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51.
Discrete, well-ordered, single-phase crystals (the Hanic phase) form when powders the bulk composition of Ca5Al6MgSiO17 are allowed to react in the absence of a melt. If, on the other hand, either the powders or crystals of Ca5Al6MgSiO17 are molten and then cooled, complex intergrowths form. These intergrowths consist of modules having the composition and structure
of Ca3Al4MgO10 and gehlenite (Ca2Al2SiO7), forming a polysomatic series in which the modules occur in varied proportions. High-resolution transmission electron microscope
(HRTEM) images reveal the details of these intergrowths. Mass balance is achieved by the formation of this polysomatic series
together with Mg-substituted mayenite (Ca12−xMgxAl14O33). The tetrahedral sheets in gehlenite have close structural similarities to those of Ca5Al6O14. Based on our HRTEM images, we hypothesize chemical substitutions between gehlenite and Ca5Al6O14, resulting in gehlenite-like modules that are depleted in Si and correspondingly enriched in Al and Ca.
Received: 2 August 2000 / Accepted: 18 January 2001 相似文献
52.
Mahmoud Fatemiaghda Habib Shahnazari Mehdi Talkhablu 《Marine Georesources & Geotechnology》2017,35(7):986-998
Aggregate crushing is a phenomenon occurring in carbonate soils under shear and compressive loads resulting in settlement of the offshore structures such as piers, bridges, waterfronts, wharfs, and oil and gas extraction platform foundations. Therefore, it is of significant importance to address the above-mentioned issue through a comprehensive study. In the present research, the texture of carbonate soil in south Iran coasts and its effect at high hydrostatic pressure (2?MPa) on aggregate crushing was studied. The physical properties of coastal soils, such as the effective grain size, shape index, and angularity were then characterized to investigate their effect on grain crushing grade. The results showed that the effective grain size, shape index, angularity, and calcium carbonate content are the main parameters affecting the crushing of grains and consequently settlement of marine soils. Based on the above parameters, a relationship is proposed to estimate grain crushing in carbonate soils. 相似文献
53.
Daniel Markewitz Ricardo de O. Figueiredo Eric A. Davidson 《Journal of Geochemical Exploration》2006,88(1-3):214
The objective of this study was to investigate the role of dissolved CO2 (H2CO3*) as a mechanism of cation removal from surface soils under secondary land uses in the tropics. Soil leachate columns were prepared with 0–10 cm soils from mature and secondary forest, and managed pastures, and extracted with H2CO3* from deionized water equilibrated with 0%, 0.5%, 1%, and 10% CO2 (g). Extraction of soil cations slowed over time following an exponential form for the cumulative data. The rate of cation concentration decline varied as a function of CO2 concentration with the 10% solution resulting in a greater percent decline with extraction volume. Potassium removal from the exchange sites of all soils and for all solutions was nearly complete ranging from 85% to 97% while removals of Mg (31% to 71%) and Ca (12% to 42%) were lower. The asymptotic patterns of cation loss observed in this study suggest that H2CO3* acid-driven losses of cations may become self-limiting over time. Other stronger acids from atmospheric deposition or organic sources may serve to perpetuate cation removal, and re-forestation on these cleared lands would certainly re-distribute cations from soils to vegetation. 相似文献
54.
55.
Laura Schaefer 《Icarus》2004,169(1):216-241
We modified the MAGMA chemical equilibrium code developed by Fegley and Cameron (1987, Earth Planet. Sci. Lett. 82, 207-222) and used it to model vaporization of high temperature silicate lavas on Io. The MAGMA code computes chemical equilibria in a melt, between melt and its equilibrium vapor, and in the gas phase. The good agreement of MAGMA code results with experimental data and with other computer codes is demonstrated. The temperature-dependent pressure and composition of vapor in equilibrium with lava is calculated from 1700 to 2400 K for 109 different silicate lavas in the ONaKFeSiMgCaAlTi system. Results for five lavas (tholeiitic basalt, alkali basalt, Barberton komatiite, dunite, and a molten type B1 Ca, Al-rich inclusion) are discussed in detail. The effects of continuous fractional vaporization on chemistry of these lavas and their equilibrium vapor are presented. The predicted abundances (relative to Na) of K, Fe, Si, Al, Ca, and Ti in the vapor equilibrated with lavas at 1900 K are lower than published upper limits for Io's atmosphere (which do not include Mg). We predict evaporative loss of alkalis, Fe, and Si during volcanic eruptions. Sodium is more volatile than K, and the Na/K ratio in the gas is decreased by fractional vaporization. This process can match Io's atmospheric Na/K ratio of 10±3 reported by Brown (2001, Icarus 151, 190-195). Silicon monoxide is an abundant species in the vapor above lavas. Spectroscopic searches are recommended for SiO at IR and mm wavelengths. Reactions of metallic vapors with S- and Cl-bearing volcanic gases may form other unusual gases including MgCl2, MgS, MgCl, FeCl2, FeS, FeCl, and SiS. 相似文献
56.
