首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 265 毫秒
1.
Semi‐pelitic rocks ranging in grade from the prehnite–pumpellyite to the greenschist facies from south‐eastern Otago, New Zealand, have been investigated in order to evaluate the reactions leading to formation and breakdown of stilpnomelane. Detrital grains of mica and chlorite along with fine‐grained authigenic illite and chlorite occur in lower‐grade rocks with compactional fabric parallel to bedding. At higher grades, detrital grains have undergone dissolution, and metamorphic phyllosilicates have crystallized with preferred orientation (sub)parallel to bedding, leading to slaty cleavage. Stilpnomelane is found in metapelites of the pumpellyite–actinolite facies and the chlorite zone of the greenschist facies, but only rarely in the biotite zone of the greenschist facies. Illite or phengite is ubiquitous, whereas chlorite occurs only rarely with stilpnomelane upgrade of the pumpellyite‐out isograd. Chemical and textural relationships suggest that stilpnomelane formed from chlorite, phengite, quartz, K‐feldspar and iron oxides. Stilpnomelane was produced by grain‐boundary replacement of chlorite and by precipitation from solution, overprinting earlier textures. Some relict 14 Å chlorite layers are observed by TEM to be in the process of transforming to 12 Å stilpnomelane layers. The AEM analyses show that Fe is strongly partitioned over Mg into stilpnomelane relative to chlorite (KD≈2.5) and into chlorite relative to phengite (KD≈1.9). Modified A′FM diagrams, projected from the measured phengite composition rather than from ideal KAl3Si3O10(OH)2, are used to elucidate reactions among chlorite, stilpnomelane, phengite and biotite. In addition to pressure, temperature and bulk rock composition, the stilpnomelane‐in isograd is controlled by variations in K, Fe3+/Fe2+, O/OH and H2O contents, and the locus of the isograd is expected to vary in rocks of different oxidation states and permeabilities. Biotite, quartz and less phengitic muscovite form from stilpnomelane, chlorite and phengite in the biotite zone. Projection of bulk rock compositions from phengite, NaAlO2, SiO2 and H2O reveals that they lie close to the polyhedra defined by the A′FM minerals and albite. Other extended A′FM diagrams, such as one projected from phengite, NaAlO2, CaAl2O4, SiO2 and H2O, may prove useful in the evaluation of other low‐grade assemblages.  相似文献   

2.
This investigation deals with the Nigerian iron-formations and their host rocks and is based on about 560 mineral analyses (electron-microprobe) and 93 whole-rock analyses (64 iron-formations and 29 host rocks). The manganese-rich and Al-bearing iron-formations occurring in various schist belts of the northern and southern part of West-Nigeria consist of the magnetite-free silicate, the magnetite–silicate and the quartz-rich hematite facies.Iron-formations and host rocks originated from submarine-volcanogenic exhalations enriched in Fe, Mn and CO2 and from Al2O3, SiO2 and alkali (K2O and Na2O)-rich continental-derived pelitic to psammitic material. From these sources and their interaction and controlled by the volcanogenic activity, differently composed protoliths were deposited in the marine basin during the Birimian time. Subsequent metamorphism of greenschist to low amphibolite facies conditions during the Eburnian time led to the formation of the metaprotoliths of the magnetite–silicate (consisting of predominantly magnetite and quartz and subordinate of garnet and amphibole), the silicate facies (consisting of garnet, amphibole and rarely Mn-bearing ilmenite and quartz) and the metasediment phyllite. Garnets are predominantly almandine–spessartine solid solutions, whereas amphiboles are Mn and Ca-bearing grunerite–cummingtonite solid solutions. In the course of a second tectono-metamorphic event of Pan-African age, the magnetite–silicate facies iron-formation/phyllite association was transformed into the hematite facies and muscovite/biotite schists, whereas the silicate facies is characterized by extensive silicification features. The hematite facies and the silicified silicate facies are restricted to southern Nigeria where the second and heterogeneous tectono-metamorphic event is more pronounced (amphibolite facies conditions) than in northern Nigeria.The genesis, summarized as the metamorphic model, shows that the carbonate-rich (siderite, rhodochrosite and subordinate magnesite and calcite) protoliths were metamorphically transformed into the silicate and magnetite–silicate facies. The separation of Mn and Fe, leading to manganese-bearing iron-formations and iron-bearing manganese-formations was explained by varying pH-conditions, under which siderite (pH: 6.8–9.4) and rhodochrosite (pH: 9–11) precipitated.Similar to the Gunfit and Biwabik iron-formations of Minnesota, USA, the iron-formation of Bingi (Maru schist belt), now present in the form of the fayalite bearing silicate facies, was overprinted by contact metamorphism caused by a gabbro intrusion.  相似文献   

