Devitrite (Na2Ca3Si6O16)—structural, spectroscopic and computational investigations on a crystalline impurity phase in industrial soda-lime glasses     

Volker Kahlenberg  Daniela Girtler  Erik Arroyabe  Reinhard Kaindl  Daniel M. Többens 《Mineralogy and Petrology》2010,100(1-2):1-9

Single crystals of devitrite (Na₂Ca₃Si₆O₁₆) were synthesized as pale-yellow transparent needle shaped crystals using a Na₂MoO₄-flux. Experiments aiming to prepare the K-equivalent of devitrite from the corresponding K₂MoO₄-flux were unsuccessful. The crystal structure of devitrite was determined from single-crystal X-ray diffraction data (Mo-K_α radiation, 2θ_max.?=?25.34°, R_int?=?2.66%) and refined in space group P $ \bar{1} $ (no. 2) to R(|F|)?=?3.08% using 2,513 observed reflections with I?>?2σ(I). Unit-cell parameters are: a?=?7.2291(8), b?=?10.1728(12), c?=?10.6727(12) Å, α?=?95.669(9), β?=?109.792(10), γ?=?99.156(9)°, V?=?719.19(14) ų, Z?=?2. The structure belongs to the group of multiple chain silicates consisting of dreier quadruple chains, i.e. the crystallochemical formula can be written as $ {\hbox{N}}{{\hbox{a}}_2}{\hbox{C}}{{\hbox{a}}_3}\left\{ {{\mathbf{uB}}{,4}_\infty^1} \right\}\left[ {^3{\hbox{S}}{{\hbox{i}}_6}} \right.\left. {{{\hbox{O}}_{16}}} \right\} $ . Linkage between the bands running along [100] is provided by double chains of edge sharing CaO₆-octahedra as well as additional more irregularly coordinated Na- and Ca-cations located in the tunnel-like cavities of the mixed tetrahedral-octahedral framework. Structural investigations were completed by Raman and infrared spectroscopical studies. The allocation of the bands to certain vibrational species was aided by density functional theory (DFT) calculations.  相似文献   

The role of the minor substitutions in the crystal structure of natural challacolloite, KPb2Cl5, and hephaistosite, TlPb2Cl5, from Vulcano (Aeolian Archipelago, Italy)     

D. Mitolo  D. Pinto  A. Garavelli  L. Bindi  F. Vurro 《Mineralogy and Petrology》2009,96(1-2):121-128

Crystals of challacolloite, KPb₂Cl₅, and hephaistosite, TlPb₂Cl₅, from volcanic sublimates formed on the crater rim of the “La Fossa Crater” at Vulcano, Aeolian Archipelago, Italy, were investigated. Chemical compositions were ${\left( {{\text{K}}_{{0.93}} {\text{Tl}}_{{0.02}} } \right)}_{{\Sigma = 0.95}} {\text{Pb}}_{{2.04}} {\left( {{\text{Cl}}_{{4.90}} {\text{Br}}_{{0.11}} } \right)}_{{\Sigma = 5.01}} $ and ${\text{Tl}}_{{0.94}} {\text{Pb}}_{{2.01}} {\left( {{\text{Cl}}_{{4.91}} {\text{Br}}_{{0.14}} } \right)}_{{\Sigma = 5.05}} $ , respectively. Single crystal X-ray measurements showed monoclinic symmetry for both phases, space group P2₁/c, with the following unit-cell parameters: a = 8.8989(4), b = 7.9717(5), c = 12.5624(8) Å, β = 90.022(4)°, V = 891.2(1) ų, Z = 4 (challacolloite) and a = 9.0026(6), b = 7.9723(6), c = 12.5693(9) Å, β = 90.046(4)°, V = 902.1(1) ų, Z = 4 (hephaistosite). The structure refinements converge to R = 3.99% and R = 3.86%, respectively. The effects of Br?Cl and K?Tl substitutions on the structure of these natural compounds have been discussed.  相似文献   

An experimental study of Ti and Zr partitioning among zircon, rutile, and granitic melt     

Amy E. Hofmann  Michael B. Baker  John M. Eiler 《Contributions to Mineralogy and Petrology》2013,166(1):235-253

In order to evaluate the effect of trace and minor elements (e.g., P, Y, and the REEs) on the high-temperature solubility of Ti in zircon (zrc), we conducted 31 experiments on a series of synthetic and natural granitic compositions [enriched in TiO₂ and ZrO₂; Al/(Na + K) molar ~1.2] at a pressure of 10 kbar and temperatures of ~1,400 to 1,200 °C. Thirty of the experiments produced zircon-saturated glasses, of which 22 are also saturated in rutile (rt). In seven experiments, quenched glasses coexist with quartz (qtz). SiO₂ contents of the quenched liquids range from 68.5 to 82.3 wt% (volatile free), and water concentrations are 0.4–7.0 wt%. TiO₂ contents of the rutile-saturated quenched melts are positively correlated with run temperature. Glass ZrO₂ concentrations (0.2–1.2 wt%; volatile free) also show a broad positive correlation with run temperature and, at a given T, are strongly correlated with the parameter (Na + K + 2Ca)/(Si·Al) (all in cation fractions). Mole fraction of ZrO₂ in rutile $ \left( {\mathop X\nolimits_{{{\text{ZrO}}_{ 2} }}^{\text{rt}} } \right) $ in the quartz-saturated runs coupled with other 10-kbar qtz-saturated experimental data from the literature (total temperature range of ~1,400 to 675 °C) yields the following temperature-dependent expression: $ {\text{ln}}\left( {\mathop X\nolimits_{{{\text{ZrO}}_{ 2} }}^{\text{rt}} } \right) + {\text{ln}}\left( {a_{{{\text{SiO}}_{2} }} } \right) = 2.638(149) - 9969(190)/T({\text{K}}) $ , where silica activity $ a_{{{\text{SiO}}_{2} }} $ in either the coexisting silica polymorph or a silica-undersaturated melt is referenced to α-quartz at the P and T of each experiment and the best-fit coefficients and their uncertainties (values in parentheses) reflect uncertainties in T and $ \mathop X\nolimits_{{{\text{ZrO}}_{2} }}^{\text{rt}} $ . NanoSIMS measurements of Ti in zircon overgrowths in the experiments yield values of ~100 to 800 ppm; Ti concentrations in zircon are positively correlated with temperature. Coupled with values for $ a_{{{\text{SiO}}_{2} }} $ and $ a_{{{\text{TiO}}_{2} }} $ for each experiment, zircon Ti concentrations (ppm) can be related to temperature over the range of ~1,400 to 1,200 °C by the expression: $ \ln \left( {\text{Ti ppm}} \right)^{\text{zrc}} + \ln \left( {a_{{{\text{SiO}}_{2} }} } \right) - \ln \left( {a_{{{\text{TiO}}_{2} }} } \right) = 13.84\left( {71} \right) - 12590\left( {1124} \right)/T\left( {\text{K}} \right) $ . After accounting for differences in $ a_{{{\text{SiO}}_{2} }} $ and $ a_{{{\text{TiO}}_{2} }} $ , Ti contents of zircon from experiments run with bulk compositions based on the natural granite overlap with the concentrations measured on zircon from experiments using the synthetic bulk compositions. Coupled with data from the literature, this suggests that at T ≥ 1,100 °C, natural levels of minor and trace elements in “granitic” melts do not appear to influence the solubility of Ti in zircon. Whether this is true at magmatic temperatures of crustal hydrous silica-rich liquids (e.g., 800–700 °C) remains to be demonstrated. Finally, measured $ D_{\text{Ti}}^{{{\text{zrc}}/{\text{melt}}}} $ values (calculated on a weight basis) from the experiments presented here are 0.007–0.01, relatively independent of temperature, and broadly consistent with values determined from natural zircon and silica-rich glass pairs.  相似文献   

