Dolomite Structure Research Articles

Thermodynamic and Kinetic Studies of Dolomite Formation: A Review

The “dolomite problem”, which has confused scientists for nearly two centuries, is an important fundamental geological problem. The mineralogical characteristics of carbonate minerals show that the dolomite structure consists of an ordered arrangement of alternating layers of Ca2+ and Mg2+ cations interspersed with CO32− anion layers normal to the c-axis. The dolomite structure violates the c glide plane in the calcite structure, which means that dolomite has R3¯ space group symmetry. The ordered dolomite has superlattice XRD reflections [e.g., (101), (015) and (021)], which distinguish it from calcite and high-Mg calcite. The calculation of thermodynamic parameters shows that modern seawater has a thermodynamic tendency of dolomite precipitation and the dolomitization reaction can be carried out in standard state. However, the latest thermodynamic study shows that modern seawater is not conducive to dolomitization, and that seawater is favorable for dolomitization in only a few regions, such as Abu Dhabi, the Mediterranean and the hypersaline lagoons in Brazil. The kinetic factors of dolomite formation mainly consist of the hydration of Mg2+, the presence of sulfate and the activity of carbonate. Current studies have shown that the presence of microorganisms, exopolymeric substances (EPS), organic molecules, carboxyl and hydroxyl functional groups associated with microorganisms and organic molecules, clay minerals with negative charges and dissolved silica facilitate magnesium ions to overcome hydration and thus promote Mg2+ incorporation into growing Ca-Mg carbonates. Similarly, the metabolic activity of microorganisms is conducive to the increase in alkalinity. However, the inhibitory effect of sulfate on dolomite formation seems to be overestimated, and sulfate may even be a catalyst for dolomite formation. Combining the carbonate crystallization mechanism with thermodynamic and kinetic factors suggests that the early stage of dolomite precipitation or the dolomitization reaction may be controlled by kinetics and dominated by unstable intermediate phases, while metastable intermediate phases later transform to ordered dolomite via an Ostwald’s step rule.

First-Principle Studies of the Structural, Electronic, Mechanical, and Vibrational Properties of Double Carbonates with a Dolomite Structure

Density functional theory with a PBE gradient functional and a dispersion correction D3(BJ) in the basis of localized orbitals of the CRYSTAL17 package are used to calculate crystal structure parameters, electronic and vibrational spectra, elastic constants of rhombohedral double carbonates М1М2 (СО3)2 (М1, М2: Mg, Ca, Mn, Cd, Zn) with a dolomite-type structure. The paper demonstrates the possibility for establishing the linear dependences for lattice constants, interatomic distances, elastic constants and polycrystalline moduli, wave numbers of individual vibrational modes on the radii of cations. The same possibility is shown for the dependences of the parameters of chemical bonds and formation energies on their electronegativities. The lattice parameters increase along with the average cation radius, but the elastic constants and moduli demonstrate their decrease. The highest compressibility of carbonates is predicted in the direction of the c axis, which is consistent with the nature of the chemical bond, where stronger carbon — oxygen bonds are present in the ab plane, and weaker metal — oxygen bonds are in the direction of the c axis. The formation energy of binary carbonates CaMg(CO3)2 produced from solid oxides and gaseous CO2 is equal to -2.82 eV. It is equal to -2.71 eV for CaCd(CO3)2, and -0.054 eV, 0.023 eV for simple carbonates, respectively. The resulting formulas can be used to assess and predict the physical properties of solid solutions of carbonates of variable composition.

