Synthetic Calcite Research Articles

High-temperature creep of synthetic calcite aggregates: influence of Mn-content

To investigate the role of impurities on the deformation of calcite rocks, we performed creep experiments on fine-grained synthetic calcite aggregates with varying Mn-content. The tests were performed in a uniaxial creep apparatus at temperatures of 700–830 °C and stresses between 15 and 65 MPa, resulting in strain rates of 10 −7 to 10 −4 s −1. Varying the oxygen fugacity, f O 2 , between 10 −18 and 10 −4 MPa did not affect the creep strength. Applying a power law constitutive relationship on the creep data, two distinct creep regimes were observed, a low stress regime (<15 MPa) and a high stress regime (>20–25 MPa). In the low stress regime, a stress exponent n of 1.7±0.3 and an activation energy of Q=300±45 kJ/mol were found. The mechanical data and microstructural observations suggest that grain-boundary sliding accommodated by grain-boundary diffusion is the dominant creep mechanism. At high stresses, we obtained a stress exponent of n=3.1±0.6 and an activation energy of Q=490±50 kJ/mol, indicating dislocation creep. Transmission-electron microscopy (TEM) observations show a dominance of recovery-related dislocation structures suggesting that dislocation climb is important. Increasing Mn-concentration from 10 to 670 ppm leads to an enhancement in creep rate in both creep regimes, but the effect is more substantial in the diffusion creep regime (one order of magnitude). In the diffusion creep regime, the enhancement of creep rate by Mn is attributed to the substitution of Ca by Mn, which may increase grain-boundary diffusivity of calcite. In the dislocation regime, it is interpreted as a combination of dislocation climb and glide with the exchange of Ca by aliovalent Mn, which may affect the point defect concentration.

Strontium incorporation into calcite generated by bacterial ureolysis

Strontium incorporation into calcite generated by bacterial ureolysis was investigated as part of an assessment of a proposed remediation approach for 90Sr contamination in groundwater. Urea hydrolysis produces ammonium and carbonate and elevates pH, resulting in the promotion of calcium carbonate precipitation. Urea hydrolysis by the bacterium Bacillus pasteurii in a medium designed to mimic the chemistry of the Snake River Plain Aquifer in Idaho resulted in a pH rise from 7.5 to 9.1. Measured average distribution coefficients (D EX) for Sr in the calcite produced by ureolysis (0.5) were up to an order of magnitude higher than values reported in the literature for natural and synthetic calcites (0.02–0.4). They were also higher than values for calcite produced abiotically by ammonium carbonate addition (0.3). The precipitation of calcite in these experiments was verified by X-ray diffraction. Time-of-flight secondary ion mass spectrometry (ToF SIMS) depth profiling (up to 350 nm) suggested that the Sr was not merely sorbed on the surface, but was present at depth within the particles. X-ray absorption near edge spectra showed that Sr was present in the calcite samples as a solid solution. The extent of Sr incorporation appeared to be driven primarily by the overall rate of calcite precipitation, where faster precipitation was associated with greater Sr uptake into the solid. The presence of bacterial surfaces as potential nucleation sites in the ammonium carbonate precipitation treatment did not enhance overall precipitation or the Sr distribution coefficient. Because bacterial ureolysis can generate high rates of calcite precipitation, the application of this approach is promising for remediation of 90Sr contamination in environments where calcite is stable over the long term.

Oxygen isotope exchange and disequilibrium between calcite and tremolite in the absence and presence of an experimental C?O?H fluid

Oxygen isotope exchange between minerals during metamorphism can occur in either the presence or the absence of aqueous fluids. Oxygen isotope partitioning among minerals and fluid is governed by both chemical and isotopic equilibria during these processes, which progress by intragranular and intergranular diffusion as well as by surface reactions. We have carried out isotope exchange experiments in two- and three-phase systems, respectively, between calcite and tremolite at high temperatures and pressures. The two-phase system experiments were conducted without fluid either at 1 GPa and 680 °C for 7 days or at 500 MPa and 560 °C for 20 days. Extrapolated equilibrium fractionations between calcite and tremolite are significantly lower than existing empirical estimates and experimental determinations in the presence of small amounts of fluid, but closely match calculated fractionations by means of the increment method for framework oxygen in tremolite. The small fractionations measured in the direct calcite–tremolite exchange experiments are interpreted by different rates of oxygen isotope exchange between hydroxyl oxygen, framework oxygen and calcite during the solid–solid reactions where significant recrystallization occurs. The three-phase system experiments were accomplished in the presence of a large amount of fluid (CO2+H2O) at 500 MPa and 560 °C under conditions of phase equilibrium for 5, 10, 20, 40, 80, 120, 160, and 200 days. The results show that oxygen isotope exchange between minerals and fluid proceeds in two stages: first, through a mechanism of dissolution-recrystallization and very rapidly; second, through a mechanism of diffusion and very slowly. Synthetic calcite shows a greater rate of isotopic exchange with fluid than natural calcite in the first stage. The rate of oxygen diffusion in calcite is approximately equal to or slightly greater than that in tremolite in the second stage. A calculation using available diffusion coefficients for calcite suggests that grain boundary diffusion, rather than volume diffusion, has been the dominant mechanism of oxygen transport between the fluid and the mineral grains in the later stage.

