Experimente zur Zeolithbildung durch hydrothermale Umwandlung

The formation of zeolites by hydrothermal alteration has been investigated by taking trass from the Laach volcanic area as a sample. Zeolites to be found are chabazite, phillipsite and analcime, all of which originated from the same phonolitic glass. This paper aims at explaining the formation of zeolites by means of experimental alteration of the pumice with various solutions. NaOH and KOH solutions were used in the experiments, these limited the formation conditions of chabazite, phillipsite, analcime in alkaline environments. Moreover, experiments were carried out with H₂O dist and with solutions that formed during the alteration of pumice by reacting with H₂O. These experiments were conducted to supply clues concerning the formation of zeolites in the Laach volcanic area. With NaOH solutions zeolites were formed from pumice within a temperature range of 70 to 250° C and a concentration range of 0.001 to 1.0 n; with KOH solutions they were formed in the same concentration range within a temperature range of 100 to 250° C. The formation of zeolites proved to depend much on temperature and concentration: At low temperatures high concentrations are necessary, higher temperatures need lower concentrations. With NaOH solutions the zeolites are formed in the succession, chabazite, phillipsite, analcime at increasing temperatures and increasing concentrations. With KOH solutions the same succession is to be found with rising temperature. With rising concentration, however, this succession is changed at higher temperatures: Phillipsite appears in place of analcime at high temperatures and concentrations. By the alteration of pumice with H₂O, zeolites are formed from 200° C onward. With solutions that had been formed during the alteration of pumice by reacting with H₂O, the minimum temperature for the formation of zeolites was 180° C. Concerning the genesis of zeolites in the Laach volcanic area, the experiments showed that the temperature for the formation of analcime was about 250° C; for the formation of chabazite and phillipsite it was between 150 and 190° C. The pH of the reacting solutions may have been between 7 and 8. Thus the experiments proved that from the same material different zeolites, chabazite, phillipsite, analcime may be formed by temperature change of the reacting solutions (H₂O to slightly alkaline solutions).     

Kinetics of formation of zeolite Na-A [LTA] from natural kaolinites

 The kinetics and mechanism of hydrothermal formation of zeolite A from natural kaolinites have been studied using as starting materials two international kaolinite standards (KGa-1 and KGa-2 from Georgia, USA) exhibiting a different degree of stacking disorder. Precursors utilized for the synthesis were prepared by heating the kaolinites at 800 °C. Metakaolinite was also prepared from KGa-1 by thermal activation at 600 °C. The hydrothermal syntheses were accomplished by heating the samples in NaOH solutions at temperatures between 70 and 110 °C. The kinetic experiments were performed by time-resolved synchrotron powder diffraction in isothermal mode using a transmission geometry and an Image Plate detector. The results of the kinetic analysis are interpreted in the light of the structural state of the starting kaolinite, and of the temperature of activation of the precursor material. For kaolinite activated at high temperature the nucleation and crystallization of zeolite A is essentially independent of the defect density of the original kaolinite, and the thermal history of the precursor seems to be the main controlling parameter. The formation process of zeolite A from metakaolinite materials obtained at lower activation temperatures shows significantly faster reaction rates and lower apparent activation energies. This is again interpreted in the light of the short range inhomogeneities present in metakaolinite. As the reaction proceeds metastable zeolite A transforms into hydroxy-sodalite. Received April 18, 1996 / Revised, accepted September 27, 1996     

Experimente zum Einfluß des Gesteinsglas-Chemismus auf die Zeolithbildung durch hydrothermale Umwandlung

In the formation of zeolites by hydrothermal alteration volcanic glasses are the starting material in most cases. The experiments aimed at demonstrating in what way the chemistry of the volcanic glass influences:

1. the alteration rate of the volcanic glass to zeolites,
2. the kind of zeolites being formed and their formation conditions.