Activity-composition relations of the magnesiowüstite solid solution series in equilibrium with metallic iron in the temperature range 1050–1400 K     

I. Srečec  A. Ender  E. Woermann  W. Gans  E. Jacobsson  G. Eriksson  E. Rosén 《Physics and Chemistry of Minerals》1987,14(6):492-498

The equilibrium $${\text{(1}} - y{\text{)Fe}}_{(s)} + \tfrac{{\text{1}}}{{\text{2}}}{\text{O}}_{{\text{2(g)}}} \rightleftarrows {\text{Fe}}_{{\text{1}} - y} {\text{O}}_{{\text{(}}s,{\text{ in MW)}}} $$ was studied by measuring oxygen potentials for a range of different magnesiowüstite compositions relative to those of the iron-wüstite system in an oxygen concentration cell involving yttria stabilized zirconia as the solid electrolyte. The temperature range covered was 1050 to 1400 K. Separate measurements of the mole fraction of trivalent iron in magnesiowüstite (x(Fe³⁺)) were made and the composition dependence ofx(Fe³⁺) was taken into account in calculations of the activity-composition relations of FeO, Fe_2/3O and MgO.  相似文献   

Energetics of hydration of cordierite and water barometry in cordierite-granulites     

A. Bhattacharya  S. K. Sen 《Contributions to Mineralogy and Petrology》1985,89(4):370-378

The Gibbs free energy and volume changes attendant upon hydration of cordierites in the system magnesian cordierite-water have been extracted from the published high pressure experimental data at \(P_{{\text{H}}_{\text{2}} {\text{O}}} \) =P _total, assuming an ideal one site model for H₂O in cordierite. Incorporating the dependence of ΔG and ΔV on temperature, which was found to be linear within the experimental conditions of 500°–1,000°C and 1–10,000 bars, the relation between the water content of cordierite and P, T and \(f_{{\text{H}}_{\text{2}} {\text{O}}} \) has been formulated as $$\begin{gathered} X_{{\text{H}}_{\text{2}} {\text{O}}}^{{\text{crd}}} = \hfill \\ \frac{{f_{{\text{H}}_{\text{2}} {\text{O}}}^{{\text{P, T}}} }}{{\left[ {{\text{exp}}\frac{1}{{RT}}\left\{ {64,775 - 32.26T + G_{{\text{H}}_{\text{2}} {\text{O}}}^{{\text{1, }}T} - P\left( {9 \times 10^{ - 4} T - 0.5142} \right)} \right\}} \right] + f_{{\text{H}}_{\text{2}} {\text{O}}}^{{\text{P, T}}} }} \hfill \\ \end{gathered} $$ The equation can be used to compute H₂O in cordierites at \(P_{{\text{H}}_{\text{2}} {\text{O}}} \) <1. Our results at different P, T and partial pressure of water, assuming ideal mixing of H₂O and CO₂ in the vapour phase, are in very good agreement with the experimental data of Johannes and Schreyer (1977, 1981). Applying the formulation to determine \(X_{{\text{H}}_{\text{2}} {\text{O}}}^{{\text{crd}}} \) in the garnet-cordierite-sillimanite-plagioclase-quartz granulites of Finnish Lapland as a test case, good agreement with the gravimetrically determined water contents of cordierite was obtained. Pressure estimates, from a thermodynamic modelling of the Fe-cordierite — almandine — sillimanite — quartz equilibrium at \(P_{{\text{H}}_{\text{2}} {\text{O}}} = 0\) and \(P_{{\text{H}}_{\text{2}} {\text{O}}} \) =P_total, for assemblages from South India, Scottish Caledonides, Daly Bay and Hara Lake areas are compatible with those derived from the garnetplagioclase-sillimanite-quartz geobarometer.  相似文献   

