Magnesium stable isotope fractionation in marine biogenic calcite and aragonite

This survey of magnesium stable isotope compositions in marine biogenic aragonite and calcite includes samples from corals, sclerosponges, benthic porcelaneous and planktonic perforate foraminifera, coccolith oozes, red algae, and an echinoid and brachiopod test. The analyses were carried out using MC-ICP-MS with an external repeatability of ±0.22‰ (2SD for δ²⁶Mg; n = 37), obtained from a coral reference sample (JCp-1).Magnesium isotope fractionation in calcitic corals and sclerosponges agrees with published data for calcitic speleothems with an average Δ²⁶Mg_{calcite-seawater} = −2.6 ± 0.3‰ that appears to be weakly related to temperature. With one exception (Vaceletia spp.), aragonitic corals and sclerosponges also display uniform Mg isotope fractionations relative to seawater with Δ²⁶Mg_{biogenic aragonite-seawater} = −0.9 ± 0.2.Magnesium isotopes in high-Mg calcites from red algae, echinoids and perhaps some porcelaneous foraminifera as well as in all low-Mg calcites (perforate foraminifera, coccoliths and brachiopods) display significant biological influences. For planktonic foraminifera, the Mg isotope data is consistent with the fixation of Mg by organic material under equilibrium conditions, but appears to be inconsistent with Mg removal from vacuoles. Our preferred model, however, suggests that planktonic foraminifera synthesize biomolecules that increase the energetic barrier for Mg incorporation. In this model, the need to remove large quantities of Mg from vacuole solutions is avoided. For the high-Mg calcites from echinoids, the precipitation of amorphous calcium carbonate may be responsible for their weaker Mg isotope fractionation.Disregarding superimposed biological effects, it appears that cation light isotope enrichments in CaCO₃ principally result from a chemical kinetic isotope effect, related to the incorporation of cations at kink sites. In this model, the systematics of cation isotope fractionations in CaCO₃ relate to the activation energy required for cation incorporation, which probably reflects the dehydration of the cation and the crystal surface and bond formation at the incorporation site. This kinetic incorporation model predicts (i) no intrinsic dependence on growth rate, unless significant back reaction upon slow growth reduces the isotope fractionation towards that characteristic for equilibrium isotope partitioning (this may be observed for Ca isotopes in calcites), (ii) a small decrease of isotope fractionation with increasing temperature that may be amplified if higher temperatures promote back reaction and (iii) a sensitivity to changes in the activation barrier caused by additives such as anions or biomolecules or by the initial formation of amorphous CaCO₃.     

Boron isotopes as pH proxy: A new look at boron speciation in deep-sea corals using B MAS NMR and EELS

Dissolved boron in modern seawater occurs in the form of two species, trigonal boric acid B(OH)₃ and tetrahedral borate ion . One of the key assumption in the use of boron isotopic compositions of carbonates as pH proxy is that only borate ions, , are incorporated into the carbonate. Here, we investigate the speciation of boron in deep-sea coral microstructures (Lophelia pertusa specimen) by using high field magic angle spinning nuclear magnetic resonance (¹¹B MAS NMR) and electron energy-loss spectroscopy (EELS). We observe both boron coordination species, but in different proportions depending on the coral microstructure, i.e. centres of calcification versus fibres. These results suggest that careful sampling is necessary before performing boron isotopic measurements in deep-sea corals. By combining the proportions of B(OH)₃ and determined by NMR and our previous ion microprobe boron isotope measurements, we propose a new equation for the relation between seawater pH and boron isotopic composition in deep-sea corals.     