Three volcanic glasses were used, a basaltic, a phonolitic, and a rhyolitic one. The experimental conditions were as similar as possible to the natural alteration conditions. Solutions being used: H₂O dist (pH ～5.5), 0.01 n NaOH (pH ～10.5), and solutions of similar chemistry to the natural ones. The temperatures were 180 °, 200 °, 250 ° C. The experiments were carried out both in closed and in open systems. The experimental results show a difference in the alteration rate and in the zeolites being formed between the basaltic and the phonolitic glasses on the one hand and the rhyolitic one on the other. In case of the closed system the SiO₂-poor volcanic glasses react more rapidly than the SiO₂-rich one. The zeolites being formed are chabazite, phillipsite, analcime respectively mordenite, analcime. In case of the open system the influence of the chemistry of the volcanic glass on the alteration rate and the zeolite being formed is less significant. Which zeolite is formed at a given temperature depends on: the chemistry of the starting material, the chemistry of the reacting solution and wether there is a closed or an open system.     

Hydrothermal origin for the 2 billion year old Mount Tom Price giant iron ore deposit, Hamersley Province, Western Australia

Giant iron-ore deposits, such as those in the Hamersley Province of northwestern Australia, may contain more than a billion tonnes of almost pure iron oxides and are the world's major source of iron. It is generally accepted that these deposits result from supergene oxidation of host banded iron formation (BIF), accompanied by leaching of silicate and carbonate minerals. New textural evidence however, shows that formation of iron ore at one of those deposits, Mount Tom Price, involved initial high temperature crystallisation of magnetite-siderite-iron silicate assemblages. This was followed by development of hematite- and ferroan dolomite-bearing assemblages with subsequent oxidation of magnetite, leaching of carbonates and silicates and crystallisation of further hematite. Preliminary fluid inclusion studies indicate both low and high salinity aqueous fluids as well as complex salt-rich inclusions with the range of fluid types most likely reflecting interaction of hydrothermal brines with descending meteoric fluids. Initial hematite crystallisation occurred at about 250 °C and high fluid pressures and continued as temperatures decreased. Although the largely hydrothermal origin for mineralisation at Mount Tom Price is in conflict with previously proposed supergene models, it remains consistent with interpretations that the biosphere contained significant oxygen at the time of mineralisation. Received: 16 February 1999 / Accepted: 14 May 1999     

Third dimension of a presently forming VMS deposit: TAG hydrothermal mound, Mid-Atlantic Ridge, 26°N

ODP drilling of the active TAG hydrothermal mound at 26°N on the Mid-Atlantic Ridge provided the first insights into the third dimension of a volcanic-hosted massive sulfide (VMS) deposit on a sediment-free mid-ocean ridge. Sulfide precipitation at this site started at least 20,000 years ago and resulted in the formation of a distinctly circular, 200-m diameter, 50-m-high pyritic mound and a silicified stockwork complex containing approximately 3.9 million tonnes of sulfide-bearing material with an average of 2.1 wt% Cu and 0.6 wt% Zn in 95 samples collected from 1–125 m below the seafloor. The periodic release of high-temperature hydrothermal fluids at the same location for several thousand years with intermittent periods of hydrothermal quiesence is the dominating process in the formation of the TAG hydrothermal mound. Distinct geochemical, mineralogical and isotopic zonation as well as a complex assemblage of sulfide-anhydrite-quartz bearing breccias can be related to this process. Geochemical depth profiles indicate extremely low base and trace element concentrations for the interior of the mound, which clearly contrasts with published analyses of samples collected from the surface of the TAG mound. This is explained by continued zone refining during which metals were mobilized from the interior of the mound by upwelling, hot (>350 °C) hydrothermal fluids. Mixing of these fluids with infiltrating ambient seawater subsequently caused redeposition of metals close to the mound-seawater interface. The sulfur isotopic composition of bulk sulfides (+4.4 to +8.2‰δ³⁴S; average +6.5‰) is unusually heavy when compared to other sediment-free mid-ocean ridge deposits and implies the introduction of heavy seawater sulfur to the hydrothermal fluid. The slight increase in sulfur isotope ratios with depth and distinct variations between early, disseminated sulfides related to wallrock alteration, and massive as well as late vein sulfides indicates widespread entrainment of seawater deep into the system. Fluid inclusion measurements in quartz and anhydrite reveal high formation temperatures throughout the TAG mound (up to 390 °C) at one time with an overall increase in trapping temperatures with depth. Lower formation temperatures close to the paleo-seafloor indicate local entrainment of seawater into the mound. Formation temperatures for a central anhydrite-bearing zone range from 340–360 °C and are slightly lower than the exit temperature of hydrothermal fluids presently venting at the Black Smoker Complex (360–369 °C). Fluid inclusions in quartz and anhydrite from the stockwork zone are characterized by formation temperatures higher than 375 °C, indicating conductive cooling of the hydrothermal fluids or mixing with ambient seawater prior to venting. Formation temperatures for quartz from an area of extremely low heat flow at the western side of the mound reach up to 390 °C, implying that this area was once part of a high-temperature hydrothermal upflow zone. The low heat flow and the absence of anhydrite within this part of the mound are strong indications that the recent pulse of high-temperature hydrothermal activity is not affecting this area and provides evidence for significant changes in the fluid flow regime underneath the deposit between hydrothermal cycles. Received: 16 November 1998 / Accepted: 19 August 1999     