The system Fe-Si-O: Oxygen buffer calibrations to 1,500K   总被引：1，自引：0，他引：1  

J. Myers  H. P. Eugster 《Contributions to Mineralogy and Petrology》1983,82(1):75-90

The five solid-phase oxygen buffers of the system Fe-Si-O, iron-wuestite (IW), wuestite-magnetite (WM), magnetite-hematite (MH), quartz-iron-fayalite (QIF) and fayalite-magnetite-quartz (FMQ) have been recalibrated at 1 atm pressure and temperatures from 800°–1,300° C, using a thermogravimetric gas mixing furnace. The oxygen fugacity, \(f_{{\text{O}}_{\text{2}} }\) was measured with a CaO-doped ZrO₂ electrode. Measurements were made also for wuestite solid solutions in order to determine the redox behavior of wuestites with O/Fe ratios varying from 1.05 to 1.17. For FMQ, additional determinations were carried out at 1 kb over a temperature range of 600° to 800° C, using a modified Shaw membrane. Results agree reasonably well with published data and extrapolations. The reaction parameters K, ΔG _r ^o , ΔH _r ^o , and ΔS _r ^o were calculated from the following log \(f_{{\text{O}}_{\text{2}} }\) /T relations (T in K): $$\begin{gathered} {\text{IW }}\log f_{{\text{O}}_{\text{2}} } = - 26,834.7/T + 6.471\left( { \pm 0.058} \right) \hfill \\ {\text{ }}\left( {{\text{800}} - 1,260{\text{ C}}} \right), \hfill \\ {\text{WM }}\log f_{{\text{O}}_{\text{2}} } = - 36,951.3/T + 16.092\left( { \pm 0.045} \right) \hfill \\ {\text{ }}\left( {{\text{1,000}} - 1,300{\text{ C}}} \right), \hfill \\ {\text{MH }}\log f_{{\text{O}}_{\text{2}} } = - 23,847.6/T + 13.480\left( { \pm 0.055} \right) \hfill \\ {\text{ }}\left( {{\text{1,040}} - 1,270{\text{ C}}} \right), \hfill \\ {\text{QIF }}\log f_{{\text{O}}_{\text{2}} } = - 27,517.5/T + 6.396\left( { \pm 0.049} \right) \hfill \\ {\text{ }}\left( {{\text{960}} - 1,140{\text{ C}}} \right), \hfill \\ {\text{FMQ }}\log f_{{\text{O}}_{\text{2}} } = - 24,441.9/T + 8.290\left( { \pm 0.167} \right) \hfill \\ {\text{ }}\left( {{\text{600}} - 1,140{\text{ C}}} \right). \hfill \\ \end{gathered}$$ These experimentally determined reaction parameters were combined with published 298 K data to determine the parameters G_f, H_f, and S_f for the phases wuestite, magnetite, hematite, and fayalite from 298 K to the temperatures of the experiments. The T? \(f_{{\text{O}}_{\text{2}} }\) data for wuestite solid solutions were used to obtain activities, excess free energies and Margules mixing parameters. The new data provide a more reliable, consistent and complete reference set for the interpretation of redox reactions at elevated temperatures in experiments and field settings encompassing the crust, mantle and core as well as extraterrestrial environments.  相似文献   

The crystal structure of galgenbergite-(Ce), CaCe2(CO3)4∙H2O     

Franz Walter  Hans-Peter Bojar  Christine E. Hollerer  Kurt Mereiter 《Mineralogy and Petrology》2013,107(2):189-199

Galgenbergite-(Ce) from the type locality, the railroad tunnel Galgenberg between Leoben and St. Michael, Styria, Austria, was investigated. There it occurs in small fissures of an albite-chlorite schist as very thin tabular crystals building rosette-shaped aggregates associated with siderite, ancylite-(Ce), pyrite and calcite. Electron microprobe analyses gave CaO 9.49, Ce₂O₃ 28.95, La₂O₃ 11.70, Nd₂O₃ 11.86, Pr₂O₃ 3.48, CO₂ 30.00, H₂O 3.07, total 98.55 wt.%. CO₂ and H₂O calculated by stoichiometry. The empirical formula (based on Ca + REE ∑3.0) is $ \mathrm{C}{{\mathrm{a}}_{1.00 }}{{\left( {\mathrm{C}{{\mathrm{e}}_{1.04 }}\mathrm{L}{{\mathrm{a}}_{0.42 }}\mathrm{N}{{\mathrm{d}}_{0.42 }}\mathrm{P}{{\mathrm{r}}_{0.12 }}} \right)}_{2.00 }}{{\left( {\mathrm{C}{{\mathrm{O}}_3}} \right)}_4}\cdot {{\mathrm{H}}_2}\mathrm{O} $ , and the simplified formula is $ \mathrm{CaC}{{\mathrm{e}}_2}{{\left( {\mathrm{C}{{\mathrm{O}}_3}} \right)}_4}\cdot {{\mathrm{H}}_2}\mathrm{O} $ . According to X-ray single crystal diffraction galgenbergite-(Ce) is triclinic, space group $ P\overline{1},a=6.3916(5) $ , b?=?6.4005(4), c?=?12.3898(9) Å, α?=?100.884(4), β?=?96.525(4), γ?=?100.492(4)°, V?=?483.64(6) ų, Z?=?2. The eight strongest lines in the powder X-ray diffraction pattern are [d _calc in Å/(I)/hkl]: 5.052/(100)/011; 3.011/(70)/0-22; 3.006/(66)/004; 5.899/(59)/-101; 3.900/(51)/1-12; 3.125/(46)/-201; 2.526/(42)/022; 4.694/(38)/-102. The infrared absorption spectrum reveals H₂O (OH-stretching mode at 3,489 cm^?1, HOH bending mode at 1,607 cm^?1) and indicates the presence of distinctly non-equivalent CO₃-groups by double and quadruple peaks of their ν1, ν2, ν3 and ν4 modes. The crystal structure of galgenbergite-(Ce) was refined with X-ray single crystal data to R1?=?0.019 for 2,448 unique reflections (I?>?2σ(I)) and 193 parameters. The three cation sites of the structure Ca(1), Ce(2) and Ce(3) have a modest mixed site occupation by Ca and small amount of REE (Ce, La, Pr, Nd) and vice versa. The structure is based on double layers parallel to (001), which are composed of Ca(1)Ce(2)(CO₃)₂ single layers with an ordered chessboard like arrangement of Ca and Ce, and with a roof tile-like stacking of the CO₃ groups. Perpendicular to (001) the double layers are connected to a triclinic framework structure with good cleavage parallel to (001) by a differently organized and more open part of the structure formed by Ce(3)(CO₃)₂(H₂O). Based on the topology of the CaCe(CO₃)₂ single layer in galgenbergite-(Ce), structural relationships to rutherfordine, to aragonite and ancylite type minerals, and to lanthanite are outlined.  相似文献   