The Effect of Stoichiometry, Mg-Ca Distribution, and Iron, Manganese, and Zinc Impurities on the Dolomite Order Degree: A Theoretical Study

The determination of the degree of Mg-Ca order in the dolomite structure is crucial to better understand the process or processes leading to the formation of this mineral in nature. I01.5/I11.0 intensity ratios in the X-ray powder diffractograms are frequently measured to quantify dolomite cation order in dolomites. However, the intensity of diffraction peaks can be affected by factors other than the Mg-Ca distribution in the dolomite structure. The most relevant among these factors are (i) deviations from the ideal dolomite stoichiometry, and (ii) the partial substitution of Mg and Ca atoms by Fe, Mn, and Zn impurities. Using the VESTA software, we have constructed crystal structures and calculated I01.5/I11.0 ratios for dolomites with Mg:Ca ratios ranging from 0.5 to 1.5, and with Fe, Mn, and Zn contents up to 30%. Our results show that both deviations from dolomite ideal stoichiometry and the presence of impurities in its structure lead to a significant decrease in I01.5/I11.0 intensity ratios, an effect which must be considered when cation orders of natural dolomites from different origins are compared.

Research of cation dependences of structural and elastic properties of metal carbonates series by density functional theory calculations

Using the density functional theory methods with the PBE gradient functional, and taking into account the van der Waals interaction in the basis of localized atomic orbitals, using the CRYSTAL 17 program code, calculations of the structure and elastic constants for MgCO3, ZnCO3, CdCO3 CaCO3 in the calcite structure, CdMg(CO3)2, CaMg(CO3)2, CaZn(CO3)2 in the dolomite structure, BaMg(CO3)2 – norsethite, and CaCO3, SrCO3, PbCO3, BaCO3 in the aragonite structure are calculated. It is shown that the lattice constants, interatomic distances linearly depend on the cation radius RM, and the band gap, atomic charges, overlap populations of electron shells, and crystal density depend on cation electronegativity χM. Elastic compression constants and linear elastic moduli along the c axis are also described by RM. For polycrystalline bulk modulus BH, shear modulus GH, and Young's modulus EH, linear dependences with a good correlation coefficient were obtained: BH (GPa) = 175.95−84.89·RM, GH (GPa) = 87.57−46.05·RM, EH (GPa) = 224.36−103.59·RM. Similar dependences are established for the longitudinal and transverse speeds of sound, the Debye temperature and the coefficient of minimum thermal lattice conductivity. These results can be used to predict the elastic and mechanical characteristics of carbonates with the chemical formula MCO3 and solid solutions based on them.

Kutnohorite CaMn(CO3)2 crystal growth at high pressure-temperature

High quality kutnohorite CaMn(CO3)2 single crystals up to 100 μm in size were successfully achieved under high pressure-temperature (P-T) conditions. Electron microprobe analyses revealed the average wt% of CaO was 25.98% and that of MnO was 32.88%, correspondingly well to the ideal formula of Ca1.0Mn1.0(CO3)2. Accurate crystalline structural data were determined from single-crystal X-ray diffraction (XRD), with the R3 space group and R3c space group used to refine the crystal structure of CaMn(CO3)2. The Ca-O and Mn-O bond lengths were slightly different when using the R3 space group, which were clearly distinguished from those in the dolomite structure. Therefore, R3c is the most probable space group for the CaMn(CO3)2 crystal structure. Thermogravimetric (TG) analysis and differential scanning calorimetry (DSC) showed that CaMn(CO3)2 decomposed from 620–750 °C, but only one endothermic peak was observed during the decomposition process. It indicated that the octahedral units in CaMn(CO3)2 have the same thermal stability due to the complete miscibility of Ca and Mn. The results of single crystal XRD and thermal analysis provided direct evidence that CaMn(CO3)2 has a calcitetype structure, not dolomite-type layered structure, which was in good agreement with the rigid model of rhombohedral carbonates.