Shear-enhanced compaction during non-linear viscous creep of porous calcite–quartz aggregates

In this paper, we extend the previous studies of semi-brittle flow of synthetic calcite–quartz aggregates to a range of temperatures and effective pressures where viscous creep occurs. Triaxial deformation experiments were performed on hot-pressed calcite–quartz aggregates containing 10, 20 and 30 wt% quartz at confining pressure of 300 MPa, pore pressures of 50–290 MPa, temperatures of 673–1073 K and strain rates of 3.0×10−5/s, 8.3×10−5/s and 3.0×10−4/s. Starting porosity varied from 5 to 9%. We made axial and volumetric strain measurements during the mechanical tests. Pore volume change was measured by monitoring the volume of pore fluid that flows out of or into the specimen at constant pore pressure. Yield stress increased with decreasing porosity and showed a dependence on effective pressure. Thus, the yield stress versus effective pressure can be described as a yield surface with negative slope that expands with decreasing porosity and increasing strain hardening, gradually approaching the envelope of strength at 10% strain, which has a positive slope. Creep of porous rock can be modeled to first order as an isolated equivalent void in an incompressible nonlinear viscous matrix. An incremental method is used to calculate the stress–strain curve of the porous material under a constant external strain rate. The numerical simulations reproduce general trends of the deformation behavior of the porous rock, such as the yield stress decreasing with increasing effective pressure and significant strain hardening at high effective pressure. The drop of yield stress with increasing porosity is modeled well, and so is the volumetric strain rate, which increases with increasing porosity.

Unusual morphological and microstructural features of synthetic calcite induced by acrylate/styrene copolymers

Crystallisation of calcite has been induced at the air–water interface by added water-soluble acrylate-based copolymers. Two unusual calcite morphologies are reported, one c-axis needles the other spherical particles. Both appear to comprise submicron crystallites.

Cathodoluminescence of synthetic and natural calcite: the effects of manganese and iron on orange emission

The orange cathodoluminescence (CL) of calcite is known to be due to the presence of Mn2+ cations. It has been demonstrated here using CL and electron paramagnetic resonance (EPR) crossed analysis from synthetic calcite that neither Fe2+ nor Fe3+ ions influence this luminescence emission. More complex natural calcium carbonates have been investigated to check whether or not this conclusion can be applied to them. For this purpose, different white marbles from Greek quarries were analysed with CL. The data are completed with neutron activation analysis (NAA) for manganese and iron contents. Again it is shown that only manganese plays a role in the orange CL of these white marbles. This result provides an important clue in the wide field of provenance determination of calcium carbonate used in ancient art.

The effect of dissolved magnesium on diffusion creep in calcite

We experimentally tested a series of synthetic calcite marbles with varying amounts of dissolved magnesium in a standard triaxial deformation machine at 300 MPa confining pressure, temperatures between 700 and 850°C, stresses between 2 and 100 MPa, and strain rates between 10 −7 and 10 −3 s −1. The samples were fabricated by hot isostatic pressing of a mixture of calcite and dolomite at 850°C and 300 MPa. The fabrication protocol resulted in a homogeneous, fine-grained high-magnesian calcite aggregate with minimal porosity and with magnesium contents between 0.07 and 0.17 mol% MgCO 3. At stresses below 40 MPa the samples deformed with linear viscosity that depended inversely on grain size to the 3.26±0.51 power, suggesting that the mechanisms of deformation were some combination of grain boundary diffusion and grain boundary sliding. Because small grain sizes tended to occur in the high-magnesium calcite, the strength also appeared to vary inversely with magnesium content. However, the strength at constant grain size does not depend on the amount of dissolved magnesium, and thus, the impurity effect seems to be indirect. At stresses higher than 40 MPa, the aggregates become non-linearly viscous, a regime we interpret to be dislocation creep. The transition between the two regimes depends on grain size, as expected. The activation energy for diffusion creep is 200±30 kJ/mol and is quite similar to previous measurements in natural and synthetic marbles deformed at similar conditions with no added magnesium.

