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Research Paper |
Department of Earth and Environmental Sciences, Hashemite University, P.O. Box 150459, Zarqa 13115, Jordan
* E-mail: ibrahim{at}hu.edu.jo
(Received 28 May 2003; revised 10 October 2003)
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ABSTRACT |
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 TOP ABSTRACT GEOLOGICAL SETTINGMETHODS OF STUDYRESULTS AND DISCUSSIONCONCLUSIONACKNOWLEDGMENTSREFERENCES |
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phillipsite chabazite natrolite analcime calcite. The natrolite studied is chemically similar to those reported in the literature with minor variations. The phillipsite and chabazite studied are chemically different from the other Jordanian phillipsite and chabazite reported. The latter are chemically equivalent to those formed under open hydrological systems, whereas the phillipsite and chabazite in this study are chemically equivalent to those formed in saline lakes and arid soil environments. This conclusion is based on the (Na+K)/(Na+K+Ca+Mg) ratio and the Si/Al ratio. KEYWORDS: natrolite, phillipsite, chabazite, zeolite, Jordan
Natrolite has been known for a long time, mainly as ‘fibrous zeolite’. Klaproth introduced the name for a mineral from Hegau, Germany in 1803 (Gottardi & Galli, 1985). The unit-cell content of natrolite is Na16Al16Si24O80.16H2O with a Si/Al ratio of 1.5 and a total pore volume of 22%. Basic mineralogical and crystallographic data for natrolite were compiled by Tsitsishvili et al. (1992). Natrolite is very common as a hydrothermally deposited mineral filling vesicles (Demant et al., 1998). A few occurrences have been described, however, where a sedimentary genesis has been proposed (Gottardi & Galli, 1985). Hay (1966) suggested a closed hydrological system, as typified by a saline-lake basin in a hot semi-arid climate, for natrolite found as an alteration product of nepheline from Pleistocene sediments of Olduvai Gorge, Tanzania. An ‘open system’, however, was proposed by Hay & Iijima (1968a) for the natrolite of the palagonite tuff of Oahu-Hawaii. In Jordan, natrolite was first discovered by Ibrahim (1996) in a natrolite-bearing basaltic tuff. The following study investigates the mineralogy, chemistry and origin of a natrolite tuff which occurs in Tilal al Hisnawat ~55 km to the east of As Safawi in northeast Jordan (Fig. 1
). The northeastern region of Jordan is an arid area with <50 mm of annual precipitation.
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GEOLOGICAL SETTING |
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TOPABSTRACTGEOLOGICAL SETTINGMETHODS OF STUDYRESULTS AND DISCUSSIONCONCLUSIONACKNOWLEDGMENTSREFERENCES |
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METHODS OF STUDY |
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TOPABSTRACTGEOLOGICAL SETTINGMETHODS OF STUDYRESULTS AND DISCUSSIONCONCLUSIONACKNOWLEDGMENTSREFERENCES |
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radiation. The operating conditions were 45 kV and 40 mA. The XRD patterns were obtained by step-scanning from 3 to 50°2
in steps of 0.015°2 with a counting time of 1.00 s per step. Polished and normal thin sections were prepared for each sample for petrographic study and electron microscopy. A scanning electron microscope (SEM) study was performed using an Hitachi S-2400, and electron microprobe analysis using a Cambridge Microscan Electron Probe Microanalyser, with Link analytical system. The instrument was operated in the energy dispersive mode at 15 kV with a defocused beam, a sample current of 4.2–1 nA and counting time of 60 s. The reliability of the analyses of the zeolites is confirmed by the number of framework cations (Si + Al) being very close to half the number of oxygen atoms and the low balance errors (E = ±7), as suggested by Passaglia (1970). Data from the microprobe were obtained by averaging several point analyses from one sample. The XRD and SEM analyses were carried out in the laboratories of the Mineralogy and Petrology Division of the British Geological Survey, Nottingham, UK. |
RESULTS AND DISCUSSION |
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TOPABSTRACTGEOLOGICAL SETTINGMETHODS OF STUDYRESULTS AND DISCUSSIONCONCLUSIONACKNOWLEDGMENTSREFERENCES |
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), is ~10 m thick, and is confined to a low-lying closed basin ~5.25 km long and 3.5 km wide. This morphological setup along with the lithological features may indicate water-laid sedimentation. New field evidence indicates that the natrolite-bearing basaltic tuff is not part of the Aritayn Volcaniclastic Formation as was previously suggested by Ibrahim (1996). Based on field observations, it appears that natrolite tuff is older, and is overlain by the Aritayn Volcaniclastic Formation and ~3 m thick young basalt flows. The natrolite tuff consists of stratified, hard, light brown to grey fine ash tuff laminae a few cm thick. The tuff comprises a varying amount of medium- to coarse-grained vitric clasts, megacrysts, crystal fragments and lithic clasts in the ash matrix. Clasts have different shapes and sizes but are usually <10 mm, subangular, angular and sometimes subrounded. Vesicles vary from 20–30% by volume, and are of different sizes, mostly rounded to sub-rounded. They are partially or totally filled with zeolites and/or calcite.