3.
Sulfur and carbon contents and isotope ratios are reported for five Archean iron-formations, Helen, Nakina and Finlayson, Lumby and Bending Lake areas, distributed across 850 km of the Canadian shield all 2.7 Ga-old.A δ34S profile through a complete stratigraphic column (oxide facies excluded) of the Helen iron-formation shows a δ34S range of 30.2‰, mean δ34S value of 2.5‰ and a standard deviation (δi) of 7.3‰ In sharp contrast to the sulfide and siderite facies, the oxide facies in the column shows a uniform δ34S value close to zero. The δ34S values obtained for the other four iron-formations are again wide ranging, highly variable in the sulfide and pyrite—siderite facies, but uniform and close to zero for the oxide facies.The carbon in the oxide, siderite, chert facies has δ13C values of +2.3 to −1.1‰ in the range of Phanerozoic marine carbonates. However, the carbonates in the graphite rich sulfide facies have δ13C values as low as −7.6‰. The mixing of reduced carbon with marine carbonate is suggested to explain the light carbonate values. The reduced carbon associated with the light carbonate is also relatively light at up to δ13Corg = 33.5‰, but is in the range of other Precambrian values. Distal, high temperature, abiogenic sulfate reduction as a source of highly fractionated sulfides in the Archean iron-formations is ruled out on the basis of both isotopic and geologic evidence. It is concluded that only the bacterial reduction of sulfate at low temperatures could produce the wide ranging, highly variable δ34S values exhibited by these sulfides over large areas.  相似文献   

4.
The structure of H2O-saturated silicate melts, coexisting silicate-saturated aqueous solutions, and supercritical silicate liquids in the system Na2O·4SiO2–H2O has been characterized with the sample at high temperature and pressure in a hydrothermal diamond anvil cell (HDAC). Structural information was obtained with confocal microRaman and with FTIR microscopy. Fluids and melts were examined along pressure-temperature trajectories defined by the isochores of H2O at nominal densities, ρfluid, (from EOS of pure H2O) of 0.90 and 0.78 g/cm3. With ρfluid = 0.78 g/cm3, water-saturated melt and silicate-saturated aqueous fluid coexist to the highest temperature (800 °C) and pressure (677 MPa), whereas with ρfluid = 0.90 g/cm3, a homogeneous single-phase liquid phase exists through the temperature and pressure range (25–800 °C, 0.1–1033 MPa). Less than 5 vol% quartz precipitates near 650 °C in both experimental series, thus driving Na/Si-ratios of melt + fluid phase assemblages to higher values than that of the Na2O·4SiO2 starting material.Molecular H2O (H2O°) and structurally bonded OH groups were observed in coexisting melts and fluids as well as in supercritical liquids. Their OH/(H2O)-ratio is positively correlated with temperature. The OH/(H2O)° in melts is greater than in coexisting fluids. Structural units of Q3, Q2, Q1, and Q0 type are observed in all phases under all conditions. An expression of the form, 12Q3 + 13H2O2Q2 + 6Q1 + 4Q0, describes the equilibrium among those structural units. This equilibrium shifts to the right with increasing pressure and temperature with a ΔH of the reaction near 425 kJ/mol.  相似文献   

5.
Prograde mineral assemblages and compositions have been predicted for pelitic schist in the 10 component system Na2O–K2O–CaO–MnO–FeO–MgO–Al2O3–SiO2–CO2–H2O for three cases of prograde metamorphism and fluid-rock interaction: (1) increasing temperature (T) at constant pressure (P) and constant pore fluid volume (1%) without infiltration (no-infiltration case); (2) increasing T at constant P accompanied by sufficient fluid infiltration that fluid composition is at all times constant (large-flux case); and (3) increasing T at constantP accompanied by a timeintegrated fluid flux f 104 cm3 cm 2 (intermediate-flux case). Stable mineral assemblages and compositions were calculated by solving a system of non-linear equations that specify mass balance and chemical equilibrium between minerals and fluid. The model pelitic system includes quartz, muscovite, plagioclasc, chlorite, ankerite, siderite, biotite, garnet, staurolite, andalusite, kyanite, sillimanite, K-feldspar, and a coexisting, binary H2O–CO2 fluid. Specifically, prograde thermal metamorphism was modelled for Shaw's (1956) average low-grade pelite and for a moderate range of bulk rock compositions at P=3, 5, and 7 kb and initial fluids with Xco 2 o =0.02–0.40. The model predicts a carbonate-bearing mineral assemblage for average pelite under chlorite zone conditions composed of quartz, muscovite, albite, chlorite, ankerite, and siderite. The mineral assemblages predicted for the noinfiltration case are unlike those typically observed in regional metamorphic terranes. Simulations of metamorphism for the large-flux and intermediate-flux cases, however, reproduce the sequence of mineral assemblages observed in normal Barrovian regional metamorphic terranes. These results suggest that regional metamorphism of pelitic schists is typically associated with infiltration of significant quantities of aqueous fluid.  相似文献   