Spontaneously Induced Reduction of Trivalent Ytterbium in Synthesized Crystal of Calcite

Abstract XANES spectra showed the occurrence of divalent ytterbium in the synthetic calcite (CaCO3). Approximately 15% ytterbium ion existed as divalent in calcite, although ytterbium ion was trivalent in the starting solutions.

Rheology of calcite–quartz aggregates deformed to large strain in torsion

High‐temperature, high‐pressure torsion experiments have been conducted on Solnhofen limestone and synthetic calcite–quartz aggregates to investigate the evolution of mechanical strength and microstructure to large strains. Hot isostatic pressing of powders containing 1–30 wt% quartz particles produced synthetic two‐phase aggregates containing trace amounts of wollastonite. The experiments were performed at constant twist rate, 300 MPa confining pressure, and temperatures ranging from 900 to 1300 K in the stability fields of calcite plus quartz and wollastonite plus carbon dioxide. The mechanical data from torsion tests of most samples show a pronounced peak stress at shear strains &lt;1 and subsequent weakening. For most samples, a steady state stress is reached only at shear strains &gt;5. A distinct shape and lattice preferred orientation developed in Solnhofen limestone and in the calcite matrix of the synthetic aggregates. However, at large shear strains, flattened porphyroclasts were almost entirely consumed by recrystallization. Elongation parallel to the specimen axis at temperature below 1100 K is possibly due to rotation of minerals into a preferred crystallographic orientation. Stress exponent and activation energy for creep of the calcite–quartz aggregates increase substantially with increasing quartz content. The strength of the two‐phase aggregates increases with quartz fraction up to 20 wt% by a factor of 2–5 depending on temperature and finite strain. Continuum models underestimate particle strengthening of the calcite–quartz aggregates. An alternative microphysical mechanism for the observed strengthening may be related to the reduced mobility of dislocations through diffusion of silicon into dislocation cores.

Pitfalls in the determination of empirical dissolution rate equations of minerals from experimental data and a way out: an iterative procedure to find valid rate equations, applied to Ca-carbonates and -sulphates

Empirical rate equations such as R= k(1− c/ c eq) n in the dissolution of minerals are common in nature, e.g. limestone. The quantity c is the concentration of a major ion contained in the mineral, and c eq its concentration at equilibrium. If experimental data obey such a rate equation, by plotting log( R) versus log(1− c/ c eq) straight lines are found from which k and n can be determined. In many experiments, however, especially for natural minerals c eq is not known exactly. If one uses wrong values of c eq that deviate only a few percent from true equilibrium such plots are severely distorted and one may conclude that above some value c s the true order n changes to a new value, even when a rate equation as given above is valid. We present an iterative computational procedure, which allows to find the valid rate equation from experimental data, even when c eq is not known. The method is applied to limestone and synthetic calcite as well as to natural and synthetic gypsum. New experimental data are given for the dissolution rates of anhydrite (CaSO 4). By use of our new method, we find that this mineral exhibits a surface controlled rate equation with k=5.0±1.0×10 −6 mmol cm −2 s −1, n=4.5±0.2 and c eq=23.5±0.1 mmol/l at T=10 °C.

Quantitative cathodoluminescence (CL) spectroscopy of minerals: possibilities and limitations

¶The luminescence spectrum of a mineral contains complex information related to the intrinsic crystal and the defect structure. For quantitative analysis of cathodoluminescence (CL) the spectra have to be deconvoluted by fitting and filtering procedures to identify and measure individual peaks. Peak-width, peak-position and transition probability of the luminescence centres are influenced by effects such as interactions within the defects themselves, and interaction between defects and the surrounding crystal lattice. For calcite and feldspar a linear correlation between the defect concentration of manganese and the Mn2+-activated CL-intensity is documented. Combined Micro-Particle Induced X-ray Emission (μ-PIXE) and CL-spectroscopy analyses of REE-doped synthetic calcite suggest a linear correlation between REE-activated CL intensity and REE-concentration at REE-concentration levels below approximately 500 ppm. Sensitising and quenching by other REE are dominant effects yielding strong variations in the correlation between the REE-activated CL-intensity and the REE-content.