Statistical mechanics of coupled solid solutions in the dilute limit     

Ronald E. Cohen 《Physics and Chemistry of Minerals》1986,13(3):174-182

A statistical mechanical analysis of the limiting laws for coupled solid solutions shows that the random model, in which the configurational entropy is calculated as if atoms mix randomly on each crystallographic site, is correct as a first approximation. In coupled solid solutions, since atoms of different valence substitute on the same sites, significant short-range order which reduces the entropy can be expected. A first-order correction is rigorously obtained for the entropy in dilute binary short-range ordered coupled solid solutions: $$\bar S^{{\text{XS}}} {\text{/R = }}Q\left( {{\text{e}}^{--H_{\text{A}} /{\text{R}}T} \left( {\frac{{H_{\text{A}} }}{{{\text{R}}T}} + 1} \right) - 1} \right)N_2^a N_4^b ,$$ where Q is the number of positions an associated cation pair can assume per formula unit, H _A is the association energy per formula unit, and N ₂ ^a and N ₄ ^b are the site occupancy fractions for atoms 2 and 4 that are dilute on sites a and b. S ^XS is the configurational entropy minus the random model entropy. Aluminous pyroxenes on the joints diopside-jadeite and diopside-CaTs are examined as examples. A generalization for dilute multiple component solutions, including possible long-range ordering variations is given by: $$\frac{{\bar S^{{\text{XS}}} }}{{\text{R}}}{\text{ = }}\sum\limits_i {\sum\limits_j {\sum\limits_k {Q_i } } \left( {{\text{e}}^{--H_{\text{A}}^{j{\text{ }}k{\text{, }}i} /{\text{R}}T} \left( {\frac{{H_{\text{A}}^{j{\text{ }}k{\text{, }}i} }}{{{\text{R}}T}} + 1} \right) - 1} \right)N_j^l N_k^m ,} $$ where i labels each crystallographically distinct pair, j and k label atomic species, l and m label crystallographic sites, and the N's are site occupancy fractions for the solute atoms. A total association model is examined as well as the partial association and random models. Real solution behavior must lie between the total association model and the random model. Molecular models in which the ideal activity is proportional to a mole fraction, which in itself is not always unambiguously defined, do not lie in this range and furthermore have no physical justification.  相似文献   

Experimental calibration of Forsterite–Anorthite–Ca-Tschermak–Enstatite (FACE) geobarometer for mantle peridotites     

P.?FumagalliEmail authorView author&#;s OrcID profile  G.?Borghini  E.?Rampone  S.?Poli 《Contributions to Mineralogy and Petrology》2017,172(6):38

  相似文献   

Redox-controlled iron isotope fractionation during magmatic differentiation: an example from the Red Hill intrusion, S. Tasmania   总被引：1，自引：1，他引：0  

Paolo A. Sossi  John D. Foden  Galen P. Halverson 《Contributions to Mineralogy and Petrology》2012,164(5):757-772

This study presents accurate and precise iron isotopic data for 16 co-magmatic rocks and 6 pyroxene–magnetite pairs from the classic, tholeiitic Red Hill sill in southern Tasmania. The intrusion exhibits a vertical continuum of compositions created by in situ fractional crystallisation of a single injection of magma in a closed igneous system and, as such, constitutes a natural laboratory amenable to determining the causes of Fe isotope fractionation in magmatic rocks. Early fractionation of pyroxenes and plagioclase, under conditions closed to oxygen exchange, gives rise to an iron enrichment trend and an increase in $ f_{{{\text{O}}_{2} }} $ of the melt relative to the Fayalite–Magnetite–Quartz (FMQ) buffer. Enrichment in Fe³⁺/ΣFe_melt is mirrored by δ⁵⁷Fe, where ^VIFe²⁺-bearing pyroxenes partition ⁵⁷Fe-depleted iron, defining an equilibrium pyroxene-melt fractionation factor of $ \Updelta^{57} {\text{Fe}}_{{{\text{px}} - {\text{melt}}}} \le - 0.25\,\permille \times 10^{6} /T^{2} $ . Upon magnetite saturation, the $ f_{{{\text{O}}_{2} }} $ and δ⁵⁷Fe of the melt fall, commensurate with the sequestration of the oxidised, ⁵⁷Fe-enriched iron into magnetite, quantified as $ \Updelta^{57} {\text{Fe}}_{{{\text{mtn}} - {\text{melt}}}} = + 0.20\,\permille \times 10^{6} /T^{2} $ . Pyroxene–magnetite pairs reveal an equilibrium fractionation factor of $ \Updelta^{57} {\text{Fe}}_{{{\text{mtn}} - {\text{px}}}} \approx + 0.30\,\permille $ at 900–1,000?°C. Iron isotopes in differentiated magmas suggest that they may act as an indicator of their oxidation state and tectonic setting.  相似文献   

The origin of mafic rocks in the Naqadeh intrusive complex, Sanandaj-Sirjan Zone, NW Iran     

Seyed Ali Mazhari  Sadraldin Amini  Jalil Ghalamghash  Fernando Bea 《Arabian Journal of Geosciences》2011,4(7-8):1207-1214

The Naqadeh mafic plutonic rocks are located on a plutonic assemblage and include different granitoid rocks related to ～40 Ma. U-Pb SHRIMP data shows different ages of 96?±?2.3 Ma for mafic rocks. Naqadeh mafic plutonic rocks consist of diorite to diorite-gabbros with relatively high contents of incompatible elements, low Na2O, and $ {\hbox{Mg\# }} = \left[ {{\hbox{molar}}\;{100} \times {\hbox{MgO/}}\left( {{\hbox{MgO}} + {\hbox{FeO}}} \right)} \right] > 44.0 $ . These features suggest that the Naqadeh mafic rocks originate from enriched lithospheric mantle above subducted slab during Neotethys subduction under Iranian plate.  相似文献   

A multi-isotope approach for understanding sources of water,carbon and sulfur in natural springs of the Central Appalachian region     

Andrea L. Sack  Shikha Sharma 《Environmental Earth Sciences》2014,71(11):4715-4724

Natural springs have been reliable sources of domestic water and have allowed for the development of recreational facilities and resorts in the Central Appalachians. The structural history of this area is complex and it is unknown whether these natural springs receive significant recharge from modern precipitation or whether they discharge old water recharged over geological times scales. The main objective of this study was to use stable isotopes of water ( $\delta^{18} {\text{O}}_{{{\text{H}}_{2} {\text{O}}}}$ and $\delta^{2} {\text{H}}_{{{\text{H}}_{2} {\text{O}}}}$ ), dissolved inorganic carbon ( $\delta^{13} {\text{C}}_{\text{DIC}}$ ) and dissolved sulfate ( $\delta^{34} {\text{S}}_{{{\text{SO}}_{4} }}$ and $\delta^{18} {\text{O}}_{{{\text{SO}}_{4} }}$ ) to delineate sources of water, carbon and sulfur in several natural springs of the region. Our preliminary isotope data indicate that all springs are being recharged by modern precipitation. The oxygen isotope composition indicates that waters in thermal springs did not encounter the high temperatures required for O isotope exchange between the water and silicate/carbonate minerals, and/or the residence time of water in the aquifers was short due to high flow rates. The carbon isotopic composition of dissolved inorganic carbon and sulfur/oxygen isotopic composition of dissolved sulfate provide evidence of low-temperature water–rock interactions and various biogeochemical transformations these waters have undergone along their flow path.  相似文献   