Fragmentation of dolomite bed material at elevated temperature in the presence of H2O & CO2: Implications for fluidized bed gasification

With the aims of improving the understanding of the use of dolomite bed materials in a fluidized bed (FB) gasifier, dolomite primary fragmentation behaviors and mechanisms at elevated temperatures were thoroughly investigated by means of quantitative analysis of particles production, size distribution, and particles shape (in terms of HS circularity and convexity). Crystalline Glanshammar and amorphous Sala dolomites of different original particle sizes (300–350 μm, 350–500 μm, and 500–1000 μm) were treated under various conditions, such as temperatures of 650, 750, and 950 °C and gas conditions of N2, N2 & steam, and H2O & CO2. Crystalline Glanshammar dolomite exhibited severe fragmentation, while amorphous Sala dolomite only fragmented slightly. Fragmentation mechanisms were proposed for amorphous and crystalline dolomite. In case of the Glanshammar dolomite, the release of H2O trapped in the crystal lattices and trace level of CO2 at structure defects, contributed to 10% and 55% of the primary fragmentation, respectively. The presence of CO2 can significantly mitigate the fragmentation of crystalline Glanshammar dolomite, which is probably either due to the prevention of CO2− dissociation at the defects or filling of cations by interchanging CO2− with the dolomite, consequently, allowing for a stabilization of dolomite structure. The analysis shows that elutriation in fluidized bed gasifier can be reduced significantly when either using amorphous Sala dolomite as bed material or treatment of the Glanshammar dolomite in the presence of CO2 before its utilization in a fluidized bed.

Incorporation of Co In the Dolomite Structure: Coupled EPMA and Single Crystal Structural Studies of Co-Rich Dolomite From the Tenke-Fungurume District, Democratic Republic of Congo

Incorporation of Co In the Dolomite Structure: Coupled EPMA and Single Crystal Structural Studies of Co-Rich Dolomite From the Tenke-Fungurume District, Democratic Republic of Congo

Ca and Mg isotope fractionation during the stoichiometric dissolution of dolomite at temperatures from 51 to 126 °C and 5 bars CO2 pressure

Natural polycrystalline hydrothermal Sainte Colombe dolomite was dissolved in stirred titanium closed system reactors in aqueous 0.1mol/kg NaCl solutions at 51, 75, 121, and 126°C and a pressure of 5bars CO2. In total, 52, 27, 16, and 12%, respectively, of the dolomite placed in the reactors dissolved into the fluid phase during these experiments. Each experiment lasted from 12 to 47days and the fluid phase in each evolved towards, but did not exceed, ordered dolomite equilibrium at a pH of 5.9±0.3. All aqueous reactive fluids were undersaturated with respect to all potential secondary phases including calcite and magnesite. The reactive fluid compositions at the end of the experiments had a molar Ca/Mg ratio equal to that of the dissolving dolomite, and the dolomite recovered after the experiments contained only pure dolomite as verified by scanning electron microscopy. The Ca and Mg isotopic ratios of the reactive fluids remained within uncertainty equal to that of the dissolving dolomite in the experiments performed at 51 and 75°C. In contrast, the Ca isotopic composition of the reactive fluid in the experiment performed at 121 and 126°C was significantly greater such that Δ44/42Casolid-fluid=−0.6±0.1‰, whereas that of Mg was within uncertainty equal to that of the dissolving mineral. The equilibrium fractionation factors for both divalent cations favor the incorporation of isotopically light metals into the dolomite structure. Our results at 121 and 126°C, therefore, are consistent with the one-way transfer of Mg from dolomite to the fluid but the two-way transfer of Ca from and to dolomite as equilibrium is approached during its stoichiometric dissolution. The lack of Mg returning to the dolomite structure at these conditions is attributed to the slow dehydration kinetics of aqueous Mg. As more than 12% of the dolomite dissolved during the 121 and 126°C closed system experiments, our observations indicate a significant change in the Ca isotopic signature of the dolomite during its stoichiometric dissolution. Moreover, as there is no visual evidence for dolomite recrystallization during this experiment, it seems likely that the resetting of Ca isotopic signatures of carbonate minerals can be readily overlooked in the interpretation of natural systems.