Etude de la resonance paramagnetique electronique (RPE) de l'ion CO−3 distordu et associe a l'impurete lithium dans la calcite synthetique

Electronic spin resonance (ESR) study of distorted CO−3 ions associated with lithium impurities in synthetic calcite. A paramagnetic centre was created in a single crystal of synthetic calcite by X ray irradiation at 77 K. It is characterized by an extreme thermal instability since at 100 K it disappears completely. The shapes of the ESR spectra, as well as the values of the components of the g̃ tensor and the superhyperfine interaction tensor, allowed to identify this defect as a distorted CO−3 ion, associated with a Li+ ion located in an interstitial site in the plane of the Ca2+ ions. Cet article présente les résultats de l'étude d'un défaut ponctuel observé par RPE dans la calcite synthétique. Ce centre paramagnétique a été créé dans un monocristal de calcite synthétique par irradiation X à 77 K. Il se caractérise par une extrême instabilité thermique, puisqu'à 100 K il blanchit totalement. L'allure des spectres RPE, ainsi que les valeurs des composantes du tenseur g̃ et du tenseur d'interaction superhyperfine, ont permis d'identifier ce défaut comme étant un ion CO−3 distordu et associé à un ion Li+ placé en site interstitiel dans le plan des ions Ca2+.

Effect of impurities on grain growth in synthetic calcite aggregates

We investigated grain growth of calcite aggregates fabricated from crushed natural single crystals with different impurity content. The total trace-element concentration of the starting powders varied from about 170 ppm to more than 930 ppm with Mn as the major component. Samples were produced by hot-isostatic pressing of the different powders at 300 MPa confining pressure at 600 °C for 2 h. The starting material for the anneals was dry and had a uniform microstructure with an average grain size of about 5 μm and a porosity of <2.1%. Three disks with Mn concentrations of 10, 350, and 670 ppm, respectively, were annealed in the same run at a confining pressure of 300 MPa, and temperatures between 700 and 900 °C for up to 20 h. Grain growth was fastest in samples with the highest Mn concentrations. A multivariable fit to the data yields grain-growth exponents of 2.0 ± 0.3 for samples with 10 ppm Mn and 2.3 ± 0.2 for those with 670 ppm Mn. The activation energies for grain growth vary from 99 ± 12 kJ mol−1 to 147 ± 14 kJ mol−1 for the respective calcite compositions.

Electrokinetic behaviour of calcite—the relationship with other calcite properties

Six carbonate samples of different origin were investigated for their properties: composition, grain size (Mz), specific surface area (SSA) and electrokinetic ( ζ) potential. Four of the samples were of natural origin—one sample was sea sediment, one lake sediment and two limestone rocks. The remaining two samples were synthetic calcites. The results of characterisation have shown that the samples consisted mostly of calcite. Traces of impurities (Si, Al, Fe, Mg) and organic matter were found in natural samples. The grain size of samples ranged between 2.0 and 10.3 μm, and SSA values were between 17.0 and 1.0 m 2 g −1 accordingly. The electrokinetic mobility measurements were performed in the pH range from 6 to 10. The results have shown two different types of behaviour: synthetic calcites had positive ζ-potential at the point of equilibrium (pH eq=8.4±0.1), which decreased with either the increase or the decrease of the pH. Natural calcites, in contrast, had negative ζ-potential throughout the entire investigated pH range. Electrokinetic behaviour of synthetic samples was the result of the exchange between the constituent ions in the solution and the available surface sites. The pH decrease caused the dissolution of the particles, and consequently the diminution of the grain size and surface area. Lower values of the ζ-potential were then the result of the specific adsorption of constituent anions on such reduced surface area. Going towards higher pH values, the decrease in the ζ-potential (and finally the charge reversal) was caused by the increasing concentration of the CO 3 2−. The isoelectric point (pH IEP) was reached at about pH≈9.5. The electrokinetic behaviour of natural samples on the other hand was not the result of the exchange of constituent ions between the surface and the solution. Their ζ-potential was governed by the organic matter attached to the surface or incorporated in the structure of calcite.