Multiple-pumped-well aquifer test to determine the anisotropic properties of a karst limestone aquifer in Pasco County, Florida, USA   总被引：1，自引：0，他引：1  

Louis H. Motz 《Hydrogeology Journal》2009,17(4):855-869

Groundwater-level data from an aquifer test utilizing four pumped wells conducted in the South Pasco wellfield in Pasco County, Florida, USA, were analyzed to determine the anisotropic transmissivity tensor, storativity, and leakance in the vicinity of the wellfield. A weighted least-squares procedure was used to analyze drawdowns measured at eight observation wells, and it was determined that the major axis of transmissivity extends approximately from north to south and the minor axis extends approximately from west to east with an angle of anisotropy equal to N4.54°W. The transmissivity along the major axis ${\left( {T_{{\xi \xi }} } \right)}$ is 14,019 m² day^–1, and the transmissivity along the minor axis ${\left( {T_{{\eta \eta }} } \right)}$ is 4,303 m² day^–1. The equivalent transmissivity $T_{e} = {\left( {T_{{\xi \xi }} T_{{\eta \eta }} } \right)}^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-0em} 2}} = 7,767{{\text{m}}^{2} } \mathord{\left/ {\vphantom {{{\text{m}}^{2} } {{\text{day}}^{{ - {\text{1}}}} }}} \right. \kern-0em} {{\text{day}}^{{ - {\text{1}}}} }$ , and the ratio of anisotropy is 3.26. The storativity of the aquifer is 7.52?×?10^?4, and the leakance of the overlying confining unit is 1.37?×?10^?4 day^?1. The anisotropic properties determined for the South Pasco wellfield in this investigation confirm the results of previous aquifer tests conducted in the wellfield and help to quantify the NW–SE to NE–SW trends for regional fracture patterns and inferred solution-enhanced flow zones in west-central Florida.  相似文献   

Light scattering in jadeite melt: Strain relaxation measurements by photon correlation spectroscopy     

R. Siewert  M. Rosenhauer 《Physics and Chemistry of Minerals》1994,21(1-2):18-23

Photon correlation spectroscopy has been applied to the study of longitudinal strain relaxation of vitreous Jadeite (NaAlSi₂O₆) in the temperature range 811–1014° C. The correlation function $\left| {g^{\left( 1 \right)} \left. {\left( t \right)} \right|^2 \propto \exp \left( {\left( { - 2t/\tau _\beta } \right)^\beta } \right)} \right.$ obeys a Kohlrausch type function with β=0.64±0.01. Individual correlation functions fit altogether a master relaxation curve, thus demonstrating thermorheological simplicity (TRS). The temperature dependence of the measured relaxation times shows Arrhenian behaviour with $\log \left( \tau \right) = - 21.4 \pm 0.3{\text{s}} {\text{ + }} {\text{471}}{\text{.6}} \pm {\text{22}} {\text{kJmol}}^{{\text{ - 1}}} /RT$ . The time scale of longitudinal strain relaxation is consistent with the existing data on shear relaxation derived from shear viscosity and structural relaxation calculated from calorimetric C _pmeasurements. Comparison with oxygen diffusion indicates that network forming elements relax at about the same time scale as viscoelastic properties. On the other hand, Na⁺ relaxation times derived from impedance spectroscopy are short compared to viscoelastic relaxation times at low temperatures. This difference is decreasing with increasing temperature and possibly disappearing at approximately 1100° C.  相似文献   

The Hydrolysis of Al(III) in NaCl solutions: A Model for M(II), M(III), and M(IV) Ions     

Ryan J. Woosley  Frank J. Millero 《Aquatic Geochemistry》2010,16(3):317-324

Understanding the identity and stability of the hydrolysis products of metals is required in order to predict their behavior in natural aquatic systems. Despite this need, the hydrolysis constants of many metals are only known over a limited range of temperature and ionic strengths. In this paper, we show that the hydrolysis constants of 31 metals [i.e. Mn(II), Cr(III), U(IV), Pu(IV)] are nearly linearly related to the values for Al(III) over a wide range of temperatures and ionic strengths. These linear correlations allow one to make reasonable estimates for the hydrolysis constants of +2, +3, and +4 metals from 0 to 300°C in dilute solutions and 0 to 100°C to 5 m in NaCl solutions. These correlations in pure water are related to the differences between the free energies of the free ion and complexes being almost equal $$ \Updelta {\text{G}}^\circ \left( {{\text{Al}}^{3 + } } \right) - \Updelta {\text{G}}^\circ \left( {{\text{Al}}\left( {\text{OH}} \right)_{j}^{{\left( {3 - j} \right)}} } \right) \cong \Updelta {\text{G}}^\circ \left( {{\text{M}}^{n + } } \right) - \Updelta {\text{G}}^\circ \left( {{\text{M}}\left( {\text{OH}} \right)_{j}^{{\left( {n - j} \right)}} } \right) $$ The correlation at higher temperatures is a result of a similar relationship between the enthalpies of the free ions and complexes $$ \Updelta {\text{H}}^\circ \left( {{\text{Al}}^{3 + } } \right) - \Updelta {\text{H}}^\circ \left( {{\text{Al}}\left( {\text{OH}} \right)_{j}^{3 - j} } \right) \cong \Updelta {\text{H}}^\circ \left( {{\text{M}}^{n + } } \right) - \Updelta {\text{H}}^\circ \left( {{\text{M}}\left( {\text{OH}} \right)_{j}^{n - j} } \right) $$ The correlations at higher ionic strengths are the result of the ratio of the activity coefficients for Al(III) being almost equal to that of the metal. $$ \gamma \left( {{\text{M}}^{n + } } \right)/\gamma \left( {{\text{M}}\left( {\text{OH}} \right)_{j}^{n - j} } \right) \cong \gamma \left( {{\text{Al}}^{3 + } } \right)/\gamma \left( {{\text{Al}}\left( {\text{OH}} \right)_{j}^{3 - j} } \right) $$ The results of this study should be useful in examining the speciation of metals as a function of pH in natural waters (e.g. hydrothermal fresh waters and NaCl brines).  相似文献   