6.
The system KAlO2–MgO–SiO2–H2O–CO2 has long been used as a model for the processes of granulite-facies metamorphism and the development of orthopyroxene-bearing mineral assemblages through the breakdown of biotite-bearing assemblages. There has been considerable controversy regarding the role of carbon dioxide in metamorphism and partial melting. We performed new experiments in this system (at pressures of 342 to 1500 MPa with T between 710 and 1045 °C and X Fl H2O between 0.05 and 1.00), accurately locating most of the dehydration and melting equilibria in P-T-X Fl H2O space. The most important primary result is that the univariant reaction Phl + Qtz + Fl = En + Sa + melt must be almost coincident with the fluid-absent reaction (Phl + Qtz = En + Sa + melt) in the CO2-free subsystem. In conjunction with the results of previous measurements of CO2 solubility in silicate melts and phase equilibrium experiments, our theoretical analysis and experiments suggest that CO2 cannot act as a flux for partial melting. Crustal melting in the presence of H2O–CO2 mixed fluids will always occur at temperatures higher than with pure H2O fluid present. Magmas produced by such melting will be granitic (s.l.) in composition, with relatively high SiO2 and low MgO contents, irrespective of the H2O–CO2 ratio in any coexisting fluid phase. We find no evidence that lamprophyric magmas could be generated by partial fusion of quartz-saturated crustal rocks. The granitic melts formed will not contain appreciable dissolved CO2. The channelled passage of hot CO2-rich fluids can cause local dehydration of the rocks through which they pass. In rock-dominated (as opposed to fluid-dominated) systems, minor partial melting can also occur in veins initially filled with CO2-rich fluid, as dehydration and local disequilibrium drive the fluid towards H2O-rich compositions. However, CO2 is unlikely to be a significant agent in promoting regional granulite-grade metamorphism, melting, magma generation, metasomatism or long-range silicate mass transfer in Earth's crust. The most viable model for the development of granulite-facies rocks involves the processes of fluid-absent partial melting and withdrawal of the melt phase to higher crustal levels. Received: 28 November 1996 / Accepted: 25 June 1997  相似文献   

7.
Water Content of Basalt Erupted on the ocean floor   总被引：1，自引：0，他引：1  
Deep sea pillow basalts dredged from the ocean floor show that vesicularity changes with composition as well as with depth. Alkalic basalts are more vesicular than tholeiitic basalts erupted at the same depth. The vesicularity data, when related to experimentally determined solubility of water in basalt, indicate that K-poor oceanic tholeiites originally contained about 0.25 percent water, Hawaiian tholeiites of intermediate K-content, about 0.5 percent water, and alkali-rich basalts, about 0.9 percent water. Analyses of fresh basalt pillows show a systematic increase of H2O+ as the rocks become more alkalic. K-poor oceanic tholeiites contain 0.06–0.42 percent H2O+, Hawaiian tholeiites, 0.31–0.60 percent H2O+, and alkali rich basalts 0.49–0.98 percent H2O+. The contents of K2O, P2O5, F, and Cl increase directly with an increase in H2O+ content such that at 1.0 weight percent H2O+, K2O is 1.58 percent, P2O5 is 0.55 percent, F is 0.07 percent, and Cl is 0.1 percent. The measured weight percent of deuterium on the rim of one Hawaiian pillow is –6.0 (relative to SMOW); this value, which is similar to other indications of magmatic water, suggests that no appreciable sea water was absorbed by the pillow during or subsequent to eruption on the ocean floor.Concentrations of volatile constituents in the alkali basalt melts relative to tholeiitic melts can be explained by varying degrees of partial melting of mantle material or by fractional crystallization of a magma batch.Publication authorized by the Director, U.S. Geological Survey.  相似文献   

8.
Temperature and H2O activity can be determined with high precision using metamorphic mineral assemblages that define both a dehydration equilibrium and a temperature-sensitive cation-exchange equilibrium. Such determinations are obtained by applying the Gibbs method and then integrating two resulting differential equations, as illustrated here for the assemblage garnet-chlorite-quartz. The first equation, a geothermometer that monitors temperature based upon Fe–Mg exchange between garnet and chlorite, was calibrated using rocks at Pecos Baldy, New Mexico: 0=0.05 P(bars)–19.02 T(K)+4607 ln K D+24,156 with errors of ±8°C based upon analytical precision. The second equation monitors differences in the activity of water between specimens (1) and (2): 0=(0.1 X Mg–chl, 1 – 2.05)(P 2 – P 1) +[–33.02+5.96 ln(X Fe–chl, 1/X alm, 1)][T 2–T 1 –2.67 RT 1ln[a(H2O)2/a(H2O)1] +5.96 T 1ln(X Fe–chl, 2 X alm, 1/X Fe–chl, 1 X alm, 2).For samples equilibrated at the same pressure and temperature, microprobe analytical errors of 1% limit precision to ±0.01 a(H2O). For samples equilibrated at the same pressure but variable temperature, uncertainty of ±8°C limits precision to ±0.06 a(H2O). Extreme presure sensitivity requires that the H2O-barometer be applied only to rocks where pressure gradients are absent or well-constrained. The geothermometer gives temperatures in agreement with two other garnet-chlorite geothermometers (Dickenson and Hewitt 1986; Ghent et al. 1987) and with garnet-biotite geothermometry (ferry and Spear 1978) over the temperature range 350–520°C. Application of the relative H2O barometer shows variations in the activity of water approaching 0.30 in several study areas. Either pelitic schists commonly equilibrate with a fluid that is not pure H2O, or some pelitic rocks undergo metamorphism in the absence of a free fluid phase.  相似文献   