Calcium isotope fractionation in calcite and aragonite

Calcium isotope fractionation was measured on skeletal aragonite and calcite from different marine biota and on inorganic calcite. Precipitation temperatures ranged from 0 to 28°C. Calcium isotope fractionation shows a temperature dependence in accordance with previous observations: 1000 · ln(α_cc) = −1.4 + 0.021 · T (°C) for calcite and 1000 · ln(α_ar) = −1.9 + 0.017 · T (°C) for aragonite. Within uncertainty the temperature slopes are identical for the two polymorphs. However, at all temperatures calcium isotopes are more fractionated in aragonite than in calcite. The offset in δ^44/40Ca is about 0.6‰. The underlying mechanism for this offset may be related to the different coordination numbers and bond strengths of the calcium ions in calcite and aragonite crystals, or to different Ca reaction behavior at the solid-liquid interface. Recently, the observed temperature dependence of the Ca isotope fractionation was explained quantitatively by the temperature control on precipitation rates of calcium carbonates in an experimental setting (Lemarchand et al., 2004). We show that this mechanism can in principle also be applied to CaCO₃ precipitation in natural environments in normal marine settings. Following this model, Ca isotope fractionation in marine Ca carbonates is primarily controlled by precipitation rates. On the other hand the larger Ca isotope fractionation of aragonite compared to calcite can not be explained by different precipitation rates. The rate control model of Ca isotope fractionation predicts a strong dependence of the Ca isotopic composition of carbonates on ambient CO₃²⁻ concentration. While this model is in general accordance with our observations in marine carbonates, cultured specimens of the planktic foraminifer Orbulina universa show no dependence of Ca-isotope fractionation on the ambient CO₃²⁻ concentration. The latter observation implies that the carbonate chemistry in the calcifying vesicles of the foraminifer is independent from the ambient carbonate ion concentration of the surrounding water.     

Uptake of dissolved Cd by biogenic and abiogenic aragonite: a comparison with sorption onto calcite

The uptake of Cd²⁺ by aragonite and calcite is investigated by combining macroscopic measurements with some qualitative sorption experiments performed in a hydrogel medium. Both biogenic and abiogenic aragonites were studied in order to evaluate the process on materials with different textures. Assuming that sorption occurs by surface precipitation of metal-bearing solids, the gel produces a drastic decrease in the nucleation density, which allows for the precipitation of crystallites that are large enough to be analysed by scanning electron microscopy and characterized by glancing-incidence X-ray techniques. The macroscopic study reveals that aragonite is a powerful sorbent for cadmium in aqueous environments. Microscopic observations indicate that cadmium is sorbed onto aragonite by surface precipitation of (Cd, Ca)CO₃ solid solutions with a calcite-type structure. The precipitating individuals grow randomly oriented on the surface to reach sizes in the micrometre range. As a consequence, the concentration of cadmium in the aqueous solution decreases dramatically to values controlled by the low solubility of the cadmium-rich end member. This mechanism involves simultaneous dissolution-crystallization and is the same for both abiogenic and biogenic aragonites, the only difference being a result of the higher specific surface area of the biogenic starting material. Long-term uptake of cadmium by calcite occurs through a similar dissolution-crystallization mechanism, the final outcome being virtually the same, that is, surface precipitation of (Cd,Ca)CO₃ solid solutions. In this case, however, substrate and precipitate are isostructural and the process occurs by oriented overgrowth of thin lamellar crystallites, which spread to quickly cover the surface by a layer a few nanometers thick. This epitaxial layer armors the substrate from further dissolution, so that the process stops when only a small amount of cadmium has been removed from the fluid. As a result, the “sorption capacity” of calcite is considerably lower than that of aragonite. The study illustrates reaction pathways and “partial” equilibrium endpoints in surface-precipitation processes involving solid solutions.     

Americium interaction with calcite and aragonite surfaces in seawater

Observations of the distribution of ²⁴¹Am in the marine environment indicate that Am has a high affinity for solid surfaces. The adsorption of Am onto calcite and aragonite surfaces from seawater and related solutions has been studied, in order to establish the interaction of Am with a major component of many marine sediments. Results indicate that Am is rapidly and strongly adsorbed. This occurs even when both dissolved Am concentrations and solid to solution ratios are low. The minimum value for $\text{K}{}_{\mathrm{D}}$ determined is 2 × 10⁵. Measurements of reaction kinetics established that Am is adsorbed from seawater at 40 times the rate per unit surface area on synthetic aragonite that it is on synthetic calcite. Approximately 15% of the difference is attributable to epitaxial influences, with the remainder being due to enhanced site competition by Mg on calcite relative to aragonite. The adsorption rate is first order with respect to Am concentration, but follows approximately the square root of the solid surface area to solution volume ratio.Adsorption rate of Am on biogenic aragonite and Mg-calcites are, within a given particle size range, close to equal. It is not possible to normalize these adsorption rates to surface area due to the differing microporous structure of biogenic carbonates. The Am adsorption rates on a shallow water calcium carbonate-rich sediment gave results which were predicted from, its mineralogie mixture of components.     