The combined effect of F and H2O on interdiffusion between peralkaline dacitic and rhyolitic melts     

Don R. Baker  Hélène Bossányi 《Contributions to Mineralogy and Petrology》1994,117(2):203-214

The effective binary diffusion coefficient (EBDC) of silicon has been measured during the interdiffusion of peralkaline, fluorine-bearing (1.3 wt% F), hydrous (3.3 and 6 wt% H₂O), dacitic and rhyolitic melts at 1.0 GPa and temperatures between 1100°C and 1400°C. From Boltzmann-Matano analysis of diffusion profiles the diffusivity of silicon at 68 wt% SiO₂ can be described by the following Arrhenius equations (with standard errors): $$\begin{gathered} {\text{with 1}}{\text{.3 wt\% F and 3}}{\text{.3\% H}}_{\text{2}} {\text{O:}} \hfill \\ {\text{D}}_{{\text{Si}}} = \begin{array}{*{20}c} { + {\text{3}}{\text{.59}}} \\ {{\text{3}}{\text{.66}} \times {\text{10}}^{ - {\text{9}}} } \\ { - {\text{1}}{\text{.86}}} \\ \end{array} {\text{exp}}\left( {{{ - {\text{86}}{\text{.1}} \pm {\text{8}}{\text{.9}}} \mathord{\left/ {\vphantom {{ - {\text{86}}{\text{.1}} \pm {\text{8}}{\text{.9}}} {{\text{RT}}}}} \right. \kern-\nulldelimiterspace} {{\text{RT}}}}} \right) \hfill \\ {\text{with 1}}{\text{.3 wt\% F and 6}}{\text{.0\% H}}_{\text{2}} {\text{O:}} \hfill \\ {\text{D}}_{{\text{Si}}} = \begin{array}{*{20}c} { + {\text{3}}{\text{.59}}} \\ {{\text{3}}{\text{.51}} \times {\text{10}}^{ - {\text{8}}} } \\ { - {\text{1}}{\text{.77}}} \\ \end{array} {\text{exp}}\left( {{{ - {\text{109}}{\text{.5}} \pm {\text{8}}{\text{.9}}} \mathord{\left/ {\vphantom {{ - {\text{109}}{\text{.5}} \pm {\text{8}}{\text{.9}}} {{\text{RT}}}}} \right. \kern-\nulldelimiterspace} {{\text{RT}}}}} \right) \hfill \\ \end{gathered} $$ where D is in m²s^?1 and activation energies are in kJ/mol. Diffusivities measured at 64 and 72 wt% SiO₂ are only slightly different from those at 68 wt% SiO₂ and frequently all measurements are within error of each other. Silicon, aluminum, iron, magnesium, and calcium EBDCs were also calculated from diffusion profiles by error function inversion techniques assuming constant diffusivity. With one exception, silicon EBDCs calculated by error function techniques are within error of Boltzmann-Matano EBDCs. Average diffusivities of Fe, Mg, and Ca were within a factor of 2.5 of silicon diffusivities whereas Al diffusivities were approximately half those of silicon. Alkalies diffused much more rapidly than silicon and non-alkalies, however their diffusivities were not quantitatively determined. Low activation energies for silicon EBDCs result in rapid diffusion at magmatic temperatures. Assuming that water and fluorine exert similar effects on melt viscosity at high temperatures, the viscosity can be calculated and used in the Eyring equation used to determine diffusivities, typically to within a factor of three of those measured in this study. This correlation between viscosity and diffusivity can be inverted to calculate viscosities of fluorine- and water-bearing granitic melts at magmatic temperatures; these viscosities are orders of magnitude below those of hydrous granitic melts and result in more rapid and effective separation of granitic magmas from partially molten source rocks. Comparison of Arrhenius parameters for diffusion measured in this study with Arrhenius parameters determined for diffusion in similar compositions at the same pressure demonstrates simple relationships between Arrhenius parameters, activation energy-E_a, kJ/mol, pre-exponential factor-D_o, m²s^?1, and the volatile, X=F or OH^?, to oxygen, O, ratio of the melt {(X/X+O)}: $$\begin{gathered} {\text{E}}a = - {\text{1533\{ }}{{\text{X}} \mathord{\left/ {\vphantom {{\text{X}} {\left( {{\text{X}} + {\text{O}}} \right)}}} \right. \kern-\nulldelimiterspace} {\left( {{\text{X}} + {\text{O}}} \right)}}{\text{\} }} + {\text{213}}{\text{.3}} \hfill \\ {\text{D}}_{\text{O}} = {\text{2}}{\text{.13}} \times {\text{10}}^{ - {\text{6}}} {\text{exp}}\left[ { - {\text{6}}{\text{.5\{ }}{{\text{X}} \mathord{\left/ {\vphantom {{\text{X}} {\left( {{\text{X}} + {\text{O}}} \right)}}} \right. \kern-\nulldelimiterspace} {\left( {{\text{X}} + {\text{O}}} \right)}}{\text{\} }}} \right] \hfill \\ \end{gathered} $$ These relationships can be used to estimate diffusion in various melts of dacitic to rhyolitic composition containing both fluorine and water. Calculations for the contamination of rhyolitic melts by dacitic enclaves at 800°C and 700°C provide evidence for the virtual inevitability of diffusive contamination in hydrous and fluorine-bearing magmas if they undergo magma mixing of any form.  相似文献   

(Ca,Mg)2Si2O6 clinopyroxenes: A solution model based on nonconvergent site-disorder     

Paula M. Davidson  John Grover  Donald H. Lindsley 《Contributions to Mineralogy and Petrology》1982,80(1):88-102