Determination of the mechanism of cation ordering in magnesioferrite (MgFe2O4) from the time- and temperature-dependence of magnetic susceptibility

The kinetics of non-convergent cation ordering in MgFe₂O₄ have been studied by measuring the Curie temperature (T _c) of synthetic samples as a function of isothermal annealing time. The starting material was a synthetic sample of near-stoichiometric MgFe₂O₄, synthesised from the oxides in air and quenched from 900 °C in water. Ordering experiments were performed using small chips of this material and annealing them at temperatures between 450 °C and 600 °C. The chips were periodically removed from the furnace, and their Curie temperatures were determined from measurements of alternating-field magnetic susceptibility (χ) as a function of temperature (T) to 400 °C. The Curie temperature of MgFe₂O₄ is very sensitive to the intracrystalline distribution of Fe³⁺ and Mg cations between tetrahedral and octahedral sites of the spinel crystal structure, and hence provides a very sensitive probe of the cation ordering process. The χ-T curve for the starting material displays a single sharp magnetic transition at a temperature of 303 °C. During isothermal annealing, the χ-T curve develops two distinct magnetic transitions; the first at a temperature corresponding to T _c for the disordered starting material and the second at a higher temperature corresponding to T _c for the equilibrium ordered phase. The size of the magnetic signal from the ordered phase increases smoothly as a function of time, until equilibrium is approached and the shape of the χ-T curve corresponds to a single sharp magnetic transition for the homogeneous ordered phase. These observations demonstrate that cation ordering in MgFe₂O₄ proceeds via a heterogeneous mechanism, involving the nucleation and growth of fine-scale domains of the ordered phase within a matrix of disordered material. Disordering experiments were performed by taking material equilibrated at 558 °C and annealing it at 695 °C. The mechanism of isothermal disordering is shown to involve nucleation and growth of disordered domains within an ordered matrix, combined with continuous disordering of the ordered matrix. This mixed mechanism of disordering may provide an explanation for the difference between the rates of ordering and disordering observed in MgFe₂O₄ using X-ray diffraction. The origin of the heterogeneous ordering/disordering mechanism is discussed in terms of the Ginzburg-Landau rate law. It is argued that heterogeneous mechanisms are likely to occur in kinetic experiments performed far from equilibrium, whereas a homogeneous mechanism may operate under slow equilibrium cooling. The implications of these observations for geospeedometry are discussed. Received: 12 May 1998 / Accepted: 25 June 1998     

Zeolite studies III: On natural phillipsite,gismondite, harmotome,chabazite, and gmelinite