9.
An experimental study of the solubility of Pt and Pd sulfides and Au metal in aqueous bisulfide solutions was conducted at temperatures from 200° to 350 °C and at saturated vapor pressure. A 500-mL Bridgemantype pressure vessel constructed of titanium, and equipped with a motor-driven magnetic stirrer was employed. The pH and the oxidation state were buffered by the coexistence of H2S/HS–/SO inf4 sup2– . The pH at temperature was calculated to be in the range 5.91–9.43, and S was 0.3–2.2 m. Under the experimental conditions, the measured solubility of gold is about two to three orders of magnitude greater than that of either platinum and palladium, and the measured solubility of platinum is, in general, approximately equal to that of palladium, in molal units. The solubilities are found to be in the range: platinum 4–800 ppb, palladium 1–400 ppb, and gold 2–300 ppm. The solubility data can be modeled adequately using the following reactions: Au+H2S+HH–=Au(HS) 2 – +1/2H2 (K14); PtS+HS–+H+=Pt (HS) 2 0 (K15); PdS+HS–+H+=Pd (HS) 2 0 (K16); PtS2+H2=Pt (HS) 2 0 (K21).With equilibrium constants determined as follows (errors represent two standard deviations): Preliminary measurements of the solubilities of metallic Pt, Pd and Au as hydroxide complexes were also conducted using a second titanium pressure vessel, at temperatures of 200° to 350 °C and vapor saturation pressure, with pH and the oxidation state controlled or buffered by adding known amounts of NaOH and H2 gas. The concentration of NaOH was in the range 0.01–1.3 m, and the partial pressure of H2 at 200 °C was 62–275 bars, initially. Under the temperature and pressure conditions of these experiments, the solubility of platinum in 1 m NaOH solution is less than 100 ppb, that of palladium is less than 10 ppb and that of gold is less than 0.2 ppm; and in 0.01 m NaOH solutions, both Pt and Pd solubilities are less than 1 ppb. These data indicate that the contributions of hydroxide complexes to the total solubilities in the bisulfide runs, where the pH was in the range of 5.9–9.4, are negligible. The concentrations of both Pt and Pd as bisulfide complexes in the Salton Sea geothermal system predicted using the stability constants determined in this work agree very well with those values measured by McKibben et al. (1990). This calculation strongly suggests that the PGE are transported in moderately reducing, near neutral hydrothermal fluids as bisulfide complexes, as is gold. However, the much lower maximum solubility of the PGE relative to gold severely constrains models of re genesis, and may explain the relative rarity of hydrothermal PGE deposits compared to the relative abundance of hydrothermal Au deposits.  相似文献   

10.
The redox ratio of iron is used as an indicator of solution properties of silicate liquids in the system (SiO–Al2O3–K2O–FeO–Fe2O3–P2O5). Glasses containing 80–85 mol% SiO2 with 1 mol% Fe2O3 and compositions covering a range of K2O/Al2O3 were synthesized at 1400°C in air (fixed fO2). Variations in the ratio FeO/FeO1.5 resulting from the addition of P2O5 are used to determine the solution behavior of phosphorus and its interactions with other cations in the silicate melt. In 80 mol% SiO2 peralkaline melts the redox ratio, expressed as FeO/FeO1.5, is unchanged relative to the reference curve with the addition of 3 mol% P2O5. Yet, the iron redox ratio in the 85 mol% SiO2 potassium aluminosilicate melts is decreased relative to phosphorus-free liquids even for small amounts of P2O5 (0.5 mol%). The redox ratio in peraluminous melts is decreased relative to phosphorus- free liquids at P2O5 concentrations of 3 mol%. In peraluminous liquids, complexing of both Fe+3–O–P+5 and Al+3–O–P+5 occur. The activity coefficient of Fe+3 is decreased because more ferric iron can be accommodated than in phosphorus-free liquids. In peralkaline melts, there is no evidence that P+5 is removing K+ from either Al+3 or Fe+3 species. In chargebalanced melts with 3 mol% Fe2O3 and very high P2O5 concentrations, phosphorus removes K+ from K–O–Fe+3 complexes resulting in a redox increase. P2O5 should be accommodated easily in peraluminous rhyolitic liquids and phosphate saturation may be suppressed relative to metaluminous rhyolites. In peralkaline melts, phosphate solubility may increase as a result of phosphorus complexing with alkalis. The complexing stoichiometry may be variable, however, and the relative influence of peralkalinity versus temperature on phosphate solubility in rhyolitic melts deserves greater attention.  相似文献   