Experiments at high pressure and temperature indicate that excess Ca may be dissolved in diopside. If the (Ca, Mg)₂Si₂O₆ clinopyroxene solution extends to more Ca-rich compositions than CaMgSi₂O₆, macroscopic regular solution models cannot strictly be applied to this system. A nonconvergent site-disorder model, such as that proposed by Thompson (1969, 1970), may be more appropriate. We have modified Thompson's model to include asymmetric excess parameters and have used a linear least-squares technique to fit the available experimental data for Ca-Mg orthopyroxene-clinopyroxene equilibria and Fe-free pigeonite stability to this model. The model expressions for equilibrium conditions \(\mu _{{\text{Mg}}_{\text{2}} {\text{Si}}_{\text{2}} {\text{O}}_{\text{6}} }^{{\text{opx}}} = \mu _{{\text{Mg}}_{\text{2}} {\text{Si}}_{\text{2}} {\text{O}}_{\text{6}} }^{{\text{cpx}}} \) (reaction A) and \(\mu _{{\text{Ca}}_{\text{2}} {\text{Si}}_{\text{2}} {\text{O}}_{\text{6}} }^{{\text{opx}}} = \mu _{{\text{Ca}}_{\text{2}} {\text{Si}}_{\text{2}} {\text{O}}_{\text{6}} }^{{\text{cpx}}} \) (reaction B) are given by: 1 $$\begin{gathered} \Delta \mu _{\text{A}}^{\text{O}} = {\text{RT 1n}}\left[ {\frac{{(X_{{\text{Mg}}}^{{\text{opx}}} )^2 }}{{X_{{\text{Mg}}}^{{\text{M1}}} \cdot X_{{\text{Mg}}}^{{\text{M2}}} }}} \right] - \frac{1}{2}\{ W_{21} [2(X_{{\text{Ca}}}^{{\text{M2}}} )^3 - (X_{{\text{Ca}}}^{{\text{M2}}} ] \hfill \\ {\text{ + 2W}}_{{\text{22}}} [X_{{\text{Ca}}}^{{\text{M2}}} )^2 - (X_{{\text{Ca}}}^{{\text{M2}}} )^3 + \Delta {\text{G}}_{\text{*}}^{\text{0}} (X_{{\text{Ca}}}^{{\text{M1}}} \cdot X_{{\text{Ca}}}^{{\text{M2}}} )\} \hfill \\ {\text{ + W}}^{{\text{opx}}} (X_{{\text{Wo}}}^{{\text{opx}}} )^2 \hfill \\ \Delta \mu _{\text{B}}^{\text{O}} = {\text{RT 1n}}\left[ {\frac{{(X_{{\text{Ca}}}^{{\text{opx}}} )^2 }}{{X_{{\text{Ca}}}^{{\text{M1}}} \cdot X_{{\text{Ca}}}^{{\text{M2}}} }}} \right] - \frac{1}{2}\{ 2W_{21} [2(X_{{\text{Mg}}}^{{\text{M2}}} )^2 - (X_{{\text{Mg}}}^{{\text{M2}}} )^3 ] \hfill \\ {\text{ + W}}_{{\text{22}}} [2(X_{{\text{Mg}}}^{{\text{M2}}} )^3 - (X_{{\text{Mg}}}^{{\text{M2}}} )^2 + \Delta {\text{G}}_{\text{*}}^{\text{0}} (X_{{\text{Mg}}}^{{\text{M1}}} \cdot X_{{\text{Mg}}}^{{\text{M2}}} )\} \hfill \\ {\text{ + W}}^{{\text{opx}}} (X_{{\text{En}}}^{{\text{opx}}} )^2 \hfill \\ \hfill \\ \end{gathered} $$ where 1 $$\begin{gathered} \Delta \mu _{\text{A}}^{\text{O}} = 2.953 + 0.0602{\text{P}} - 0.00179{\text{T}} \hfill \\ \Delta \mu _{\text{B}}^{\text{O}} = 24.64 + 0.958{\text{P}} - (0.0286){\text{T}} \hfill \\ {\text{W}}_{{\text{21}}} = 47.12 + 0.273{\text{P}} \hfill \\ {\text{W}}_{{\text{22}}} = 66.11 + ( - 0.249){\text{P}} \hfill \\ {\text{W}}^{{\text{opx}}} = 40 \hfill \\ \Delta {\text{G}}_*^0 = 155{\text{ (all values are in kJ/gfw)}}{\text{.}} \hfill \\ \end{gathered} $$ . Site occupancies in clinopyroxene were determined from the internal equilibrium condition 1 $$\begin{gathered} \Delta G_{\text{E}}^{\text{O}} = - {\text{RT 1n}}\left[ {\frac{{X_{{\text{Ca}}}^{{\text{M1}}} \cdot X_{{\text{Mg}}}^{{\text{M2}}} }}{{X_{{\text{Ca}}}^{{\text{M2}}} \cdot X_{{\text{Mg}}}^{{\text{M1}}} }}} \right] + \tfrac{1}{2}[(2{\text{W}}_{{\text{21}}} - {\text{W}}_{{\text{22}}} )(2{\text{X}}_{{\text{Ca}}}^{{\text{M2}}} - 1) \hfill \\ {\text{ + }}\Delta G_*^0 (X_{{\text{Ca}}}^{{\text{M1}}} - X_{{\text{Ca}}}^{{\text{M2}}} ) + \tfrac{3}{2}(2{\text{W}}_{{\text{21}}} - {\text{W}}_{{\text{22}}} ) \hfill \\ {\text{ (1}} - 2X_{{\text{Ca}}}^{{\text{M1}}} )(X_{{\text{Ca}}}^{{\text{M1}}} + \tfrac{1}{2})] \hfill \\ \end{gathered} $$ where δG _E ⁰ =153+0.023T+1.2P. The predicted concentrations of Ca on the clinopyroxene Ml site are low enough to be compatible with crystallographic studies. Temperatures calculated from the model for coexisting ortho- and clinopyroxene pairs fit the experimental data to within 10° in most cases; the worst discrepancy is 30°. Phase relations for clinopyroxene, orthopyroxene and pigeonite are successfully described by this model at temperatures up to 1,600° C and pressures from 0.001 to 40 kbar. Predicted enthalpies of solution agree well with the calorimetric measurements of Newton et al. (1979). The nonconvergent site disorder model affords good approximations to both the free energy and enthalpy of clinopyroxenes, and, therefore, the configurational entropy as well. This approach may provide an example for Febearing pyroxenes in which cation site exchange has an even more profound effect on the thermodynamic properties.  相似文献   

Ephesite,Na(LiAl2) [Al2Si2O10] (OH)2: I. thermal stability and standard state thermodynamic properties     

Udo Warhus  Niranjan D. Chatterjee 《Contributions to Mineralogy and Petrology》1984,85(1):74-79