Combined high-pressure and high-temperature vibrational studies of dolomite: phase diagram and evidence of a new distorted modification

A combined high-pressure mid-infrared absorption and Raman spectroscopy study on a natural CaMg0.98Fe0.02(CO3)2 dolomite sample was performed both at ambient and high temperatures. A pressure–temperature phase diagram was constructed for all the reported dolomite ambient- and high-pressure polymorphs. In addition, a local distortion of the ambient-pressure dolomite structure was identified close to 11 GPa, just before the transition toward the first known high-pressure phase. All the Clausius–Clapeyron slopes are found to be positive with similar magnitudes. Complementary first-principles calculations suggest a metastable nature of the high-pressure dolomite polymorphs. Finally, theoretical spectroscopy is used to interpret and discuss the observed changes in the measured vibrational spectra.

Modification of Malaysian dolomite using mechanochemical treatment via different media for oil palm fronds gasification

SUMMARY Exploitation of biomass wastes gives new hopes in the exploration of clean alternative fuel source such as hydrogen energy. The application of catalysts plays a major role in the process by enhancing the production of hydrogen via reformation and tar cracking reaction. However, certain drawbacks such as catalyst cost, deactivation, and regeneration remain unsolved in this process. In this study, we focused on potential Malaysian dolomite (Perlis) as the source of catalysts for palm frond gasification. The influences of mechanochemical treatment in three different media (air, ethanol, and water) on the catalytic activity of the material were analyzed. Important changes in the properties of the dolomite along with the high catalytic performance were noted as the results of the treatment. Notable changes in the dolomite structure and porosity were observed because of the impact of the treatment. Increment in the surface area of the catalysts boosts the H2 production and catalytic activity of dolomite. Moreover, substantial CO2 reduction was also observed during reaction as a result of the absorption capacity of treated dolomite. Thus, through mechanochemical treatment, the property of Malaysian dolomite is being greatly altered, which really suits as a potential catalyst for gasification. Copyright © 2013 John Wiley & Sons, Ltd.

First-principles study of the electronic, optical and bonding properties in dolomite

The structural, electronic, optical properties and chemical bonding of dolomite CaMg(CO3)2 (rhombohedral calcite-type structure) are investigated using plane wave pseudopotential density-functional theory (DFT) method taking the local density approximation (LDA) and the generalized gradient approximation (GGA) as the exchange–correlation energy functional. The structural properties are consistent with the early experimental and theoretical results. The indirect electronic band gap is estimated to be ∼5.0eV, which is less than the optical band gap measured from the fundamental absorption edge of ∼6.0eV. The optical band gap is also consistent with the experimental band gap of similar calcite-type structure. A noticeable difference for the LDA and GGA derived transition peaks and a significant optical anisotropy are observed in the optical spectra. The analysis of electronic density of states, Mulliken charge and bonding population shows the coexistence of covalent and ionic bonding in the dolomite structure and the results are consistent with previous theoretical calculations.

II-VI oxides phase separate whereas the corresponding carbonates order: The stabilizing role of anionic groups

The formation enthalpies of isovalent, isostructural rocksalt alloys, $(A,B)$, where $X=\text{O}$ such as (Ca,Mg)O, are typically unfavorable (positive) for both ordered and random phases. Simple replacement of the single-atom anion, $X$, by a larger anionic group, such as ${\text{CO}}_{3}$ or ${\text{SO}}_{4}$, is able to induce a favorable (negative) formation enthalpy, leading to the formation of the ordered alternate monolayer, ${({\text{CaCO}}_{3})}_{1}/{({\text{MgCO}}_{3})}_{1}$, dolomite structure. The underlying cause of this behavior is analyzed by breaking down the formation process in a Born-Haber-like cycle into volume and cell-shape deformation, chemical exchange, and cell-internal relaxation using first-principles density-functional theory calculations in the generalized gradient approximation. It is found that when the anion is a group $({\text{CO}}_{3})$, rather than a single atom (O), the energy gained from the internal relaxation overcomes the energy required to compensate the volume mismatch. This explains the general experimental trends of phase separation in isovalent, isostructural alloys without internal-anion structure, compared to ordering tendencies when the anionic group removes internal strain. The importance of obtaining structural ideality in the design of stable solid solutions is highlighted.