Polycrystalline calcite to aragonite transformation kinetics: experiments in synthetic systems

The kinetics of the calcite to aragonite transformation have been investigated using synthetic polycrystalline calcite aggregates, with and without additional minerals present. The reaction progresses as a function of time were measured at four temperature/pressure conditions: (1) 550 °C/1.86 GPa; (2) 600 °C/2.11 GPa; (3) 650 °C/2.11 GPa, and (4) 700 °C/2.29 GPa. Experiments reveal that Mg-calcite and Fe-calcite transforms to aragonite at considerably slower rates than pure calcite, and that Sr-bearing calcite and calcite + quartz aggregates transform at significantly higher rates than pure calcite. The reaction progresses vs. time data for pure calcite were fitted to Cahns grain-boundary nucleation and interface-controlled growth model. Evidence for interface-controlled growth is provided by petrographic observations of grain boundaries. The activation energy for aragonite growth from the synthetic polycrystalline calcite determined in this study is significantly lower than that previously determined from a natural marble. The discrepancy in rates and activation energy may be attributed to the nature of grain boundaries, to deformational strain or the presence of impurities in the studied samples, and likely to uncertainties in experimental conditions. The results of this study imply that the variation of local petrologic conditions, in addition to temperature, pressure and grain size, may play an important role in determining the rates for the calcite to aragonite transformation in nature.Editorial responsibility: W. Schreyer     

Experimental study of the aragonite to calcite transition in aqueous solution

The experimental replacement of aragonite by calcite was studied under hydrothermal conditions at temperatures between 160 and 200 °C using single inorganic aragonite crystals as a starting material. The initial saturation state and the total [Ca²⁺]:[CO₃²⁻] ratio of the experimental solutions was found to have a determining effect on the amount and abundance of calcite overgrowths as well as the extent of replacement observed within the crystals. The replacement process was accompanied by progressive formation of cracks and pores within the calcite, which led to extended fracturing of the initial aragonite. The overall shape and morphology of the parent aragonite crystal were preserved. The replaced regions were identified with scanning electron microscopy and Raman spectroscopy.Experiments using carbonate solutions prepared with water enriched in ¹⁸O (97%) were also performed in order to trace the course of this replacement process. The incorporation of the heavier oxygen isotope in the carbonate molecule within the calcite replacements was monitored with Raman spectroscopy. The heterogeneous distribution of ¹⁸O in the reaction products required a separate study of the kinetics of isotopic equilibration within the fluid to obtain a better understanding of the ¹⁸O distribution in the calcite replacement. An activation energy of 109 kJ/mol was calculated for the exchange of oxygen isotopes between [C¹⁶O₃²⁻]_aq and [H₂¹⁸O] and the time for oxygen isotope exchange in the fluid at 200 °C was estimated at ∼0.9 s. Given the exchange rate, analyses of the run products imply that the oxygen isotope composition in the calcite product is partly inherited from the oxygen isotope composition of the aragonite parent during the replacement process and is dependent on access of the fluid to the reaction interface rather than equilibration time. The aragonite to calcite fluid-mediated transformation is described by a coupled dissolution-reprecipitation mechanism, where aragonite dissolution is coupled to the precipitation of calcite at an inwardly moving reaction interface.     

The chemistry of orthophosphate uptake from seawater on to calcite and aragonite

The chemistry of orthophosphate uptake from synthetic seawater onto the surfaces of synthetic calcite, aragonite and low-magnesium biogenic calcite has been studied, in order to elucidate the kinetics of the process (generally believed to be the major control of dissolved reactive phosphate in carbonate-rich marine sediments). Our results differ from those obtained by others, who have studied orthophosphate uptake in low ionic strength solutions and at much higher supersaturations relative to apatite.In both ‘free drift’ and chemostat experiments, Mg and F have only a minor effect on the reaction rate. Even at constant solution composition the rate of orthophosphate uptake was found to decrease by 10⁶ over a two week period. The data from the ‘free drift’ experiments can be fitted to the Elovich equation. This indicates that the kinetics observed for this reaction can be explained by an exponential decrease in available surface reaction sites and/or a linear increase in the activation energy associated with chemisorption as the reaction proceeds.     