11.
Iron-formation was deposited as a distal facies of ferruginous carbonate turbidites in an open shelf environment in front of a shallow-water carbonate platform at the time of deposition of the Campbellrand carbonate sequence. A subsequent transgression resulted in the deposition of open shelf iron-formation on top of the Campbellrand carbonate platform. Progradational sedimentation coupled with shoaling followed, and an iron-formation sequence represented by the Kuruman and Griquatown Iron-formations was deposited. This sequence consists from the base upwards of stacked open-shelf cycles of altered volcanic ash stilpnomelane lutite beds and autochthonous banded ferhythmite units; toe-of-slope greenalite—siderite rhythmites; platform slope greenalite—siderite rhythmites with grainflow bands; platform edge sideritic orthochemical and allochemical iron-formations; epeiric sea orthochemical and allochemical sideritic, hematitic and greenalitic iron-formations; supratidal disclutites and lacustrine banded greenalite lutite. Landwards, the lacustrine felutites were followed by deltaic chloritic claystone, siltstone and quartz wacke of the Koegas Subgroup.Autochthonous ferhythmites of the Kuruman Iron-formation reach a maximum development in a basin near Prieska, whereas orthochemical and allochemical units are more abundant on the Kaapvaal craton. Iron mineral and chert microbanding in the ferhythmites is attributed to seasonal changes in Eh and pH in the depository and may be related to biological activity. Chert mesobanding in the iron-formations is essentially of an early diagenetic origin.  相似文献   

12.
Samples of granitic rock from south-central Maine contain primary igneous minerals altered by hydrothermal fluids. The reaction mechanisms (by which the over-all mineralogical change during the alteration was accomplished) involve several different mineral-fluid reactions at different reaction sites in the rock. The reactions involve both molecular and charged species in solution. The different reaction sites correspond to alteration of different primary igneous minerals. Biotite is partially converted to chlorite+sphene; microcline to muscovite; plagioclase to various combinations of muscovite, epidote, and calcite. The different reaction sites are linked by exchange of ions: some reaction sites produce ions consumed at other sites and vice versa. Physical conditions during the hydrothermal event are estimated from mineralogical and thermochemical data: P = 3,500 (±300) bars; T =425 ° (± 25 °)C. The fluid was characterized by X CO 2 = 0–0.13; ln([K+]/[H+ ]) = 10.0; ln([Ca2+]/[H+]2)=9.1; ln([Na+]/[H+]) = 10.5; Fe/(Fe+Mg) = 0.95. Amounts of secondary minerals in altered rock, when compared to the inferred mineral reactions that formed them, indicate that small but significant amounts (0.01–0.3mol/ 1,000cm3 altered rock) of CO2, H2O, H+, and K+ were added to the granites by fluids during the alteration, as well as lesser amounts (< 0.01–0.03 mol/1,000cm3 altered rock) of Mg2+, Fe2+, Fe3+, Mn2+, Na+, and Ti4+. The sole element leached from the granitic rocks during alteration was Ca in amounts 0.1–0.3 mol/1,000 cm3 rock. By estimating the composition of the hydrothermal fluids before and after reaction with the granites and by measuring the amount of material added to or subtracted from the granites during the alteration, the amount and volume of hydrothermal fluid involved can be calculated. Two independent calculations require minimum volumes in the range 100–1,000 cm3 fluid/1,000cm3 altered rock to participate in the hydrothermal event.  相似文献   

13.
In the Ultevis District, Northern Sweden, viridine occurs in a potassium rich, microcline-bearing leptite together with quartz, plagioclase, phlogopite, opaques, and a retrogradely formed muscovite. The formation of viridine in this rock may be described by a dealkalization process of microcline, in which opaque minerals are involved. A diffusion of K+ and H2O out of the centres of viridine formation and of H+ into these localities, which is a prerequisite of such type of reaction, may have been favoured by a contemporaneous migmatization of the rock, since the granite-like melts can be regarded as sinks for potassium and water. The following decomposition of viridine is initiated by the reverse reaction. Texturally this is indicated by small microcline seams along the contacts between viridine and quartz grains. The final alkalization of the viridine-quartz assemblage to form muscovite reflects increasing K+/H+-ratios and H2O-contents of the vapor phase and/or decreasing temperatures. The potassium and water amounts necessary for the viridine decomposition reactions may have been generated by the cooling of the granitic liquids. An intergrowth of sillimanite and the muscovite can be described by a degenerated reaction. It can be interpreted as giving way to an equilibration of the alumosilicates und the actual metamorphic conditions, that is here P, T, , a k +, a h + and a Mn 3+.The viridine investigated has the lowest Mn-contents recorded so far for andalusite-type minerals with viridine optics (1.3–2.2 mol-% Mn2SiO5-component). Since no andalusite with lower Mn-contents occurs in the rock and since no other Al2SiO5-phase formed at this metamorphic stage it may be assumed that the incorporation of manganese stabilizes the andalusite structure under the P-, T-, –, a k +-, a h +- and a Mn 3+-conditions verified in this particular leptite.The temperature of viridine formation is estimated to be about 600 ° C, the decomposition of viridine +quartz into the muscovite-sillimanite assemblage may have happened between 650–500 ° C at 6–2.5 kb.  相似文献   