Description of the host material
Vitric clasts or granules are made of palagonite (hydrated volcanic glass), which is made of cellular, isotropic to weakly birefringent hypohyaline groundmass with olivine microphenocrysts. Acicular microlites and crystallites of pyroxene, plagioclase and opaque minerals are abundant in the groundmass. Megacrysts and crystal fragments are made mainly of olivine with smaller amounts of ortho- and clinopyroxene, and minor spinel. Angular to subrounded ultramafic xenoliths and crustal inclusions ranging from 1 mm to >10 cm are abundant. Ultramafic xenoliths include, in addition to spinel peridotite, garnet pyroxenite and spinel pyroxenite.
X-ray diffraction
Mineral identification by XRD indicated that the mean zeolite content of the samples is ~35% and showed that natrolite is the most abundant (Fig. 2), varying from ~17 to 22%. Minor amounts of chabazite (9%) and phillipsite (7%) are recorded. Analcime and smectite were recognized as accessory phases. The amount of the zeolites is generally proportional to the amount of palagonite in the tuff. Calcite occurs in almost all of the analysed samples along with olivine, pyroxene and traces of hematite.
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Phillipsite.
Phillipsite is always found as a thin rim inside the vesicles. Phillipsite occurs mainly as white rosettes of radiating and spherulitic crystal aggregates (Fig. 3). It is also present as isolated euhedral stout prisms. Crystals range from <50 µm to 200 µm long. The spherulites are 150–900 µm in diameter. Phillipsite crystals are terminated by a two-sided dome indicating pseudo-orthorhombic symmetry (Mumpton & Ormsby, 1976).
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). The mean value of the Si/Al ratio is 2.86, which lies in the range of the phillipsite from the saline lake environment (2.6–3.4) of Tsitsishvili et al. (1992) as illustrated in Fig. 4. Phillipsite, as a matter of fact, has a higher Si/Al ratio than the other two zeolites reported in the study area. The K content varies between 1.79 and 2.28 atoms per unit cell with an average value of 2.03. This value is the highest among Jordanian phillipsites. Sodium is also high and varies between 1.45 and 2.29 atoms per unit cell. Calcium is either absent or very low (up to 0.32 atoms per unit cell, Fig. 5). The (Na+K)/(Na+K+Ca+Mg) ratio is consequently close to 1.
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), the Tilal al Hisnawat phillipsite occupies areas outside the field of the Jordanian phillipsite and is believed to have formed under an open hydrological system and is similar to phillipsite of saline lakes. A triangular plot of the exchangeable cations in the Tilal al Hisnawat phillipsite and the Jordanian phillipsite is shown in Fig. 5 and confirms the differences in the composition of the phillipsites.
Chabazite.