An improved experimental procedure of separating a composite thermoluminescence glow curve into its components

We present an improved experimental procedure of separating a composite thermoluminescence glow curve into its components. Careful monitoring of the isothermal cleaning process using the initial rise method ensures the complete thermal removal of TL peaks. Digital subtraction of two experimental TL glow curves yields individual experimental TL glow peaks. Several standard methods (initial rise and whole glow curve) are used to obtain the energy values and frequency factors of the traps. The method has been used successfully to analyze the well-known composite TL glow curve of the dosimetric material LiF (TLD-100). The limitations of the method are illustrated by analyzing the highly complex TL glow curve of a UV irradiated synthetic calcite consisting of at least 6 TL peaks. Although the method works best for TL glow curves described by first order kinetics, it should also be applicable to more general kinetics.

Electronic paramagnetic resonance as a tool for studying the blackening of Carrara marble due to irradiation by a Q-switched YAG laser

Several authors or practitioners have remarked that Q-switched lasers have an immediate blackening effect on mineral pigments such as vermilion (HgS), minium (Pb 3O 4), white lead [2PbCO 3. Pb(OH) 2], malachite [Cu 2CO 3.(OH) 2] and yellow ochre (FeOH 2). A similar blackening effect can also be obtained on veined Carrara marble. This stone contains several trace elements, particularly iron and manganese. The latter is responsible for a particular response in electronic paramagnetic resonance spectroscopy (EPRS). We investigated the effect of a Q-switched Nd:YAG laser irradiation on the EPRS trace of Carrara marble. Further experiments were performed with the same analytical tool on synthetic calcite powders that had been doped with iron or manganese. It appeared that no significant change of EPRS trace could be put into evidence after laser irradiation of both bulk Carrara marble or doped calcite powders, although some samples had actually blackened. It is concluded that the blackening effect is not due to an oxidation of manganese ions. Colour measurements were also performed within the framework of this investigation. They showed that the colour shift to black is proportional to the fluence applied to the stone surface

Nanotopography of synthetic and natural dolomite crystals

Examination with scanning electron microscopy (SEM) and scanning force microscopy (SFM) revealed etch pits, layers and islands on dolomite crystal faces synthesized from calcite in Ca-Mg-Cl solutions at 200 °C and a wide variety of natural dolomites. Layers are broad, flat structures bounded by steps less than 100 nm high and greater than 1 μm wide. Islands are rounded topographic highs <20 nm high and <200 nm wide. The nanotopography of synthetic dolomite changed from islands throughout most of the reaction to layers at 100% dolomite. Island nanotopography formed on both Ca-rich and near-stoichiometric dolomite. Analyses of reaction products from dolomite synthesis indicates that there are no SFM-detectable products formed in <10 h. SEM-detectable products formed in 15 h. X-ray diffraction (XRD)-detectable products formed in ≈18 h, and the reaction went to completion in ≈40 h. Based on SFM analyses, the induction period for dolomitization in these experiments accounts for ≈20% of the total reaction time necessary to dolomitize CaCO3 completely under the experimental conditions used here. Island nano- topography is inferred to occur at higher degrees of supersaturation than layer nanotopography for three reasons. First, island nanotopography on synthetic calcite and gypsum forms at higher supersaturations than layer nanotopography. Secondly, island nanotopography formed in solutions with higher degrees of supersaturation with respect to dolomite. Thirdly, the greater surface roughness of a crystal face composed of islands compared with layers indicates that island surfaces have higher surface energy than layer surfaces. Therefore, the stability of island surfaces requires a higher degree of supersaturation. Because islands and layers form under a wide range of conditions, their presence provides broadly applicable criteria for evaluating relative degrees of supersaturation under which ancient dolomite formed. Comparison of synthetic dolomites with natural dolomites demonstrates (1) similar nanotopography on natural and synthetic dolomites and (2) both natural planar and non-planar dolomite may have island nanotopography.

Novel TEM approaches to imaging of microstructures in carbonates; clues to growth mechanisms in calcite and dolomite