Thermal behavior of Zefreh dolomite mine (Central Iran)

Dolomite has a large amounts of MgO and is an important raw materials for steel, iron and refractory industries. In this paper thermal behavior of Zefreh dolomite at various temperatures was studied by differential thermal analysis (DTA), Thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT - IR) and powder X-ray diffraction (XRD) techniques. The DTA curve shows two endothermic peaks at 772 and 834?C. The first peak is associated with the formation of magnesia (MgO), calcite(CaCO3) and CO2. The second peak represents the decomposition of calcite with formation of CaO and further CO2 release. At 772?C quantitative XRD analysis in atmospheric condition shows small amounts of CaO (less than 2%) which form simultaneously or later than calcite. TGA curve shows total weight loss for decomposition of dolomite is 46.2%. By increasing heating time of samples in furnace, dolomite decomposes in lower temperatures. X-ray diffraction analysis of heat treated samples up to 750?C indicates that dolomite structure changes into calcite. The increases in quartz content accelerated the mechanochemical deformation and amorphization of dolomite phase. At high temperature ( more than 1000?C) Ca3Mg(SiO4)2 and CaSiO4 were formed. XRD and FTIR confirm dolomite decomposition reactions.

Identification of transcrystalline microcracks observed in microscope images of a dolomite structure using image analysis methods based on linear structuring element processing

This paper presents an image analysis algorithm for transcrystalline microcracks recognition. The algorithm is applied for the analysis of microscope images of a dolomite structure. Linear structures are derived using an image analysis method based on mathematical morphology functions utilizing linear structuring elements. Next, the standard deviation function of microcracks and grain boundaries of surrounding pixel values is used to determine transcrystalline microcracks. All operations were executed in CIELab colour representations of dolomite images. The results obtained confirm the adequacy of this approach as a robust segmentation tool. Accordingly, an algorithm was derived that allows a fully automatic segmentation of transcrystalline microcracks. This is followed by geometrical and statistical analysis. The developed algorithm may facilitate petrographical and stereological studies of rock structures observed under a polarizing microscope.

Raman and infrared spectra of carbonates of calcite structure

AbstractThe Raman and mid‐range infrared spectra have been measured on natural limestone and dolomite minerals. The carbonate minerals show four prominent absorption bands in the regions 1450–1420, 890–870, 720–700 and 1000–1100 cm−1. The positions of the wavenumbers are unique for each carbonate mineral and are thus diagnostic of their mineralogy. Calcite and dolomite groups are characterized by the Raman wavenumbers at 288 and 309 cm−1 and the infrared absorption bands at 712 and 728 cm−1, respectively. The principal wavenumber at 1092 cm−1 in the limestone spectra is accompanied by two satellites with values of 1062 and 1075 cm−1. The observed non‐split peaks ν2 and ν4 in the infrared spectra of limestone indicate the presence of calcite structure in all these samples. The principal reflections occurring at the d‐spacings, 3.03482, 1.91658 and 1.87962 Å, confirm the presence of calcite structure in limestone minerals. The principal reflections occurring at the d‐spacings, 3.037, 1.79179 and 2.19750 Å, confirm the existence of dolomite structure in the dolomite minerals. The calculated lattice parameters for the limestone minerals are: a = 4.9781 Å, c = 17.1188 Å and V = 367.392(Å)3 and the corresponding values for the dolomite minerals are: a = 4.8247 Å, c = 15.9868 Å and V = 322.28 (Å)3. Copyright © 2006 John Wiley & Sons, Ltd.