Mariusz Pełechaty Andrzej Pukacz Karina Apolinarska Aleksandra Pełechata Marcin Siepak 《Sedimentology》2013,60(4):1017-1035
A significant portion of calcium carbonate is deposited in lake sediments as a result of biological processes related to the photosynthetic activity of phytoplankton in the pelagic realm and, in addition, macrophytes in the littoral zone. Lake Wigry, one of the largest lakes in Poland (north‐east Poland), is characterized by: (i) carbonate sediments with a CaCO3 content exceeding 80% within the littoral zone; and (ii) large areas of submerged vegetation dominated by charophytes (macroscopic green algae, Characeae family). It is claimed that charophytes are highly effective in utilizing HCO3? and forming thick CaCO3 encrustations. Thus, this study was aimed at evaluating the CaCO3 production by dense Chara stands overgrowing the lake bottom reaching a depth of 4 m. In late July 2009, the fresh and dry mass of plants, the percentage contribution of calcium carbonate and the production of CaCO3 per 1 m2 were investigated along three transects at three depths (1 m, 2 m and 3 m, with each sample area equal to 0·0625 m2) per transect. The composition and structure of phytoplankton and the physico‐chemical properties of the water analysed in both the littoral and pelagic zones served as the environmental background and demonstrated moderately low fertility in the lake. The greatest dry plant mass exceeded 1000 g m?2 and CaCO3 encrustations constituted from 59% to over 76% of the charophyte dry weight. Thus, the maximum and average values of carbonates precipitated by charophytes were 685·5 and 438 g m?2, respectively, which exceeded previously reported results. A correlation of carbonate production with the depth of Chara stands was detected, and intermediate depths offered the most favourable conditions for carbonate precipitation (589 g m?2 on average). As precipitated carbonates are ultimately stored in bottom deposits, the results highlight the significance of charophytes in lacustrine CaCO3 sedimentation. 相似文献
57.
The monoclinic titanite-like high-pressure form of calcium disilicate has been synthesized and quenched to ambient conditions
to form the triclinic low-pressure phase containing silicon in four-, five- and sixfold coordination. The enthalpy of formation
of the quench product has been measured by high-temperature oxide melt calorimetry. The value obtained from samples from a
series of several synthesis experiments is ΔH
f
= (−26.32 ± 4.27) kJ mol−1 for the formation from the component oxides, or ΔH
f
= (−2482.81 ± 4.59) kJ mol−1 for the formation from the elements. The result is identical within experimental error to available estimates, although the
previously predicted energy difference between the monoclinic and triclinic phases could not be verified.
Received: 16 February 2000 / Accepted: 14 July 2000 相似文献
58.
Internationally there is considerable interest in utilizing hydrogen as an energy carrier. The use of hydrogen offers considerable potential benefits such as reducing greenhouse emissions, reducing urban pollution, increased energy security and increased efficiencies from the use of advanced energy conversion technologies.One of the most important questions when considering the development of a hydrogen economy is “where will the hydrogen come from?” Possible answers include electrolysis of water, steam reforming of methane and the gasification of coal. Given the high costs associated with electrolysis of water, and the increase in the cost of methane predicted over time, the gasification of coal is viewed by many as being the cheapest method of hydrogen production in the foreseeable future. These considerations are particularly relevant to New Zealand where gas supplies are dwindling but where there is sufficient coal to last for many centuries at present utilization rates. This, along with the current high international interest in hydrogen energy, has been recognized by the New Zealand Government in the form of a six-year [2002–2008] research project “Hydrogen Energy for the Future of New Zealand”.One important coal property that, in particular, determines the suitability of a particular coal for use in a fluidised bed gasifier is its reactivity towards the gasification reaction. It was found that a high percentage of New Zealand's coal resource is particularly well-suited towards fluidised bed gasification, reacting at anywhere between 0.9 to 1.75 times the rate of Australian brown coals. It was found the New Zealand lignites contained significant levels of organically bound calcium, which was shown to be responsible for not only the high reactivity of the New Zealand lignites, but also a product gas composition with higher than expected hydrogen concentrations. These findings are discussed along with their implications for the gasifier and gas clean-up design. 相似文献
59.