14.
The reaction chloritoid (ctd)=almandine (alm)+diaspore+H2O (CAD) has been reversed using Fe3+-free synthetic chloritoid and almandine, under fO2 conditions of the solid oxygen buffer Fe/FeO (CADWI), and using partially oxidized synthetic minerals under fO2 conditions of the solid oxygen buffer Ni/NiO (CADNNO). Experiments have been conducted between 550 and 700°C, 25 and 45 kbar. The equilibrium pressure and temperature conditions are strongly dependent on the fO2 conditions (CADNNO lies some-what 50°C higher than CADWI). This can be explained by a decrease in aH2O for experiments conducted on the Fe/FeO buffer, and a decrease in actd and aalm (through incorporation of ferric iron preferentially in chloritoid) for experiments conducted on the Ni/NiO buffer. The H2O activity has been calculated using the MRK equation of state, and the values obtained checked against the shift of the equilibrium diaspore=corundum+H2O bracketed on the Fe/FeO buffer and under unbuffered fO2 conditions. For fO2 buffered by the assemblage Fe/FeO, aH2O increases with pressure from about 0.85 at 600°C, 12 kbar to about 0.9 at 605°C, 25 kbar and 1 above 28 kbar. For fO2 buffered by the assemblage Ni/NiO, aH2O=1. The aH2O decrease from Ni/NiO to Fe/FeO is, however, too small to be entirely responsible for the temperature shift between CADNNO and CADWI. In consequence, the amount of ferric iron in almandine and chloritoid growing in the CADNNO experiments must be significant and change along the CADNNO, precluding calculation of the thermodynamic properties of chloritoid from this reaction. Our experimental data obtained on the Fe/FeO buffer are combined, using a thermodynamic analysis, with Ganguly's (1969) reversal of the reaction chloritoid=almandine+corundum +H2O (CAC) on the same oxygen buffer. Experimental brackets are mutually consistent and allow extraction of the thermodynamic parameters H o f,ctd and S octd. Our thermodynamic data are compared with others, generally calculated using Ganguly's bracketing of CACNNO. The agreement between the different data sets is relatively good at low pressure, but becomes rapidly very poor toward high pressure conditions. Using our thermodynamic data for chloritoid and KD=(Fe3+/Al)ctd/(Fe3+/Al)alm estimated from natural assemblages, we have calculated the composition of chloritoid and almandine growing from CADNNO and CACNNO. The Fe3+ content in chloritoid and almandine increases with pressure, from less than 0.038 per FeAl2SiO5(OH)2 formula unit at 10 kbar to at least 0.2 per formula unit above 30 kbar. This implies that chloritoid and almandine do contain Fe3+ in most natural assemblages. The reliability of our results compared to natural systems and thermodynamic data for Mg-chloritoid is tested by comparing the equilibrium conditions for the reaction chloritoid+quartz=garnet (gt)+kyanite+H2O (CQGK), calculated for intermediate Fe–Mg chloritoid and garnet compositions, from the system FASH and from the system MASH. For 0.65<(XFe)gt<0.8, CQKG calculated from FASH and MASH overlap for KD=(Mg/Fe)ctd/(Mg/Fe)gt=2. This is in good agreement with the KD values reported from chloritoid+garnet+quartz+kyanite natural assemblages.  相似文献   

15.
The hydrolysis of the Pd2+ ion in HClO4 solutions was examined at 25–70°C, and the thermodynamic constants of equilibrium K (1)0 and K (2)0were determined for the reactions Pd2+ + H2O = PdOH+ + H+ and Pd2+ + 2H2O = Pd(OH)20 + 2H+, respectively. The values of log K (1)0 = −1.66 ± 0.5 (25°C) and −0.65 ± 0.25 (50°C) and log K (2)0 = −4.34 ± 0.3 (25°C) and −3.80 ± 0.3 (50°C) were derived using the solubility technique at 0.95 confidence level. The values of log K (1)0 = −1.9 ± 0.6 (25°C), −1.0 ± 0.4 (50°C), and −0.5 ± 0.3 (70°C) were obtained by spectrophotometric techniques. The palladium ion is significantly hydrolyzed at elevated temperatures (50–70°C) even in strongly acidic solutions (pH 1–1.5), and its hydrolysis is enhanced with increasing temperature.  相似文献   