Static lattice energy calculations of mixing and ordering enthalpy in binary carbonate solid solutions

Enthalpies of mixing and ordering in the binaries of the quaternary carbonate (Ca,Mg,Fe,Mn)CO 3 solid solution have been modeled using static lattice energy minimization calculations. A set of self-consistent empirical potentials has been specially developed for this task. The calculations illustrate the importance of size mismatch in determining magnitudes of the enthalpies of mixing and ordering in the binaries. The enthalpy effects show positive correlation with the predicted temperatures of the order/disorder transitions in intermediate compounds with the dolomite structure. It is shown that the general behavior of the mixing enthalpy in high- and low- T limits can be constrained with a minimum set of sampled configurations.

The fate of structure-bound Mn2+during the decomposition of dolomite and in the resulting conversion products: An EPR study

Dolomite from the Jhamarkotra phosphate mine (India) contains 680 ppm Mn 2+ as indicated by electron-spin resonance spectroscopy (EPR). Dipolar broadening of the EPR signal prevents a quantitative assignment of Mn 2+ to the Ca and Mg sites. Upon heating to 700 °C, over 99% of the dolomite is decomposed and all the Mn 2+ is released from the dolomite structure. Approximately 95% of the original Mn 2+ is oxidized and forms Mn-oxide. The remaining Mn 2+ preferentially migrates into CaO. This behavior is interpreted by the Goldschmidt rule stating that smaller ions are taken up at the sites of larger ions. During subsequent hydration of CaO into Ca(OH) 2 , the Mn 2+ remains stable. The protection of Mn 2+ against oxidation is explained by the topotaxical alteration of the host minerals. Carbonation of the Ca(OH) 2 leads to Mn 2+ oxidation caused by the dissolution of the hydroxide prior to the formation of CaCO 3 . Divalent Mn monitored by EPR can be used to unravel the behavior of trace elements on the molecular level during mineral alteration

A shell model for the simulation of rhombohedral carbonate minerals and their point defects

The electronic polarization of oxygen ions has been explicitly incorporated in a shell model to better simulate the structure of calcite and related rhombohedral carbonate minerals. Pair-potentials for Ca{sup 2+} ions and C and O comprising the carbonate molecular ion were simultaneously fitted to experimental lattice, elastic, dielectric, and vibrational data for calcite, and the structure and elastic properties of aragonite. The resulting potential parameters for the CO{sub 3}{sup 2{minus}} group were then transferred to models for the structures and bulk moduli of the carbonate minerals incorporating Mn, Fe, Mg, Ni, Zn, Co, Cd, and thus a fully consistent set of interaction parameters for calculating the properties of the carbonate minerals was obtained. Defect energies for doping the divalent cations into the calcite structure, and for calcium and carbonate ion vacancies were calculated. In addition, various disorder types for dolomite, including anti-site defects, stacking defects, stacking defects, and the energy related to increasing the Ca/Mg ration in the dolomite structure were simulated. The theoretical enthalpy for domomite ordering (34.4 kJ/mol) compares very well with experimental measurements.

On sub-micrometre inclusions in diamond coat: crystallography and composition of ankerites and related rhombohedral carbonates

AbstractCrystallographic studies of micro-inclusions in diamond coat, carried out by analytical electron microscopic techniques, show that structures possessing rhombohedral symmetry form a significant fraction of the population of well-crystaUised particles. They are, however, less frequent than apatite or biotite, which were identified earlier by the same investigative methods. Thin-foil specimens of diamond coat were prepared by mechanical fine-polishing, and subsequently ion-beam milling, sawn and polished plates of coated diamonds oriented parallel to (100) or (110). The majority of crystalline inclusions analysed were ≤0.2 p,m in diameter. Data obtained on 15 individual inclusions, including composition analysis by energy-dispersive X-ray spectroscopy, are reported and discussed. The a-axis of the hexagonal unit cell, and the c/a ratio, were determined for all specimens, and are believed to be accurate to ≈1% in most cases. In 12 out of 15 specimens, the cations with Z ≥ 11 identified comprised substantially only Mg, Fe and Ca. Values of a in these specimens ranged from 4.81 to 4.92 Å; and for three of them the space group R, corresponding to the dolomite structure, was positively identified. These crystals are classed as ankerites. Representative Mg:Fe:Ca ratios in the ankerites are 28 : 18 : 54, with cell dimensions a = 4.87 Å, c/a = 3.37. The ankerites contained small amounts of Ba, typically 1-3%, and smaller amounts of Sr, typically 0.2-2%. Three out of the 15 specimens contained >10% Ba. In two of these, Ba and Ca concentrations were roughly equal, and in one of the two, which had cell parameters a = 5.11 Å, c/a = 3.50, the space group symmetry Rc, corresponding to the calcite structure, was verified.