Part I: Chemical and structural effects of cation-exchange Attempts were made to prepare, by appropriate exchange methods, homoionic samples of phillipsite, gismondite, harmotome, chabazite and gmelinite containing Ba, Ca, K, Na or Li-ions. Powdered natural samples were used as starting material. All samples were analysed chemically before and after the cation exchange. The results of the analyses demonstrated clearly that the „exchange capacity“ depends on the method used, the structure of the zeolite and the nature of the cation involved in the exchange. The analyses also disclosed the important fact that the ratio in Mole % of the sum of exchangeable cations: Al₂O₃ of the natural and of the exchanged samples is generally <1, and can be as low as 0.74. Examples are presented where cation exchange results in a substantial change in the framework structure. Part II: Dehydration behavior and structural changes at elevated temperatures Samples of the natural zeolites mentioned above, and of some of their cation exchange products were dehydrated in air of controlled humidity up to 600° C. The slopes of the weight loss curves of chabazite and gmelinite are continuous, whereas those of phillipsite, gismondite, and harmotome show a discontinuity between 90–190° C, indicating the existence of two discrete hydrated phases for each of these zeolites. High temperature x-ray studies of powdered samples confirmed this result. The high temperature hydrates of phillipsite, gismondite, and harmotome persist reversibly up to approximately 230° C. At higher temperatures, new probably anhydrous phases form. Gmelinite, at 240° C, transforms irreversibly to anhydrous gmelinite which is stable up to >700° C. The transition was studied by single crystal techniques. The chabazite structure remains intact up to >700° C. The absolute water content and the dehydration behavoir of the zeolites investigated are primarily dependent on the nature of the exchange cation. The structural changes at elevated temperatures are determined by the silica alumina framework. Part III: Hydrothermal stability* and interconversions The stability of phillipsite, gismondite, harmotome, chabazite, gmelinite, their exchange products, and of the synthetic Linde zeolites Faujasite and Type A was studied in the temperature range between 150° and 350° C at a constant pressure of 1000 atm of H₂O. Between 180° and 260° C all examined Sodium and Calcium zeolites were metastable with respect to analcite (wairakite**). Phillipsite and sodium-rich zeolites generally converted to analcite (wairakite) directly. Ca^ex-chabazite and Ca^ex-gmelinite formed phillipsite, whereas Ca-gismondite and Ca-Type A formed natrolite as intermediate phases. Li-gmelinite converted to bikitaite***. (This represents the first successful preparation of natrolite and bikitaite. Attempts starting from gels or glasses have been unsuccessful so far.) Ba-gmelinite converted to harmotome at 250° C. This transformation was studied microscopically and by single crystal x-ray techniques. The transformations that take place on hydrothermal treatment as well as on low temperature cation exchange of zeolites (see Part I) indicate that, unlike the conditions prevailing in clays, the type of cation and the ratio of cations in the exchange positions have an important influence on the structure of the silicaalumina-oxygen framework. This explains two phenomena: The lack of solid solution between two potential end members of a solid solution series (for instance phillipsite-gismondite), and the large number of different zeolites in nature, where a great variety in the ratios of available alkali and alkaline earths ions must be expected. Any classification of zeolites becomes still more difficult in view of the fact that conversions among different groups (chabazite → phillipsite) and different structures (three-dimensional framework → fibre) take place relatively easily. Contribution No. 59–92, College of Mineral Industries, The Pennsylvania State University, University Park, Pennsylvania.     

A new approach to geobarometry by combining fluid inclusion and clumped isotope thermometry in hydrothermal carbonates

This study presents a new approach to geobarometry by combining fluid inclusion and clumped isotope (Δ₄₇) thermometry on carbonate minerals. The offset between homogenisation temperatures of primary fluid inclusions with known composition and Δ₄₇ temperatures of the host mineral allows a direct estimation of the fluid pressure at the time of carbonate crystallisation. This new approach is illustrated via hydrothermal dolomite samples from the Variscan foreland fold‐and‐thrust belt in northern Spain. Clumped isotope analyses yield crystallisation temperatures (107–168°C) which are higher than homogenisation temperatures in corresponding samples (95–145°C). The calculated pressure values suggest that dolomitizing fluids were overpressured during formation of zebra dolomite textures, whereas lower pressures are obtained for dolomite cement from breccia textures. This new approach to geobarometry opens up the possibility of estimating the pressure of carbonate crystallisation and has potential applications in diagenesis, basin analysis, ore geology and tectonics.     

