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Aquatic Geochemistry (v.6, #2)
Lippmann Diagrams: Theory and Application to Carbonate Systems by Heinz Gamsjäger; Erich Königsberger; Wolfgang Preis (pp. 119-132).
Since 1980 when F. Lippmann's seminal paper appeared, ourunderstanding of solubility equilibria involving ionic solidsolutions has been advanced by theoretical considerations as wellas careful experimental studies designed to determine excess Gibbsfunctions. A unified theory of solid-solution aqueous-solutionequilibria as well as the thermodynamic background of thephenomenon of ``stoichiometric saturation'' are reviewed.It is shown that Lippmann diagrams effectively summarize thethermodynamic basis of solid-solute aqueous-solution equilibria ofsparingly soluble metal carbonate systems. Clearly, the predictivepower of these diagrams may be limited due to kineticrestrictions. Only when dissolution and precipitation areessentially reversible, favourable conditions to synthesizehomogeneous solid phases can be derived from studies of equilibria.
Keywords: Lippmann diagrams; phase equilibria; solubility; stoichiometric saturation; metal carbonates
Computing Lippmann Diagrams from Direct Calculation of Mixing Properties of Solid Solutions: Application to the Barite-Celestite System by M. Prieto; A. Fernández-González; U. Becker; A. Putnis (pp. 133-146).
The Lippmann diagram for the system(Ba, Sr)SO4-H2O was computed at 25 °Cby determining the solid-phase activity coefficientsfrom first principles calculations. Directcalculations of the mixing properties of thebarite-celestite series indicate this solid solutionbehaves as non-ideal and non-regular. At 25 °C,the enthalpy of mixing shows a minimum around 50 mole% SrSO4 due to an ordering tendency. Thefree energy of mixing shows two minima that delimit awide and symmetric miscibility gap (from 2.1 to 97.9 mole% SrSO4) at this temperature. The excessfree energy of mixing requires a Guggenheim expansionseries of 5 terms to be described, where the termswith odd exponents are null as a consequence of thesymmetric distribution of the mixing properties withcomposition. The Lippmann diagram shows a peritecticpoint that corresponds to the composition of an aqueoussolution which is simultaneously at equilibrium withthe two extremes of the miscibility gap. The largedifference between the solubility products of theendmembers involves a strong preferential partitioningof the less soluble endmember towards the solid phase,which explains the extremely Ba-poor composition ofthe aqueous solution (aqueous activity fraction forBa2+ = 0.000446 ) at the peritectic point.
Keywords: Lippmann diagram; solid solution; free energy of mixing; barite; celestite
Application of Gibbs Energy Minimization to ModelEarly-Diagenetic Solid-Solution Aqueous-Solution EquilibriaInvolving Authigenic Rhodochrosites in Anoxic BalticSea Sediments by Dmitrii A. Kulik; Michael Kersten; Uwe Heiser; Thomas Neumann (pp. 147-199).
The natural early-diagenetic environment ``anoxicporewater – authigenic mineral phases'' has beencharacterized in sediment of the Gotland Deep,Baltic Sea, by a closed-system model. Occurrence ofcarbonate precipitates as thin almost pure whitelaminae was considered as a natural experiment forlong-term equilibration between these phases andporewater. Plots of distribution coefficientsindicate that metastable equilibrium exists betweenporewater and the authigenic Ca-rich rhodochrositephases below 7 cm depth. A thermodynamic model ofporewater geochemistry at in situ P = 25 barand T = 5 °C was developed using the Gibbsenergy minimization (GEM) approach. The values of isobaric-isothermal potentials of Mn, Ca, Fe, Mg,Sr, Ba, C, and O, calculated from the porewatercomposition, were used in a new ``dual thermodynamic''calculation approach to estimate solid activitycoefficients of the end-members in the non-idealsolid solution (Mn, Ca, Mg, Sr, Ba, Fe)CO3,i.e., at full major and minor multi-componentcomplexity. The regular Margules interactionparameters for the composing binaries estimated bythis model were αMn-Ca = 1.9 ± 0.5,αMn-Mg = 0.6,αCa-Mg = 3.7,αMn-Fe = 0.2,αCa-Fe = 2.8,αMn-Sr = 9.7,αCa-Sr = 2.15,αMn-Ba = 4.0,αCa-Ba = 1.4,validating the theoretical predictions given byLippmann in his pioneering 1980's paper. Thestrictly thermodynamic equilibrium model is not onlyable to match both the measured porewater andcarbonate solid-solution composition, but also topredict that the porewater pH, pe, alkalinity, anddissolved Mn, Fe, and S concentrations arecontrolled by the authigenic mineral bufferingassemblage mackinawite-greigite-rhodochrosite. Ourmodel is only compatible with the idea of ACRformation with typical composition (XMnbetween 70–75%) in the topmost sediment layerwhich, however, needs a major source ofMnaq II. This is provided by reduction ofparticulate Mn oxides precipitated in significantamounts in the water column upon major inflow eventsin the Baltic Sea. The model enables also to set upscenarios of changing environmental conditions, e.g.,to predict the non-linear response of the carbonatesolid-solution composition to changes in Mn loading,alkalinity and salinity of the sediment-watersystem. The results suggest that the major andespecially minor element contents (Sr, Mg, Ba) inauthigenic carbonates can be applied as anenvironmental paleoproxy.