Relating microstructures imaged by transmission electron microscopy (TEM) to specific growth mechanisms requires imaging of both the surface microtopography on crystal faces and its corresponding microstructure in the bulk crystal. Pt-C replicas of as-grown surfaces are ideally suited to this task, as they provide a high-resolution, nearly three-dimensional image of surface topography that can be correlated with microstructures visible in sections at high angle to the as-grown surface. Ultramicrotomy enables the preparation of ultrathin sections more quickly than conventional ion-thinning and can be used to investigate chemical heterogeneities by analytical electron microscopy (AEM). We evaluate the potential of both techniques for the study of microstructures in calcite and dolomite. (1) TEM images were obtained from Pt-C replicas of synthetic calcite. The as-grown (101̅4) face of a Mg/Mndoped crystal growth showed growth hillocks aligned along preferred orientations. In Pb/Mn/Srdoped calcite, sections at high angle to as-grown faces showed a uniform microstructure in {101̅4} sectors. The {011̅2} sectors contained concentric zones that alternated from striated to uniform microstructures, suggesting periodic variations in growth rate and possibly in the growth mechanism. An oscillatory-zoned Mn-doped calcite showed fine-scale banding (30-150 mm) and periodic roughening of (101̅4) surfaces that suggest repetitive transitions between growth mechanisms. (2) In sedimentary dolomites, Pt-C replicas of surfaces cleaved parallel to a (101̅4) face showed a hillocky topography or smooth (101̅4) surfaces modified by non-equivalent facets. Surfaces produced by cleavage at high angle to a (101̅4) face exposed sectors with fine-scale banding crosscut by sharp boundaries, suggestive of closely spaced growth hillocks intercalated with non-equivalent subsectors. Ultrathin sections prepared by microtomy showed some disruption of the crystal structure but, in some dolomite samples, domains with distinctive microstructures similar in scale and shape to those seen in the Pt-C replica were identified. (3) The growth microstructures in {101̅4} sectors of dolomite present striking similarities to the larger scale compositional zoning patterns produced by growth spirals in calcite doped with trace elements. At this stage, Pt-C replicas and ultramicrotomy can distinguish among dolomites that are petrographically very similar.

Adsorption of Benzoic Acid on Synthetic Calcite Dispersed in Cyclohexane as a Function of Temperature

The adsorption of benzoic acid on synthetic calcite dispersed in cyclohexane has been studied between 296 and 346 K. The results can be successfully explained by a two-step model involving a Langmuir adsorption of first a layer of molecules oriented parallel to the surface and next, as the concentration of the adsorbate in the liquid phase increases, a gradual replacement of this layer with a monolayer of molecules oriented perpendicular to the surface. The maximum adsorption of benzoic acid in the perpendicular oriented monolayer has been determined to be 7.345μmol/m2, independent of the temperature, and close to the theoretical value of 7.4 μmol/m2. The Langmuir constant for the second step, in which parallel adsorbed molecules are replaced by perpendicular adsorbed ones, varies from 2 × 103L/mol at 296 K to 16 × 103L/mol at 346 K, and the enthalpy and entropy changes associated with this part of the adsorption process are 37.9 kJ/mol and 0.191 kJ/mol K, respectively. In the same temperature interval the Langmuir constant for the first step of the adsorption process involving parallely adsorbed benzoic acid molecules is estimated to be fairly constant, and within a factor of 2 from the best fit value of 33 × 103L/mol.

Oxygen grain boundary diffusion in natural and hot-pressed calcite aggregates

Oxygen grain boundary diffusion rates were experimentally determined in natural (Solnhofen limestone) and hot-pressed calcite aggregates at 300 to 500°C and 100 MPa water pressure. The Solnhofen limestone was pre-annealed for 24 h at 700°C and 100 MPa confining pressure under anhydrous conditions to produce an equilibrium microstructure for the diffusion experiments. The synthetic calcite aggregates were formed by hot isostatic pressing of an intimate mixture of 1–2 μm calcite grains plus 5 wt% 0.3 μm alumina grains at 700°C and 200 MPa for 3–10 days. Values for the product of the grain boundary diffusion coefficient ( D′) and the effective grain boundary diffusion width ( δ) were determined from 18O concentration profiles measured with an ion microprobe. There is no measurable difference between D′ δ values obtained for pre-annealed Solnhofen samples and hot-pressed calcite aggregates; the temperature dependence for oxygen grain boundary diffusion at 100 MPa is described by the Arrhenius parameters D 0′ δ=3.8×10 −14 m 3/s and Q=127±17 kJ/mol. Comparison of the results of this study with previously published data show that the rate of oxygen grain boundary diffusion in calcite aggregates is four to five orders of magnitude greater than that of volume diffusion in calcite single crystals, and the activation energy is less. Grain boundary diffusion of oxygen in calcite aggregates is five to six orders of magnitude greater than that of calcium, when the latter is extrapolated to the temperature range of this study. In addition, oxygen grain boundary diffusion rates in calcite aggregates are about a factor of 4–10 and ∼100 times greater than that in feldspar and quartz aggregates, respectively, under similar experimental conditions.