Kinetics of dehydration of Ca-montmorillonite

Isothermal thermogravimetric experiments have been carried out to determine the reaction kinetics of the dehydration processes in fuller's earth, a natural Ca-montmorillonite. Dehydration in swelling clays is a complex reaction, and analysis of the thermogravimetric data using empirical rate equations and time-transformation analysis reveals that the nature of the rate controlling mechanism is dependent upon both the temperature regime of the sample as well as the extent of reaction. For fuller's earth, we find that the dehydration kinetics are dominated by a nucleation and growth mechanism at low temperatures and fractions transformed (stage I), but above 90 °C the last stages of the reaction are diffusion controlled (stage II). The activation energy for dehydration during stage I is around 35 kJ · mol⁻¹, whereas the removal of water during stage II requires an activation energy of around 50 kJ · mol⁻¹. These two stages of dehydration are associated with primary collapse of the interlayer (stage I) and movement of water that is hydrated to cations within the interlayer (stage II). Received: 28 August 1998 / Revised, accepted: 27 January 1999     

Chemical alteration of volcanic glasses and related control by secondary minerals: Experimental studies

A detailed experimental mineralogical and geochemical study on hydrothermal alteration processes of volcanic glasses with a different chemical composition and leucites sampled in the Roman Comagmatic Region was carried out. 2g samples of different grain sizes and 50 ml of deionized water or seawater were sealed in bronze Teflon-lined autoclaves and placed in a rotating sample-holder at 200°C. The internal pressure was 16.2 bars. At arbitrary intervals, the pressure vessel was quenched to room temperature and both solids and solutions were separated by filtration. The solids were used to identify possible alteration products by means of X-ray powder diffraction, SEM and microprobe analyses. In all deionized water systems the contact solution reached pH basic values, but in the various systems the consumption of H⁺ ions occurred in different ways. This is probably linked to different concentrations of released cations. In seawater systems, however, pH values initially fell sharply but subsequently increased slightly. After 2 days, cation concentrations were clearly buffered by reaction products in all solutions. These were mainly zeolites and clay minerals. The following crystallization sequences in all glass/deionized water experiments were observed: the first reaction product was represented by phillipsite, followed by analcime and illite in the alkali-trachytic run; chabasite and analcime in the phonolitic-tephritic run; analcime and finally feldspar in shoshonitic and trachytic runs. Phillipsite and smectite crystallized together only in the basaltic run. In leucite/deionized water systems, however, the first reaction phase was illite, followed by analcime and then phillipsite. The reaction products in glass/seawater systems were smectite and anydrite. The chemical composition of synthetic zeolites was clearly controlled by the chemistry of initial glasses. These synthetic zeolites are like the natural ones in volcaniclastic products from central and southern Italy. This suggests that extensive zeolitization processes of these volcaniclastic rocks may occur through interaction of volcanic glasses with fluids at a very low salinity and a temperature close to 200°C.     

Time calibration of a P–T path from a Variscan high-temperature low-pressure metamorphic complex (Bayerische Wald, Germany), and the detection of inherited monazite