Stable Isotope Fractionation during Experimental Formation of Norsethite (BaMg[CO3]2): A Mineral Analogue of Dolomite by Michael E. Bötcher (pp. 201-212).
Stable oxygen and carbon isotopefractionation during the experimental formation ofordered norsethite (BaMg[CO3]2) from thereaction of anhydrous BaCO3 (witherite) withrelatively low concentrated sodium-magnesiumbicarbonate solutions has been studied between20° and 135 °C. In the investigatedtemperature range, 18O and 13C are enrichedin norsethite with respect to water and gaseous carbondioxide, respectively. Whereas 18O/16Opartitioning is intermediate between those of theBaCO3–H2O and MgCO3–H2O systems,13C/12C partitioning is more similar to thatfor BaCO3–CO2. Between 20° and90°C, the temperature dependences of the18O/16O and 13C/12C fractionationfactors are represented by the equations (T in °K):103 ln αBaMg[CO3]2-H2O = 2.83 106T--2.85, and 103lnαBaMg[CO3]2-CO2(gas) = 1.78 106T--10.16. The later equation considers carbon isotope fractionationbetween the dissolved carbonate ion and carbon dioxide measured by Halaset al. (1997). Under standard state conditions (25 °C) the fractionation factors in the system BaMg[CO3]2-CO2-H2O are: Oxygen isotopes: αBaMg(CO3)2-H2O = 1.02941, αBaMg(CO3)2-OH-(aq) = 1.07059,αBaMg(CO3)2-CO2(gas) = 0.98868, andαBaMg(CO3)2-H2CO3 * = 0.98843; carbon isotopes:αBaMg(CO3)2-CO2(gas) = 1.00992,αBaMg(CO3)2-H2CO3 * = 1.01099,αBaMg(CO3)2-HCO3 - = 1.00194,αBaMg(CO3)2-CO3 2- = 1.00491 or 1.00150.The spontaneous precipitation of aBaMg[CO3]2 gel at 20 °C,followed by the alteration of the products at20° or 60°C for 31 days,demonstrated isotope exchange reactions betweensolids and mother solutions dueto recrystallization. Isotope equilibrium, wasnot reached within run time.
Keywords: carbonates; norsethite; dolomite; stable isotope fractionation; experiment; low temperature
Constraints on Cation Order in Calcium-rich Sedimentary Dolomite by Richard J. Reeder (pp. 213-226).
Most sedimentary Ca-rich dolomite in pre-Holocenerocks is known to exhibit a fine-scale modulation inTEM images, reflecting a domain structure of which onecomponent is a low-symmetry variant of dolomite. Thevarious structural models proposed for thelow-symmetry component involve Ca–Mg order patternsthat are different from that in dolomite. Caution istherefore required for interpretation of X-raydiffraction data, which average over the entirestructure. The average structures of two Ca-richdolomites having a structural modulation are refinedusing single-crystal intensity data. The resultsindicate a poor fit using a dolomite model, which isconsistent with the findings from TEM and electrondiffraction indicating that one component of thedomain structure is different than dolomite. Resultsalso indicate that average model A and B cation siteshave mixed Ca–Mg occupancy, which provides someconstraints on possible models for the low-symmetrydomains.
