample No. 2 opaque and translucent can be attributed to the difference in their quartz content. The d.t.a, curves (Fig. 2(a)-(c)) show a gradual loss of hydroxyl water from 5 ~ C to 56 ~ C followed by a sharp expulsion of further hydroxyl at about 67 ~ C and a strong mullite nucleation exotherm at 98 ~ C. The mullite nucleation peak is typical of a~well-cystallized kandite mineral. The gradual dehydroxylation peak seems to take place in stages, there being rate changes at around 55 ~ and 63 ~ C. In sample No. 2 this heterogeneity of the dehydroxylation peak becomes more noticeable and clearly defined peaks occur at 64 ~ C and 67 ~ C. The shape of the dehydroxylation peak coupled with the common mullite nucleation exotherm suggests that the rock consists of both nacrite and dickite and that minor amounts of kaolinite may also be present. The X-ray data of the samples are presented in Table 3 and Figs The table shows the main lines and their relative intensities while the figures indicate the sharpness and symmetry of the main kandite peaks. The quartz lines (4-27 A and 3"35 A) are clearly evident in the diffractogram of sample No. 2 opaque. This is consistent with the chemical data (Table 1). An anatase line (3.53 A) shows up in sample No. 1; this too is consistent with the chemical data. A weak 5.69 A reflection occurs in all three samples. This line very probably represents a plagioclase feldspar, as microscopic examination indicates that the mineral is present. Sample No. 1 was further characterized by infrared analysis. The main absorp-

4 464 C. E. Davies et al. o.~.~ ~. o ~1 I 6 o ~1 I O.o > o.~. I eq oo o o No ~.~ e~ < N " = c~ < m 3~ m ~ Z d& Z d'-,

5 Dickite from Jamaica 465 (a) (c) I I I I I I I I I I I Temperature (*C) F[o. 2. D.T.A. curves of kandite samples. tion peaks are indicated in Table 4, and are characteristic of a kandite. Fig. 6(a) and (b) shows the spectra of the sample and that of Georgia kaolinite. Two main spectral differences occur; in the ease of the Job's Hill sample there are three very distinctlyresolved peaks in the range 2"7 to 2.8 t~ whereas in the case of the Georgia kaolinite, TABLE 3. 'd'-spacings and relative intensifies of Job's Hill samples Sample 1',1o. 1 Sample No. 2 (opaque) Sample No. 2 (translucent) d(a) I d(a) I d(a) ] "19 1 7"19 1 5"69 1 5" " " " " " "98 6 2"97 7 2"97 6

6 466 C. E. Davies et al. 7'19~ o 3'59A >, E 3' 35 II " Degrees 28 FIG. 3. X-ray diffraction pattern of sample No. 1. the middle peak is not so distinct, l'his, as pointed out by Roy & Roy (1957), distinguishes dickite and/or nacrite from kaolinite. A further difference noted by us is the presence of three distinct absorption peaks--indicative of hydrogen-bonded TAeLE 4. Frequency (cm -1) of infrared absorption bands for Job's Hill Dickite (Sample No. 1) Si--O stretching R--OH f bending Si--O--R3+ ( 54 and Ra+--OH f O---H 1 region O H-----O H--O---H I f

7 Dickite from Jamaica A o 3,59A c 3.35A 5'Z~ ;)5 Degrees 2 r i FIG. 4. X-ray diffraction pattern of sample No. 2 (opaque portion). 7"19A 3.59 II ililli.ld I. JlL~I-.iI.A IILJ' i J ~ ~ Degrees 2 I bit L...,L.., Lj,..A.,,a, 7 F m Tp.--~ ~ * F I W -llw-w I l I I FIG. 5. X-ray diffraction pattern of sample No. 2 (translucent portion).

8 468 C. E. Davies et al. Ca) g I 1 I I I I I I I I I I I I I ; " Microns 1~o. 6 (a) Infrared spectrum of Job's Hill dickite (Sample No. 1. (b) Infrared spr of Georgia kaolinite. hydroxyl (Keller & Pickett, 195; Nahin, 1955) in the Job's Hill sample between 3"3 and 3"5/~, while the spectrum of the Georgia kaolinite showed very little absorption in that range. CONCLUSIONS 1. The indurated kandite from Job's Hill, St Mary, Jamaica is essentially dickite probably with associated nacrite and kaolinite.

9 Dickite from Jamaica Differences between the samples can be summarized as follows: (a) Sample No. 1 has excess alumina, possibly as diaspore, feldspar and anatase in addition to traces of iron, calcium and magnesium minerals; and (b) Sample No. 2 shows excess silica, particularly in the opaque fraction, and is free from anatase. The translucent material, which is present in small amounts only, comprises about 99% kandite. ACKNOWLEDGMENTS The authors are indebted to the Scientific Research Council and Technical Director, Dr R. N. Gonzalez, for permission to publish and wish to thank Mr D. E. Thomas for chemical analyses. REFERENCES KELLER W.D. & PICKETT E.C. (195) The absorption of infrared radiation by clay minerals. Am. Y. Sci. 248, 264. NAm~ P.G. (1955) Infrared analysis of clays and related minerals. Clays and Clay Technology Bull Calif. Div. of Mines. San Francisco. RoY D.M. & RoY R. (1957) Geochim cosmochim. Acta 2, 75.