A temperature–time path was constructed for high-temperature low-pressure (HT–LP) migmatites of the Bayerische Wald, internal zone of the Variscan belt, Germany. The migmatites are characterised by prograde biotite dehydration melting, peak metamorphic conditions of approximately 850 °C and 0.5–0.7 GPa and retrograde melt crystallisation at 800 °C. The time-calibration of the pressure–temperature path is based on U–Pb dating of single zircon and monazite grains and titanite separates, on ⁴⁰Ar/³⁹Ar ages obtained by incremental heating experiments on hornblende separates, single grains of biotite and K-feldspar, and on ⁴⁰Ar/³⁹Ar spot fusion ages of biotite determined in situ from sample sections. Additionally, crude estimates of the duration of peak metamorphism were derived from garnet zoning patterns, suggesting that peak temperatures of 850 °C cannot have prevailed much longer than 2.5 Ma. The temperature–time paths obtained for two areas approximately 30 km apart do not differ from each other considerably. U–Pb zircon ages reflect crystallisation from melt at 850–800 °C at 323 Ma (southeastern area) and 326 Ma (northwestern area). The U–Pb ages of monazite mainly coincide with those from zircon but are complicated by variable degrees of inheritance. The preservation of inherited monazite and the presence of excess ²⁰⁶Pb resulting from the incorporation of excess ²³⁰Th in monazite formed during HT–LP metamorphism suggest that monazite ages in the migmatites of the Bayerische Wald reflect crystallisation from melt at 850–800 °C and persistence of older grains at these temperatures during a comparatively short thermal peak. The U–Pb ages of titanite (321 Ma) and ⁴⁰Ar/³⁹Ar ages of hornblende (322–316 Ma) and biotite (313–309 Ma) reflect cooling through the respective closure temperatures of approximately 700, 570–500 and 345–310 °C published in the literature. Most of the feldspars' ages (305–296 Ma) probably record cooling below 150–300 °C, while two grains most likely have higher closure temperatures. The temperature–time paths are characterised by a short thermal peak, by moderate average cooling rates and by a decrease in cooling rates from 100 °C/my at temperatures between 850–800 and 700 °C to 11–16 °C/my at temperatures down to 345–310 °C. Further cooling to feldspar closure for Ar was probably even slower. The lack of decompressional features, the moderate average cooling rates and the decline of cooling rates with time are not easily reconciled with a model of asthenospheric heating, rapid uplift and extension due to lithospheric delamination as proposed elsewhere. Instead, the high peak temperatures at comparatively shallow crustal levels along with the short thermal peak require external advective heating by hot mafic or ultramafic material. Received: 7 July 1999 / Accepted: 28 October 1999     

High temperature dehydroxylation of muscovite-2M1: a kinetic study by in situ XRPD

Muscovite-2M₁ shows a major phase transition at about 800°C, which is generally attributed in the literature to the structural dehydroxylation process, although a number of structural models have been proposed for the dehydroxylated phase, and different transformation mechanisms have also been put forward. The observed first order transformation involves an increase in the cell volume, and it is not clear to date how the cell expansion is related to the loss of hydroxyl groups. The phase change has been re-investigated here by in situ high temperature powder diffraction, both in non-isothermal and isothermal modes, to combine for the first time the structural and the kinetic interpretation of the transformation. The results unequivocally confirm that the reaction taking place in the temperature range 700–1000°C is truly a dehydroxylation process, involving the nucleation and growth of the high temperature dehydroxylated phase, having Al in 5-fold coordination. Structural simulations of the basal peaks of the powder diffraction patterns indicate that the model originally proposed by Udagawa et al. (1974) for the dehydroxylated phase correctly describes the high temperature phase. The kinetic analysis of the isothermal data using an Avrami-type model yields values for the reaction order compatible with a reaction mechanism limited by a monodimensional diffusion step. Apparent activation energy of the process in vacuum is about 251 kJ/mol. Experiments carried out at temperatures much higher than the onset temperature of the reaction show that the dehydroxylation reaction overlaps with the reaction of formation of mullite, the final product in the reaction pathway. Received: 24 April 1998 / Revised, accepted: 12 October 1998     

Radiation-induced defects in quartz. IV. Thermal properties and implications

The thermal stabilities and decay kinetics of three peroxy radicals (Centers #1, B and B′) and three other radiation-induced defects (#3, C′ and E₁′) in natural quartz from the high-grade McArthur River uranium deposit (Athabasca basin, Canada) have been investigated by isochronal and isothermal annealing experiments and electron paramagnetic resonance (EPR) spectroscopy. Single-crystal EPR spectra of isochronally (2 h) annealed quartz show that these centers all grow in intensity to 280°C and then decay with further increase in temperature, but their disappearance temperatures differ markedly and depend on the initial concentrations (e.g., Center #1 in a dark smoky quartz is annealed out at 380°C, B and B′ at 420°C and #3 and C′ at 580°C). The isothermal decay processes of these centers are all of the second order type. The calculated activation energies for the peroxy radicals [#1 and B + B′ at 0.36 (9) and 0.83 (8) eV, respectively] are smaller than those of Centers #3, C′ and E₁′ [1.09 (8), 1.24 (8) and 1.45 (7) eV, respectively]. Gamma-ray irradiations of thermally bleached quartz restore a fraction of the peroxy radicals, suggesting that their diamagnetic precursors are stable up to at least 800°C. The unusual decay characteristics of “peroxy radicals” in quartz reported in the literature are shown to most likely arise from multiple radiation-induced defects. These results have implications for not only applications of peroxy radicals in quartz for EPR dating but also better understanding of thermoluminescence and cathodoluminescence spectra of this mineral.     