Keywords: dolomite crystal structure; dolomite x-ray diffraction; dolomite electron microscopy; dolomite
Chemical and Isotopic Constraints for Recrystallization of Sedimentary Dolomites from the Western Canada Sedimentary Basin by Ihsan S. Al-Aasm (pp. 227-248).
Mississippian shoal carbonates of Western CanadaSedimentary Basin are important hydrocarbon hosts.Dolomitization plays a major role in the evolution ofreservoir porosity in these carbonates. This processvaries across the basin and reflects, in part, divergentsources and chemistry of pore fluids. Dolomites fromseveral petroleum reservoirs were analyzed formineralogical, geochemical and isotopic variation. Thedata clearly demonstrate the progressive and complexrecrystallization of dolomite during shallow and deepburial in modified marine, meteoric and burial fluids.These data include: change in crystal size,stoichiometry, cathodoluminscence characteristics,stable oxygen and carbon isotopic shifts and changesin radiogenic Sr isotopic composition. However,regional geology, tectonic history and fluid flowevolution play important roles in the diageneticimprints and the degree of recrystallization.Early microcrystalline dolomite formed in normalmarine and evaporative conditions in Mississippiancarbonates from Western Canada Sedimentary Basinhave undergone variable degrees of recrystallization, frompristine dolomite akin to Holocene sabkha dolomitewith preserved mineralogical and chemical attributesto highly recrystallized mesodolomite, however stillnonstoichiometric, but with highly altered chemicalsignatures. Careful attention should be made to localgeology, hydrodynamics and fluid flow when investigatingdolomite recrystallization in sedimentary basins.
Keywords: dolomite; recrystallization; chemistry; Mississippian
Temperature Dependence of Mineral Precipitation Rates Along the CaCO3–MgCO3 Join by Rolf S. Arvidson; Fred T. Mackenzie (pp. 249-256).
The large variation in precipitation rate and abundance of mineralscomprising the CaCO3–MgCO3 binary join can be understood in terms of their large differences in activation energy. Following the treatment of Lippmann (1973), activation energy isextrapolated along the join as a linear function of mole percentmagnesium. For the dolomite-type carbonates, the predicted activationenergy is compatible with recent measurements of calcian protodolomitekinetics; cation ordering in ideal dolomite can thus be seen as anadditional contribution to activation energy. Although no activationenergies are available for magnesian calcites, treatment of rate datafor these phases using the formalism of stoichiometric saturationsuggests a possible change in mechanism or rate-limiting step astemperature is decreased from 25 to 5 °C.
Keywords: precipitation kinetics; dolomite; magnesian calcite
In-situ Growth of Calcite at Devils Hole, Nevada: Comparison of Field and Laboratory Rates to a 500,000 Year Record of Near-Equilibrium Calcite Growth by L. Niel Plummer; Eurybiades Busenberg; Alan C. Riggs (pp. 257-274).
Calcite grew continuously for 500,000 years on the submerged walls of an open fault plane (Devils Hole) in southern Nevada, U.S.A. at rates of 0.3 to 1.3 mm/ka, but ceased growing approximately 60,000 years ago, even though the fault plane remained open and was continuously submerged. The maximum initial in-situ growth rate on pre-weighed crystals of Iceland spar placed in Devils Hole (calcite saturation index, SI, is 0.16 to 0.21 at 33.7 °C) for growth periods of 0.75 to 4.5 years was 0.22 mm/ka. Calcite growth on seed crystals slowed or ceased following initial contact with Devils Hole groundwater. Growth rates measured in synthetic Ca-HCO3 solutions at 34 °C, CO2 partial pressures of 0.101, 0.0156 (similar to Devils Hole groundwater) and 0.00102 atm, and SI values of 0.2 to 1.9 were nearly independent of PCO 2, decreased with decreasing saturation state, and extrapolated through the historical Devils Hole rate. The results show that calcite growth rate is highly sensitive to saturation state near equilibrium. A calcite crystal retrieved from Devils Hole, and used without further treatment of its surface, grew in synthetic Devils Hole groundwater when the saturation index was raised nearly 10-fold that of Devils Hole water, but the rate was only 1/4 that of fresh laboratory crystals that had not contacted Devils Hole water. Apparently, inhibiting processes that halted calcite growth in Devils Hole 60,000 years ago continue today.
Keywords: calcite growth; calcite precipitation; carbonate groundwater; Devils Hole; near-equilibrium rates