Chabazite is confined to the vesicles as is phillipsite, and formed after phillipsite. Chabazite rhombohedra grew on phillipsite rims and occur as transparent crystal aggregates with three sets of cleavage. The rhombohedra range from 10 to 80 µm, which represents the smallest chabazite crystals recognized in Jordan (Ibrahim, 1996). Scanning electron micrographs indicate that chabazite crystals exhibit very sharp edges. Various combinations of inter-grown rhombohedra give rise to the development of a wide variety of simple and complex penetration twinning (Fig. 6). The few chabazite analyses from Tilal al Hisnawat indicate that the Si/Al ratio is ~2.46 (Table 1). The ratio is greater than for most Jordanian chabazite, and close to chabazite formed in a hydrological closed system and arid soil (see Ibrahim, 1996, Fig. 6.7). Compared with the phillipsite, the Si/Al ratio in chabazite is lower. Chabazite is also characterized by low Ca and K content and by high Na and Mg content compared with the Jordanian and Italian chabazite (Fig. 5).
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). In most cases, natrolite and the other two zeolites (phillipsite and chabazite) do not occur together. However, the very few examples where chabazite and phillipsite may occur in contact with natrolite confirm that natrolite is always the latest zeolite phase to crystallize. This sequence of crystallization has been reported by Hay (1980) in alkali-rich, low-silica tuff of Olduvai Gorge, Tanzania, as a result of alteration in a saline, alkaline surface environment.
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indicates that the mineral has narrow chemical variations in terms of Si/Al ratio and exchangeable cations. The Si/Al ratio varies between 1.57 and 1.50, which is very similar to the Si/Al ratio reported in the literature for natrolite (Hay, 1980; Ross et al., 1992). This indicates that natrolite is the least siliceous zeolite mineral in the study samples compared with phillipsite and chabazite, which may indicate that the Si/Al ratio decreases as the zeolitization process advances. The average content of Na, Ca, K and Ba in the analysed crystals is 14.96, 0.30, 0.09 and 0.01 atoms per unit cell, respectively. Magnesium is absent which indicates that natrolite is dominated by Na, with traces of Ca and K (Fig. 5). Substitution of Fe3+ for Al3+ is also noted inside some of the tetrahedra, where the Fe3+ is between 0.15 and 0.32 atoms per unit cell. These data on the Si/Al ratio and the exchangeable cations are not sufficient to make firm statements about the origin of natrolite and whether it belongs to an open or closed hydrological system. This is because natrolite reported in the literature and formed under these different origins has similar chemistry.
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). No chemical analysis is available for analcime.
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Origin of zeolites
Palagonites usually form as a result of palagonitization of fresh volcanic glass under open or closed hydrological systems (Hay, 1978). The mechanism by which palagonite forms (palagonitization) has been proposed as a solid-state diffusion and hydration (Moore, 1966), or solution-precipitation (Hay & Iijima, 1968b). The palagonitization is accompanied by pronounced chemical changes, including extensive oxidation of iron-content and demobilization of other elements such as Si, Al, Ca, Na and K. These elements are leached out from the fresh volcanic glass into interstitial pore water and precipitated in a series of authigenic layers developed at the extreme edge of the granules (Ibrahim & Hall, 1996). The main authigenic minerals produced are smectite, zeolites (zeolitization) and calcite (Honnorez, 1978). Based on petrographic and SEM analyses aided by electron microprobe analysis, the paragenetic sequence of the authigenic minerals in the studied samples is: smectite phillipsite chabazite natrolite analcime calcite. This sequence of authigenic minerals, resembles that described by Hay & Iijima (1968b), Hay (1980) and Ibrahim & Hall (1996). No evidence was found to suggest that one zeolite replaced another except a few analcime crystals which replaced natrolite.