Ephesite, Na(LiAl₂) [Al₂Si₂O₁₀] (OH)₂, has been synthesized for the first time by hydrothermal treatment of a gel of requisite composition at 300≦T(° C)≦700 and \(P_{H_2 O}\) upto 35 kbar. At \(P_{H_2 O}\) between 7 and 35 kbar and above 500° C, only the 2M₁ polytype is obtained. At lower temperatures and pressures, the 1M polytype crystallizes first, which then inverts to the 2M₁ polytype with increasing run duration. The X-ray diffraction patterns of the 1M and 2M₁ poly types can be indexed unambiguously on the basis of the space groups C2 and Cc, respectively. At its upper thermal stability limit, 2M₁ ephesite decomposes according to the reaction (1) $$\begin{gathered} {\text{Na(LiAl}}_{\text{2}} {\text{) [Al}}_{\text{2}} {\text{Si}}_{\text{2}} {\text{O}}_{{\text{10}}} {\text{] (OH)}}_{\text{2}} \hfill \\ {\text{ephesite}} \hfill \\ {\text{ = Na[AlSiO}}_{\text{4}} {\text{] + LiAl[SiO}}_{\text{4}} {\text{] + }}\alpha {\text{ - Al}}_{\text{2}} {\text{O}}_{\text{3}} {\text{ + H}}_{\text{2}} {\text{O}} \hfill \\ {\text{nepheline }}\alpha {\text{ - eucryptite corundum}} \hfill \\ \end{gathered}$$ Five reversal brackets for (1) have been established experimentally in the temperature range 590–750° C, at \(P_{H_2 O}\) between 400 and 2500 bars. The equilibrium constant, K, for this reaction may be expressed as (2) $$log K{\text{ = }}log f_{{\text{H}}_{\text{2}} O}^* = 7.5217 - 4388/T + 0.0234 (P - 1)T$$ where \(f_{H_2 O}^* = f_{H_2 O} (P,T)/f_{H_2 O}^0\) (1,T), with T given in degrees K, and P in bars. Combining these experimental data with known thermodynamic properties of the decomposition products in (1), the following standard state (1 bar, 298.15 K) thermodynamic data for ephesite were calculated: H _f,298.15 ⁰ =-6237372 J/mol, S _298.15 ⁰ =300.455 J/K·mol, G _298.15 ⁰ =-5851994 J/mol, and V _298.15 ⁰ =13.1468 J/bar·mol.  相似文献   

A note on the significance of the assemblage calcite-quartz-plagioclase-paragonite-graphite     

E. D. Ghent  J. W. Jones  J. Nicholls 《Contributions to Mineralogy and Petrology》1970,28(2):112-116

The biotite zone assemblage: calcite-quartz-plagioclase (An₂₅)-phengite-paragonite-chlorite-graphite, is developed at the contact between a carbonate and a pelite from British Columbia. Thermochemical data for the equilibrium paragonite+calcite+2 quartz=albite+ anorthite+CO₂+H₂O yields: $$\log f{\text{H}}_{\text{2}} {\text{O}} + \log f{\text{CO}}_{\text{2}} = 5.76 + 0.117 \times 10^{ - 3} (P - 1)$$ for a temperature of 700°K and a plagioclase composition of An₂₅. By combining this equation with equations describing equilibria between graphite and gas species in the system C-H-O, the following partial pressures: \(P{\text{H}}_2 {\text{O}} = 2572{\text{b, }}P{\text{CO}}_2 = 3162{\text{b, }}P{\text{H}}_2 = 2.5{\text{b, }}P{\text{CH}}_4 = 52.5{\text{b, }}P{\text{CO}} = 11.0{\text{b}}\) are obtained for \(f{\text{O}}_2 = 10^{ - 26}\) . If total pressure equals fluid pressure, then the total pressure during metamorphism was approximately 6 kb. The total fluid pressure calculated is extremely sensitive to the value of \(f{\text{O}}_2\) chosen.  相似文献   

A thermodynamic formulation of hydrous cordierite     

J. William Carey 《Contributions to Mineralogy and Petrology》1995,119(2-3):155-165

A thermodynamic formulation of hydrous Mg-cordierite (Mg₂Al₄Si₅O₁₈·nH₂O) has been obtained by application of calorimetric and X-ray diffraction data for hydrous cordierite to the results of hydrothermal syntheses. The data include measurements of the molar heat capacity and enthalpy of hydration and the molar volume. The synthesis data are consistent with a thermodynamic formulation in which H₂O mixes ideally on a single crystallographic site in hydrous cordierite. The standard molar Gibbs free energy of hydration is-9.5±1.0 kJ/mol (an average of 61 syntheses). The standard molar entropy of hydration derived from this value is-108±3 J/mol-K. An equation providing the H₂O content of cordierite as a function of temperature and fugacity of H₂O is as follows (n moles of H₂O per formula unit, n<1): $$\begin{gathered}n = {{f_{{\text{ H}}_{\text{2}} O}^{\text{V}} } \mathord{\left/{\vphantom {{f_{{\text{ H}}_{\text{2}} O}^{\text{V}} } {\left( {f_{{\text{ H}}_{\text{2}} O}^{\text{V}} + {\text{exp}}\left[ { - {\text{3}}{\text{.8389}} - 5025.2\left( {\frac{1}{T} - \frac{1}{{298.15}}} \right)} \right.} \right.}}} \right.\kern-\nulldelimiterspace} {\left( {f_{{\text{ H}}_{\text{2}} O}^{\text{V}} + {\text{exp}}\left[ { - {\text{3}}{\text{.8389}} - 5025.2\left( {\frac{1}{T} - \frac{1}{{298.15}}} \right)} \right.} \right.}} \hfill \\{\text{ }}\left. {\left. { - {\text{ln}}\left( {\frac{T}{{{\text{298}}{\text{.15}}}}} \right) - \left( {\frac{{298.15}}{T} - 1} \right)} \right]} \right) \hfill \\\end{gathered}$$ Application of this formulation to the breakdown reaction of Mg-cordierite to an assemblage of pyrope-sillimanite-quartz±H₂O shows that cordierite is stabilized by 3 to 3.5 kbar under H₂O-saturated conditions. The thermodynamic properties of H₂O in cordierite are similar to those of liquid water, with a standard molar enthalpy and Gibbs free energy of hydration that are the same (within experimental uncertainty) as the enthalpy and Gibbs free energy of vaporization. By contrast, most zeolites have Gibbs free energies of hydration two to four times more negative than the corresponding value for the vaporization of water.  相似文献