High-temperature enthalpy at the orientational order-disorder transition in calcite: implications for the calcite/aragonite phase equilibrium

The enthalpy of calcite has been measured directly between 973 K and 1325 K by transposed-temperature- drop calorimetry. The excess enthalpy has been analysed in terms of Landau theory for this tricritical phase transition. The zero-point enthalpy and entropy allow estimates of the parameters a and C in the Landau expansion for free energy which expresses excess free energy G as a function of the order parameter Q and temperature T: G 1/2a(T _2c–T)Q ²+1/6CQ ⁶ with a=24 J·K·mol^-1, C = 30 kJ·mol^– T _c = 1260 ±5 K. The entropy of disorder below the transition has been formulated as a function of temperature allowing the calculation of the calcite/aragonite phase boundary when taking this extra entropy into account. There is remarkable agreement between the calculated equilibrium curve and previous experimental observations. The Landau theory predicts behaviour which fully accounts for the change in slope of the calcite/aragonite phase boundary, which is thus wholly due to the R¯3c –R¯3m transition in calcite.     

Thermodynamic properties of CaCO3 calcite and aragonite: A quasi-harmonic calculation

A quasi-harmonic model has been used to simulate the thermodynamic behaviour of the CaCO₃ polymorphs, by equilibrating their crystal structures as a function of temperature so as to balance the sum of inner static and thermal pressures against the applied external pressure. The vibrational frequencies and elastic properties needed have been computed using interatomic potentials based on two-body Born-type functions, with O-C-O angular terms to account for covalency inside the CO₃ molecular ion. A good agreement with experimental data is generally shown by simulated heat capacity and entropy, while the thermal expansion coefficient seems to be more difficult to reproduce. The results obtained for aragonite are less satisfactory than those of calcite, but they are improved by using a potential specifically optimized on properties of that phase itself.     

The impact of solution chemistry on Mytilus edulis calcite and aragonite

Magnesium/calcium, Sr/Ca, and Na/Ca atom ratios were determined in the calcite and aragonite regions of Mytilus edulis shells which were grown in semi-artificial ‘seawater’ solutions having varying Mg/Ca, Sr/Ca, and Na/Ca ratios. These ratios were measured by instrumental neutron activation, atomic absorption, and electron microprobe analytical techniques. Strontium/calcium ratios in both calcite and aragonite were linearly proportional to solution Sr/Ca ratios. Magnesium/calcium ratios in calcite increased exponentially when solution Mg/Ca ratios were raised above the normal seawater ratio; whereas in aragonite, Mg/Ca ratios increased linearly with increases in solution Mg/Ca ratios. Sodium/calcium and sulfur/calcium ratios in calcite covaried with Mg/Ga solution ratios. Conversely, in aragonite, Na/Ca ratios varied linearly with solution Na/Ca ratios.Magnesium is known to inhibit calcite precipitation at its normal seawater concentration. We infer from the results of the work reported here that Mytilus edulis controls the Mg activity of the outer extrapallial fluid, thus facilitating the precipitation of calcitic shell. Increases in sulfur content suggest that changes in shell organic matrix content occur as a result of environmental stress. Certain increases in Mg content may also be correlated to stress. Sodium/calcium variations, and their absolute amounts in calcite and aragonite, are best explained by assuming that a substantial amount of Na is adsorbed on the calcium carbonate crystal surface. Strontium/calcium ratios show more promise than either Mg/Ca or Na/Ca ratios as seawater paleochemistry indicators, because the Sr/Ca distribution coefficients for both aragonite and calcite are independent of seawater Ca and Sr concentrations.     