Alteration of the Callovo–Oxfordian clay from Meuse-Haute Marne underground laboratory (France) by alkaline solution. I. A XRD and CEC study

The reaction of the clay fraction of the Callovo–Oxfordian hard shale formation hosting the French underground laboratory site, with high pH NaOH, KOH and Ca(OH)₂ solutions has been investigated through closed system experiments at 60, 90 and 120 °C over 6, 24 and 168 h. The mineralogical composition of the run samples has been determined using X-ray diffraction (XRD) of randomly oriented powders showing the formation of different species of zeolites (analcime, chabazite, phillipsite) and Ca silicates (tobermorite, katoite). The phyllosilicates were studied using XRD of oriented preparations and cation exchange capacity measurements. Detrital or diagenetic mica and chlorite in the <2 μm fraction remain unchanged. On the contrary, the smectite and random illite–smectite mixed layer minerals are strongly reactive. The expandable layers of montmorillonite type are selectively dissolved while beidellitic ones survive or are transitionally formed.     

Wollastonite polytypes in the CaO-SiO2 system.

This work deals with the identification and kinetics of crystallisation of the wollastonite polytypes which form in the system CaO-SiO₂ in the temperature range 700–1000 °C. The kinetics of phase transformations in the system have been determined from analysis of in situ synchrotron X-ray powder diffraction data. Two different systems with the same compositions were investigated: one is more reactive, having grains with a larger surface area per unit volume, the other is less reactive, being composed of grains with a smaller surface area per unit volume. 1T-wollastonite forms first and progressively transforms in an intermediate 1Td-wollastonite disordered form. Both phases in turn transform into 2M-wollastonite polytype. Differences in the polytypes forming and reaction kinetics were observed for the two systems. In the more reactive system, the conversion 1T to 1Td polytypes is the dominant process. The kinetic parameters calculated using the Avrami models fully describe the reaction process of formation of both polytypes and transformation of one polytype into the other. Received: 3 August 1999 / Accepted: 20 March 2000     

X-ray single-crystal study of spinels: in situ heating

 Two MgAl₂O₄ stoichiometric spinel crystals, one natural and one synthetic, were heated from 25 to 950 °C and studied in situ by single-crystal X-ray diffraction. The natural crystal, quenched from 850 °C, was further heated and cooled. Thermal expansion was characterized, and cation partitioning at the various temperatures was determined according to a model purposely constructed for high-temperature bond lengths. It was found that the structural evolution of the samples with temperature depended on order–disorder at room temperature. At the temperatures lower than the beginning of cation exchange, thermal expansion was completely reversible and the oxygen coordinate remained stable in spite of varying temperatures. At the temperature at which cation exchange starts, the disordered samples first tend to order and then to disorder at higher temperatures, at variance with the ordered sample, which tends to disorder steadily. In general, the evolution of the spinel structural state on cooling and heating over the same temperature range and the same time intervals does not follow the same path. In particular, in the 600–950 °C range, only partially reversible order–disorder processes occurred in the time span used for the experiments. Received: 16 July 2001 / Accepted: 8 January 2002     

The evolution of the hydrothermal IOCG system in the Mantoverde district,northern Chile: new evidence from microthermometry and stable isotope geochemistry