16.
Wadeite K2ZrSi3O9 and its analogues K2TiSi3O9 and Cs2ZrSi3O9, synthesized by high-temperature solid-state sintering, have been investigated using powder X-ray diffraction coupled with Rietveld analysis and high-temperature oxide melt solution calorimetry. The crystal chemistry and energetics of these phases, together with K2SiVISi3 IVO9, a high-pressure wadeite analogue containing both tetrahedral and octahedral Si, are discussed in term of ionic substitutions. As the size of the octahedral framework cation increases, Si4+ → Ti4+ → Zr4+, the cell parameter c increases at a much higher rate than a. In contrast, increasing the interstitial alkali cation size (K+ → Cs+) results in a higher rate of increase in a compared with c. This behavior can be attributed to framework distortion around the interstitial cation. The enthalpies of formation from the constituent oxides (ΔHf,ox0) and from the elements (ΔHf,el0) have been determined from drop-solution calorimetry into 2PbO·B2O3 solvent at 975 K. The obtained values (in kJ/mol) are as follows: ΔHf,ox0 (K2TiSi3O9) = −355.8 ± 3.0, ΔHf,el0 (K2TiSi3O9) = −4395.1 ± 4.8, ΔHf,ox0 (K2ZrSi3O9) = −374.3 ± 3.3, ΔHf,el0 (K2ZrSi3O9) = −4569.9 ± 5.0, ΔHf,ox0 (Cs2ZrSi3O9) = −396.6 ± 4.4, and ΔHf,el0 (Cs2ZrSi3O9) = −4575.0 ± 5.5. The enthalpies of formation for K2SiVISi3 IVO9 were calculated from its drop-solution enthalpy of an earlier study (Akaogi et al. 2004), and the obtained ΔHf,ox0 (K2SiSi3O9) = −319.7 ± 3.4 and ΔHf,el0 (K2SiSi3O9) = −4288.7 ± 5.1 kJ/mol. With increasing the size of the octahedral framework cation or of the interstitial alkali cation, the formation enthalpies become more exothermic. This trend is consistent with the general behavior of increasing energetic stability with decreasing ionic potential (z/r) seen in many oxide and silicate systems. Further, increasing the size of the octahedral framework cation appears to induce more rapid increase in stability than increasing the interstitial alkali cation size, suggesting that framework cations play a more dominant role in wadeite stability.  相似文献   

17.
Garnet-clinopyroxene geothermometry has been applied to numerous rock types from the blueschist and eclogite terrain of the Sesia zone in the Western Alps. Discrepancies in apparent temperatures of up to 600° C have been found suggesting substantial deviation from ideal behaviour of the garnet-clinopyroxene equilibria in certain assemblages. Assemblages containing jadeitic pyroxenes rather than omphacitic or diopsidic pyroxenes yield very low K D (2.0) values and correspondingly high apparent temperatures (> 1,000° C). The garnets coexisting with high-Na pyroxenes have compositions similar to those coexisting with omphacitic pyroxenes. Departure from ideal behaviour of these garnets is likely to be minor and is accomodated in the formulation of the geothermometer. Numerous examples of coexisting garnet-clinopyroxene pairs indicate that there is no obvious relationship of K D with jadeite content in pyroxenes with jadeite content less than 70%, but at higher values, K D varies inversely with the jadeite content. The dependence of K D upon the jadeite content in the high-Na pyroxenes is believed to be due to the preference of M2 sites for Fe2+ in the enstatite-jadeite substitution (Fe2+MgNa–1Al–1). This substitution is usually very limited and has no demonstrable effect upon X Fe Cpx when the M1 (Fe2++Mg) content of the pyroxenes is high as in omphacitic and diopsidic pyroxenes. However, when the M1 (Fe2++Mg) content is low, as in jadeitic pyroxenes, the non-ideality of the enstatite type exchange has a marked effect on X Fe Cpx and consequently on garnet-clinopyroxene calculations.  相似文献   