Al speciation in tropical podzols of the upper Amazon Basin: A solid-state Al MAS and MQMAS NMR study

In the upper Amazon Basin, aluminum previously accumulated in lateritic formations is massively remobilised in soils by podzolization and exported in waters. We have investigated the speciation of aluminum in the clay-size fractions of eight horizons of waterlogged podzols lying in a depression of a plateau. The horizons illustrate the main steps involved in the podzolization of laterites. They belong to eluviated topsoil A horizons and illuviated subsoil Bhs, Bh and 2BCs horizons of weakly and better-expressed podzols located at the margin and centre of the depression. For the first time, aluminum speciation is quantitatively assessed in soils by spectroscopic methods, namely FTIR, ²⁷Al magic angle spinning (MAS) and multiple-quantum magic angle spinning (MQMAS), nuclear magnetic resonance (NMR). The results thus obtained are compared to chemical extraction data.Solid-state ²⁷Al MAS NMR spectra enable to distinguish Al bound to organic compounds from that incorporated in secondary mineral phases detected by FTIR. MQMAS experiments additionally show that both chemical shifts and quadrupolar constants are distributed for Al nuclei linked with organic compounds. Similar amounts of chelated Al are obtained from NMR spectra and chemical extractions. The study enables to highlight three major steps in the fate of aluminum. (i) Aluminum is first released by mineral weathering, feeds complexing sites of organic matter and accumulates in subsurface Bhs horizons of weakly expressed podzols (acidocomplexolysis). (ii) Complexes of aluminum with organic matter (Al-OM) then migrate downwards in sandy horizons of better-expressed podzols and accumulate at depth in less permeable 2BCs horizons. (iii) The minor amounts of aluminum present in the 2BCs horizon of the downslope podzol show that aluminum is eventually exported towards the river network, either complexed with organic matter or as Al³⁺ ions after desorption from organic compounds, due to decreasing pH or biodegradation of organic ligands. The direct spectroscopic determination of Al-speciation during the formation of podzolic soils opens new perspectives to trace metal loads in the rivers of the upper Amazon Basin.     

Strontium heterogeneity and speciation in coral aragonite: implications for the strontium paleothermometer

Sea surface temperatures (SSTs) have been inferred previously from the Sr/Ca ratios of coral aragonite. However, microanalytical studies have indicated that Sr in some coral skeletons is more heterogeneously distributed than expected from SST data. Strontium may exist in two skeletal phases, as Sr substituted for Ca in aragonite and as separate SrCO₃ (strontianite) domains. Variations in the size, quantity, or both of these domains may account for small-scale Sr heterogeneity. Here, we use synchrotron X-ray fluorescence to map Sr/Ca variations in a Porites lobata skeleton at a 5 μm scale. Variations are large and unrelated to changes in local seawater temperature or composition. Selected area extended X-ray absorption fine structure (EXAFS) spectroscopy of low- and high-Sr areas indicates that Sr is present as a substitute ion in aragonite i.e., domains of Sr carbonate (strontianite) are absent or in minor abundance. Variations in strontianite abundance are not responsible for the Sr/Ca fluctuations observed in this sample. The Sr microdistribution is systematic and appears to correlate with the crystalline fabric of the coral skeleton, suggesting Sr heterogeneity may reflect nonequilibrium calcification processes. Nonequilibrium incorporation of Sr complicates the interpretation of Sr/Ca ratios in terms of SST, particularly in attempts to extend the temporal resolution of the technique. The micro-EXAFS technique may prove to be valuable, allowing the selection of coral microvolumes for Sr/Ca measurement where strontium is incorporated in a known structural environment.     

The role of magnesium in the crystal growth of calcite and aragonite from sea water

The seeded precipitation (crystal growth) of aragonite and calcite from sea water, magnesium-depleted sea water, and magnesium-free sea water has been studied by means of the steady-state disequilibrium initial rate method. Dissolved magnesium at sea water levels appears to have no effect on the rate of crystal growth of aragonite, but a strong retarding effect on that of calcite. By contrast, at levels less than about 5 per cent of the sea water level, Mg has little or no effect on calcite growth. Extended crystal growth on pure calcite seeds in sea water of normal Mg content resulted in the crystallization of magnesium calcite overgrowths, containing 7–10 mole % MgCO₃ in solid solution. This suggests that the rate inhibition by Mg is due to its incorporation within the calcite crystal structure during growth, which causes the resulting magnesian calcite to be considerably more soluble than pure calcite. The standard free energy of formation of 8.5 mole% Mg calcite calculated on this assumption is in good agreement with independent estimates of magnesian calcite stability.From the work of Katz (Geochim. Cosmochim. Acta37, 1563–1586, 1973), Plummer and Mackenzie (Amer. J. Sci. 273, 515–522, 1974), and the present paper, it can be predicted that the most stable calcite in Ca-Mg exchange equilibrium with sea water contains between 2 and 7 mole%MgCO₃ in solid solution. Likewise, calcites containing more than 8.5 mole% MgCO₃ are less stable, and those containing less than 8.5 mole% MgCO₃ are more stable than aragonite plus Ca and Mg in sea water.     