Trace element zoning in dolomite: Proton microprobe data and thermodynamic constraints on fluid compositions

Lower Cretaceous euhedral dolomite crystals from the Gargano Peninsula, Italy, have been analyzed by scanning proton microprobe (SPM) for the trace elements Sr, Zn, Fe, Mn, Cu, Ni, Y, and Rb. Intricate, decoupled zoning patterns for Fe, Mn, and Zn are a ubiquitous feature of the outer 55 microns of the dolomite crystals: differences between peak and baseline concentrations are on the order of several hundred parts per million. In contrast, Sr concentrations are nearly constant at 110 ppm. The concentrations of the other cations are all low (Cu, 20 ppm; Y, 10 ppm; and Ni and Rb, 5 ppm). In order to investigate the origin of the Gargano dolomite, we have used the method of Tardy and Garrels to estimate equilibrium partition coefficients for divalent cations between aqueous fluids and the dolomite structure. δ 18O measurements on Gargano dolomites show that they formed at a maximum temperature of 75°C (±7°C). Estimated equilibrium constants for substitution of the cation M into dolomite, calculated baseline molar cation ratios for fluids in equilibrium with Gargano dolomite, and measured cation ratios for seawater and marine sediment pore fluids at 75°C are as follows: M/N K eq Log 10 calculated ratio Log 10 M/N measured ratio in seawater Log 10 M/N measured ratio marine pore fluids Mn/Mg 1.27 −4.62 −7.17 −2.75 Fe/Mg 12.66 −4.77 −6.57 −3.81 Zn/Mg 6.85 −5.27 −6.72 −4.84 Sr/Ca 0.17 −2.94 −1.92 −1.83 Cation ratios of fluids in equilibrium with the baseline concentrations of Mn, Fe, and Zn in the Gargano dolomite samples are bracketed by the cation ratios measured in seawater and in marine sediment pore fluids. The factor often discrepancy for Sr in the predicted fluid compared with seawater is probably due to nonideal mixing of Sr on the Ca site. The decoupled fluctuations of the Mn and Fe concentrations in these samples can be explained by Eh fluctuations in the reactant fluid. Zn fluctuations may be a product of pH-dependent oxide surface reactions. To test this hypothesis of origin, unzoned Tertiary dolomites from Enewetak atoll and from the Bahama bank have also been analyzed. The homogeneous Sr concentration in the Gargano dolomite is similar to the Sr concentrations measured in the Enewetak atoll (175 ppm) and Bahama bank (150 ppm) dolomites, samples which in both cases almost certainly grew in equilibrium with seawater. Gargano baseline Mn (20 ppm) and Fe (80 ppm) concentrations are also much the same as the Bahama bank dolomite Mn (15 ppm) and Fe (140 ppm) concentrations. The lack of Sr zoning in the Gargano samples and the similarity of their Sr, Mn, and Fe content to dolomite that formed in equilibrium with seawater indicates that the dolomitizing fluid was most likely seawater or a seawater-type brine. Low Sr concentrations (12 ppm) in pore-filling calcite from the Gargano suggests that, in contrast to the dolomite, the calcite precipitated from meteoric fluids.