J. N. Pattan Toshiyuki Masuzawa D. V. Borole G. Parthiban Pratima Jauhari Mineko Yamamoto 《Journal of Earth System Science》2005,114(1):63-74
A 2m-long sediment core from the siliceous ooze domain in the Central Indian Ocean Basin (CIOB; 13‡03′S: 74‡44′E; water depth
5099m) is studied for calcium carbonate, total organic carbon, total nitrogen, biogenic opal, major and few trace elements
(Al, Ti, Fe, K, Mg, Zr, Sc,V, Mn, Cu, Ni, Zn, Co, and Ba) to understand the productivity and intensity of terrigenous supply.
The age model of the sediment core is based on U-Th dating, occurrence of Youngest Toba Tuff of ∼ 74 ka and Australasian microtektites
of ∼ 770 ka.
Low carbonate content (< 1%) of sediment core indicates deposition below the carbonate compensation depth. Organic carbon
content is also very low, almost uniform (mean 0.2 wt%) and is of marine origin. This suggests a well-oxygenated bottom water
environment during the past ∼ 1100ka. Our data suggest that during ∼ 1100 ka and ∼ 400 ka siliceous productivity was lower,
complimented by higher supply of terrigenous material mostly derived from the metasedimentary rocks of High Himalayan crystalline.
However, during the last ∼ 400 ka, siliceous productivity increased with substantial reduction in the terrigenous sediment
supply. The results suggest that intensity of Himalayan weathering, erosion associated with monsoons was comparatively higher
prior to 400 ka. Manganese, Ba, Cu, Ni, Zn, and Co have around 90% of their supply from noncrustal (excess) source and their
burial to seafloor remained unaffected throughout the past ∼ 1100 ka. 相似文献
60.
Thermodynamic properties of high-pressure minerals that are not recoverable from synthesis experiments by conventional quenching
methods (“unquenchable” phases) usually are calculated from equation of state data and phase diagram topologies. The present
study shows that, with cryogenic methods of recovery and sample treatment, phases with a suitable decomposition rate can be
made accessible to direct thermodynamic measurements. A set of samples of Ca(OH)2-II has been synthesized in a multianvil device and subsequently recovered by cooling the high-pressure assembly with liquid
nitrogen. Upon heating from liquid nitrogen to room temperature, the material transformed back to Ca(OH)2-I. The heat effect of this backtransformation was measured by differential scanning calorimetry. A commercial differential
scanning calorimeter (Netzsch DSC 404), modified to allow sample loading at liquid nitrogen temperature was used to heat the
material from −150 to +200 °C at rates varying between 5 and 15 °C min−1. The transformation started around −50 °C very gradually, and peaked at about 0 °C. To obtain a baseline correction, each
sample was scanned under exactly the same conditions after the backtransformation was complete. Because of the relative sluggishness,
onset and offset temperatures were not well defined as compared to fast (e.g., melting) reactions. To aid in integration,
the resulting signals were successfully fitted using a generic asymmetric peak model. The enthalpy of backtransformation was
determined to be ΔH =−10.37 ± 0.50 kJ mol−1. From previous in situ X-ray diffraction experiments, the location of the direct transformation in P-T space has been constrained to 5.7 ± 0.4 GPa at 500 °C (Kunz et al. 1996). With the reaction volume known from the same study,
and assuming that ΔC
p
of the transformation remains negligible between the conditions of our measurements and 500 °C, our result gives an estimate
of the entropy of transition and the P-T slope of the reaction curve. To a first approximation, the values ΔS = −16.00 ± 0.65 J(mol · K)−1 and dP/dT = 0.0040 ± 0.0002 GPa/K have been determined. These results need to be refined by equation of state data for Ca(OH)2-II.
Received: 30 December 1999 / Accepted: 10 April 2000 相似文献