Formation of low‐magnesium calcite at cold seeps in an aragonite sea

This study investigates the conditions of occurrence and petrographic characteristics of low‐Mg calcite (LMC) from cold seeps of the Gulf of Mexico at a water depth of 2340 m. Such LMC mineral phases should precipitate in calcite seas rather than today's aragonite sea. The ¹³C‐depleted carbonates formed as a consequence of anaerobic oxidation of hydrocarbons in shallow subsurface cold seep environments. The occurrence of LMC may result from brine fluid flows. Brines are relatively Ca²⁺‐enriched and Mg²⁺‐depleted (Mg/Ca mole ratio <0.7) relative to seawater, where the Mg/Ca mole ratio is ~5, which drives high‐Mg calcite and aragonite precipitation. The dissolution of aragonitic mollusk shells, grains and cements was observed. Aerobic oxidation of hydrocarbons and H₂S is the most likely mechanism to explain carbonate dissolution. These findings have important implications for understanding the occurrence of LMC in deep water marine settings and consequently their counterparts in the geological record.     

Constraining initial Th activity in incrementally deposited, biogenic aragonite from the Bahamas

The ²³⁰Th content of subsurface Bahamas seawater has been estimated from incrementally deposited aragonite skeletons of sclerosponges using U-series age models which imply syndepositional incorporation of ²³⁰Th. First order age models, assuming constant growth rate and zero initial ²³⁰Th activity, display an average surface age of 63 ± 18 y despite the fact that the surfaces of the organisms were known to be alive at the time collection in the 1990s. A knowledge of the initial ²³⁰Th activity is necessary to incorporate the surface age constraint into an age model. In the Bahamas, this approach estimates an initial ²³⁰Th/²³²Th activity ratio of 4.3 ± 1.1 when applied to several individuals of the Caribbean sclerosponge Ceratoporella nicholsoni. These results differ from the same approach applied to previous work on Jamaican sclerosponges accreting aragonite in an environment closer to crustal sources and with less carbonate substrate. We report new measurements of Jamaican sclerosponge suggesting initial activity ratios of ²³⁰Th/²³²Th of ∼1, albeit with a high amount of uncertainty, indicating a regional difference in initial thorium activities. The estimates of initial ²³⁰Th/²³²Th activity ratios in the Bahamas are similar to the dissolved ratio rather than the particulate ratio and can be explained by mixing of open ocean water and bank-top surface water, however no information regarding temporal changes in the amount of initial ²³⁰Th/²³²Th activity available for incorporation into biogenic aragonite can be constrained using this approach. Growth rates do not change after correction for initial thorium and they display an inverse relationship with depth. Although this paper deals uniquely with sclerosponge aragonite, it is likely that the approach can be used on any incrementally deposited biogenic aragonite, provided information is known a priori about the first order growth model and there is evidence that initial thorium incorporation is representative of ambient environment rather than individual physiology.     