18.
Zusammenfassung Die Kristallstruktur von künstlichem Voltait, K2Fe5 2+Fe3 3+Al[SO4]12· ·18 H2O, kubisch hexakisoktaedrisch,Fd3c–O h 8,a 0=27,254 ,Z-16, wurde mittels photographischer Röntgendaten bestimmt. Die Aufklärung der Struktur erfolgte mit Patterson- und Fouriermethoden unter Zuhilfenahme des multiplen isomorphen Ersatzes. Die Verfeinerung nach der Methode der kleinsten Quadrate ergab mit anisotropen Temperaturfaktoren für 726 beobachteteF hkl R=0,033. Das Hauptmerkmal der Struktur ist ein 3dimensionales Gerüst aus [Fe3+O6]-Oktaedern, [Fe 5 6/2+ Fe 1 6/3+ O4(H2O)2]-Oktaedern und [K+O12]-Polyedern, die durch SO4-Tetraeder verknüpft werden. Hohlräume dieses Gerüstes werden von ungeordnet orientierten [Al(H2O)6]-Oktaedern eingenommen. Es wird gezeigt, daß Al als wesentlicher Bestandteil dieses Voltaits angesehen werden muß.
The crystal structure of voltaite, K2Fe5 2+Fe3 3+Al[SO4]12·18H2O
Summary The crystal structure of synthetic voltaite, K2Fe5 2+Fe3 3+Al[SO4]12· · 18 H2O, cubic hexakis-octahedral, space groupFd3c–O h 8,a 0=27.254 ,Z=16, was determined from photographic X-ray data. The structure was solved by Patterson and Fourier-methods with the aid of multiple isomorphic substitution. Least squares refinement with anisotropic temperature factors resulted inR=0.033 for 726 observedF hkl . The dominant structural feature is a continous framework composed of [Fe3+O6]-octahedra, [Fe 5 6/2+ Fe 1 6/3+ O4(H2O)2]-octahedra and [K+O12]-polyhedra linked by SO4-tetrahedra. The arrangement gives rise to cages occupied by disordered [Al(H2O)6]-octahedra. It is shown that Al must be considered to be a essential constituent of such voltaites.

Mit 2 Abbildungen  相似文献   

19.
The new, greatly expanded internally-consistent dataset of Holland and Powell includes thermodynamic data for a wide range of mineral end-members in common rock-forming minerals, in particular, including FeMg-1 substitutions in glaucophane, garnet, chloritoid and carpholite, and FeMg-1 and MgSiAl-1Al-1 substitutions in talc and chlorite. Moreover, we have the uncertainties and correlations for these data. With the data, we have calculated the full pressure-temperature phase diagram for the system Na2O–FeO–MgO–Al2O3–SiO2–H2O (NFMASH) for quartz (or coesite) and H2O in excess, in the range 300°–800°C and 5–50 kbars. By solving the set of non-linear equations formed by the equilibrium relationships for an independent set of equations between the end-members in an assemblage in NFMASH, the compositions of the minerals (and PT) can be calculated. Thus the changes in MgSiAl-1Al-1 along NMASH reactions, and FeSiAl-1Al-1 along NFASH reactions, are calculated, and the changes in FeMg-1 and MgSiAl-1Al-1 along NFMASH reactions are calculated. From this information it is straightforward to generate PT diagrams for specific rock compositions. Mineral assemblages and mineral compositional changes in the phase diagram are discussed in relation to greenschist, blueschist and eclogite facies assemblages in metapelitic rocks. It is found that the correspondence between the predictions of the phase diagrams and the observations on rocks is remarkably good. When semiquantitative extensions of the phase diagram to include Ca(MgFe)-1, NaSiCa-1Al-1, Fe3+Al-1 and KNa-1 substitutions are taken into account the agreement is essentially complete.  相似文献   

20.
The thermal expansion of gehlenite, Ca2Al[AlSiO7], (up to T=830 K), TbCaAl[Al2O7] (up to T=1,100 K) and SmCaAl[Al2O7] (up to T=1,024 K) has been determined. All compounds are of the melilite structure type with space group Thermal expansion data was obtained from in situ X-ray powder diffraction experiments in-house and at HASYLAB at the Deutsches Elektronen Synchrotron (DESY) in Hamburg (Germany). The thermal expansion coefficients for gehlenite were found to be: α1=7.2(4)×10−6 K−1+3.6(7)×10−9ΔT K−2 and α3=15.0(1)×10−6 K−1. For TbCaAl[Al2O7] the respective values are: α1=7.0(2)×10−6 K−1+2.0(2)×10−9ΔT K−2 and α3=8.5(2)×10−6 K−1+2.0(3)×10−9ΔT K−2, and the thermal expansion coefficients for SmCaAl[Al2O7] are: α1=6.9(2)× 10−6 K−1+1.7(2)×10−9ΔT K−2 and α3=9.344(5)×10−6 K−1. The expansion-mechanisms of the three compounds are explained in terms of structural trends obtained from Rietveld refinements of the crystal structures of the compounds against the powder diffraction patterns. No structural phase transitions have been observed. While gehlenite behaves like a ’proper’ layer structure, the aluminates show increased framework structure behaviour. This is most probably explained by stronger coulombic interactions between the tetrahedral conformation and the layer-bridging cations due to the coupled substitution (Ca2++Si4+)-(Ln 3++Al3+) in the melilite-type structure. Electronic Supplementary Material Supplementary material is available for this article at  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号