New microthermometric data combined with stable isotope geochemistry and paragenetic relationships support a previously suggested cooling–mixing model for the iron oxide–copper–gold mineralization in the Mantoverde district. Fluid inclusions show characteristics of a CO₂-bearing aqueous NaCl ± CaCl₂ salt system. The evolution of the Mantoverde hydrothermal system is characterized by (1) an early hypersaline, high to moderate temperature fluid; (2) a moderate saline, moderate temperature fluid; and (3) a low saline, moderate to low temperature fluid. Early magnetite formation took place at median temperatures of 435.0°C, whereas hematite formed at median temperatures of 334.4°C. The main sulfide mineralization texturally post-dates the iron oxides and occurred before late-stage calcite, which developed at a median temperature of 244.8°C. Boiling occurs only locally and is of no relevance for the ore formation. The microthermometric and stable isotope data are supportive for a fluid cooling and mixing model, and suggestive for a predominantly magmatic–hydrothermal fluid component during the iron oxide and main sulfide mineralization. Thereafter, the incursion of a nonmagmatic fluid of ultimately meteoric or seawater gains more importance.     

Formation and kinetics of porous cordierite from fly ash

Porous cordierite ceramics were prepared from a mixture of coal fly ash and basic magnesium carbonate at 1100-1350℃. Porosity, flexural strength and thermal expansion coefficient of the samples sintered at 1300℃ were estimated to be 26%, 65 MPa and 4.21×10^-6/℃, respectively. The kinetics of the formation progress was investigated by stepwise isothermal dilatometry （SID） accompanied with XRD, SEM and porosity measurement. It was found that the isothermal shrinkage data from SID could be well analyzed to get kinetic parameters according to the erapirical rate equation developed by Makipirtti-Meng, dY/dt=nk（T）Y（1-Y）（Y/1-Y）^（1/n）,where Y is the fractional shrinkage during the sintering process and n is a dimensionless component. The apparent activation energy △E values for 900-1000℃ and 1050-1 150℃ were 1294 and 1778 kJ/mol, respectively.     

Formation of solid bituminous matter in pegmatites: Constraints from experimentally formed organic matter on microporous silicate minerals

The formation of solid bituminous matter (SBM) on surfaces of microporous silicates was experimentally studied at pressure and temperature conditions typical of late-stage magmatic and hydrothermal processes. Aliquots of microporous silicate minerals (zorite and kuzmenkoite-Mn, Lovozero Alkaline Massif, Kola Peninsula, Russia) were exposed to solid or liquid organic carbon sources (natural brown coal and liquid 1-hexene for synthesis purposes) in a 0.1 M NaCl-solution for 7 days, at constant pressure (50 MPa), and at three individual temperatures (200, 275, and 300 °C). No thermal decomposition of the solid organic sources happened at 200 °C and only a thin film of brown coal derivatives on the silicates’ surfaces and no formation of SBM were observed at 275 °C and 300 °C. But solid bituminous matter on the surfaces of both microporous silicates were detected in experiments with liquid 1-hexene as organic carbon source and at temperatures of 275 °C and 300 °C with a more pronounced formation of SBM at 300 °C compared to 275 °C. The aromatic and aliphatic hydrocarbons, as well as alcoholic compounds of the experimentally produced SBM are similar, if not even partly identical, with natural SBM occurrences of the Khibiny and Lovozero Massifs, Kola Peninsula, Russia, and from the Viitaniemi granitic pegmatite, Finland, as shown by FT-IR and ¹H NMR spectroscopy. This strengthens the hypothesis of formation of natural solid bituminous matter by catalytic reactions between microporous Ti-, Nb- and Zr-silicates and hydrocarbons at postmagmatic hydrothermal conditions.     

Kinetics of the Coesite-Quartz Transition: Application to the Exhumation of Ultrahigh-Pressure Rocks

The kinetics of the quartz–coesite phase transition hasbeen studied in situ by X-ray diffraction in the 2·1–3·2GPa, 500–1010°C pressure–temperature range.Analysis of the data within Cahn's model of nucleation and growthat grain boundaries reveals that the prograde and retrogradereactions have different kinetics. The quartz