Aragonite crystallization from strained calcite at reduced pressures and its bearing on aragonite in low-grade metamorphism

Aragonite, the dense form of CaCO₃, grew hydrothermally at 100–300° C and dry at 300–400° C at very low pressures from calcite strained by grinding. Nearly complete inversion to aragonite occurred in some runs with Ca-Mg chloride solutions at 0–2.4 kb and 100–200° C on strained calcite having a (10¯14) reflection with a half-width of 0.48° 2 Cu K. A little aragonite grew dry at one atm. from the ground calcite at 300–400° C in a few hrs. Simultaneous shear during recrystallization of calcite in a rotating squeezer resulted in significant aragonite at 300–400° C several kb. below the stability field. No inversion occurred in any ground calcite when previously annealed in CO₂ at 500° C for a few hrs. Thermochemical data show that at least 200 cal/mole of strain energy can be produced in calcite by mild deformation. This much stored energy would lower the pressure requirements of aragonite, relative to the strained calcite by more than 3 kb, and our observation that aragonite growth was faster than strain recovery of calcite indicates that aragonite can grow in nature at reduced pressures from strained calcite.Some experiments were also carried out on highly magnesian calcites with the thought that aragonite might also form at the expense of this metastable material. No aragonite was produced, but the possibility that this mechanism could be operative in nature cannot be discounted.The microtexture of aragonitic deformed marbles from NW Washington (prehnite-pumpellyite facies rocks, courtesy of J. A. Vance) as well as electron probe microanalysis of these rocks indicates that aragonite selectively replaced highly strained calcite. The calcite-aragonite transition is thus a questionable indicator of high-pressure in certain metamorphic rocks.     

Correlations between 11B NMR parameters and structural characters in borate and borosilicate minerals investigated by high-resolution MAS NMR and ab initio calculations

Borates consisting of diverse fundamental building blocks (FBB) formed from complex polymerization of planar triangular [Bϕ₃] groups and tetrahedral [Bϕ₄] groups, where ϕ = O and OH, provide an excellent opportunity for investigation of correlations between the NMR parameters and local structures. However, previous studies suggested that the ¹¹B NMR parameters in borates are insensitive to local structural environments other than the B coordination number, in contrast to those documented for ²⁹Si, ²³Na and ²⁷Al in silicates, and no correlation between ¹¹B chemical shifts and the sum of bond valences has been established for borate minerals with hydroxyl groups or molecular water in the structures. In this study, high-resolution NMR spectra have been acquired at the ultra high field of 21 T as well as at 14 T for selected borate and borosilicate minerals, and have been used to extract high-precision NMR parameters by using combined ab initio theoretical calculations and spectral simulations. These new NMR parameters reveal subtle correlations with various structural characters, especially the effects of the ¹¹B chemical shifts from the bridging oxygen atom(s), site symmetry, symmetry of FBB, the sum of bond valences, as well as the next-nearest-neighbor cations and hydrogen bonding. Also, these results provide new insights into the shielding mechanism for ¹¹B in borate and borosilicate minerals. In particular, this study demonstrates that the small variation in ¹¹B chemical shifts can still be used to probe the local structural environments and that the established correlations can be used to investigate the structural details in borates and amorphous materials.     

Transformation kinetics of polycrystalline aragonite to calcite: new experimental data,modelling, and implications

Experimental data are used to model the transformation rate of polycrystalline aragonite (grain diameter 80 m) to calcite. Optimized values for nucleation and growth rates were obtained by numerically fitting the overall transformation rates from 280° to 380°C and 0.10 MPa to an expression for a grain-boundary-nucleated and interface-controlled transformation. The nucleation rate is 4–5 orders of magnitude faster than for calcite nucleated within aragonite grains, and the growing in rate is slower below 300°C than for calcite growing in aragonite single crystals. The activation enthalpy for growth in polycrystalline aggregate is 247kJ/mol compared to 163 kJ/mol for growth in single crystals. Permanent deformation of the phases limits the elastic strain energy due to the 7% volume change and reduces the coherency of the calcite/aragonite interace. Theoretical expressions are used to extrapolate the data for nucleation and growth to other temperatures, and data from 0.10 to 400 MPa are used to evaluate the effect of pressure on the grain-boundary nucleation rate. Because of permanent deformation of the phases, the effective strain energy for nucleation is 0.55 kJ/mol, which is less than a quarter of the value for purely elastic deformation. These data are used to predict the percent transformation for various P-T-t paths; without heating during uplift partial preservation of aragonite in dry blueschist facies rocks can occur if the calcite stability field is entered at 235° C, and the kinetic data are also consistent with published P-T-t paths which include heating during uplift. The predicted percent transformation is relatively insensitive to variations in the initial grain size of the aragonite, but strongly dependent on the effective strain energy.