Hydrolysis of Si and Al dissolved from volcanic glass raises the pH of the water and converts dissolved CO2 to HCO–3 and then, perhaps, to CO2+3 (Walton, 1975). The Si, Al and cations are leached from the volcanic glass until smectite begins to precipitate. Initially, virtually all of the Al that dissolves from the volcanic glass will immediately react with water to form sheets of aluminium hydroxide polymers. Smectite is generally favoured by a high ratio of H+ to Na+, K+ and Ca2+ and by high activities of Mg2+ (Hay, 1978). The formation of smectite consumes most of the released Si and Al but leaves some of alkalis in solution thus providing an alkaline pH (9 to 9.5) and high activity of silica (Hay, 1966; De Kimpe et al., 1964). The prevailing chemical environment leads to the formation of a porewater suitable for crystallizing zeolites. As the pH increases beyond that yielding smectite, layer silicates are no longer in evidence, being replaced by zeolites in which both Al and Si are in tetrahedral coordination with oxygen (Barrer, 1982). The crystalline zeolite is a function of the temperature, pressure and various chemical parameters including the activity ratio of Si to Al, activity of Na+, K+ and Ca2+, and the activity (or partial pressure) of H2O (Hay, 1978; Passaglia & Vezzalini, 1985). Given that Mg, Ca, K and Na are leached out at different rates with Na leached faster (Ibrahim, 1996), a series of zeolite minerals will form at the extreme edge of the granules. Phillipsite is favoured over chabazite by high K/Ca and K/Na ratios (Hay, 1966; de’ Gennaro et al., 1990). The higher K/Na values favour the crystallization of phillipsite first, as it shows a slight selectivity for K (Passaglia & Vezzalini, 1985). As the interaction proceeds, there is a progressive increase of Na and Ca in solution which gives rise to an environment favourable to the crystallization of chabazite. Experimental work by Collela & Aiello (1975) and Wirsching & Höller (1989) indicates that at higher alkalinity, chabazite is more stable than phillipsite. This may explain the formation of phillipsite from earlier solutions which were believed to have lower pH than the later solutions which formed chabazite. Due to the incorporation of K and Ca in the phillipsite and chabazite, the concentration of Na increases in the pore spaces and both Na/K and Na/Ca ratios increase, giving rise to the formation of Na-rich zeolites (natrolite). The formation of natrolite is favoured by an increase in salinity and alkalinity, because an increase in these parameters decreases the activity of H2O and Si/Al ratio and increases the Na+/H+ ratio, all of which favour the formation of Na zeolites including natrolite and anacime (Moiola, 1970; Surdam & Sheppard, 1978). This is supported by the fact that the analysed zeolites exhibit a trend of continuous decrease in the Si/Al ratio from phillipsite to chabazite to natrolite, which is the same trend of paragenetic sequence of formation as described earlier.
The chemical activity of CO2 can influence the formation of zeolites both by lowering the activity of H2O and by providing the carbonate ions. The CO2 can combine with Ca to form calcite, thus utilizing the Ca which would otherwise be used in forming Ca zeolites (Hay, 1966). Polarizing microscope and SEM investigations indicate that calcite always follows the formation of zeolite, which may indicate that the activity of CO2 was at its lowest level during the process of zeolitization prior to the formation of calcite.
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CONCLUSION |
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TOPABSTRACTGEOLOGICAL SETTINGMETHODS OF STUDYRESULTS AND DISCUSSIONCONCLUSIONACKNOWLEDGMENTSREFERENCES |
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phillipsite chabazite natrolite analcime calcite. Chemical affinities of the Tilal al Hisnawat phillipsite and chabazite are different from the other Jordanian phillipsite and chabazite. and is related to the differences in the origin. The other Jordanian phillipsite and chabazite are compositionally equivalent to those formed from mafic volcanic rocks under an open hydrological system, whereas the phillipsite studied in the Tilal al Hisnawat area is compositionally equivalent to the phillipsite of a saline lake environment, based on the (Na+K)/ (Na+K+Ca+Mg) ratio and the Si/Al ratio. The chabazite is enriched with Na and Mg and is depleted in Ca and K and is close in composition to chabazite from a hydrologically closed system and arid soil. Chemical analysis of natrolite indicates that it is Na-dominated with narrow chemical variations.
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ACKNOWLEDGMENTS |
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TOPABSTRACTGEOLOGICAL SETTINGMETHODS OF STUDYRESULTS AND DISCUSSIONCONCLUSIONACKNOWLEDGMENTSREFERENCES |
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REFERENCES |
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TOPABSTRACTGEOLOGICAL SETTINGMETHODS OF STUDYRESULTS AND DISCUSSIONCONCLUSIONACKNOWLEDGMENTSREFERENCES |
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