Ideal Dolomite Research Articles

A comparison of the proto-dolomite induced by cyanobacteria and halophilic bacteria: implications for dolomite-inducing microbe identification

This study investigates the morphological and mineralogical characteristics of proto-dolomite induced by two specific microorganisms with varying lifestyles: the extremely halophilic bacterium Vibrio harveyi QPL2 and the cyanobacterium Leptolyngbya boryana. Halophilic bacterially-induced proto-dolomite (HBD) and cyanobacterially-induced proto-dolomite (CBD) were subjected to comprehensive characterization techniques, including X-ray diffraction, scanning electron microscopy, Focused Ion Beam, thermogravimetric analysis, and X-ray photoelectron spectroscopy. The results indicate that both HBD and CBD exhibit a low degree of crystallinity and possess comparable molar ratios of MgCO3 to CaCO3. Moreover, neither of them exhibits the ordered structure of ideal dolomite. HBD and CBD exhibit notable distinctions in external morphology and internal structure. HBD forms a subunit aggregate with a less dense surface and numerous pinhole structures resulting from bacterial survival. In contrast, CBD adopts a bispherical shape with a relatively dense surface and minimal indications of cyanobacterial survival. Both HBD and CBD have an internal hollow structure. However, HBD is characterized by sparse population and loosely arranged subunits, while CBD features only a central cavity. Additionally, HBD particles are smaller compared to CBD particles. These morphological differences suggest that HBD primarily grows through bacterial surface-dependent processes, whereas the growth of CBD is not directly reliant on the surface of cyanobacteria. Compositionally, the weight percentage of crystalline water in CBD exceeded that of HBD with a value of 29.42 % compared to 5.9 %. This increase in internal crystalline water enables a faster conversion of CBD to the ordered ideal dolomite in a specific diagenetic environment. This study implies that the morphology and composition of microbial proto-dolomite may aid in identifying the type of dolomite-inducing microbes.

Organomineralization of dolomite in hypersaline microbial mats from Qatar sabkhas visualized by TEM & STXM

Deep insight into the low-temperature mineralization mechanism of dolomite in sediments has remained elusive. This issue is popularly termed “The Dolomite Problem” due to its multifactorial nature. Dolomite has been observed to mineralize in the exopolymeric substances produced by microbial mat communities (Bontognali et al. 2013), where productivity is high. One working hypothesis suggests that degrading organic matter in hypersaline environments releases the necessary component ions, increasing saturation with respect to dolomite (DiLoreto et al. 2019, Dupraz et al. 2009, Petrash et al. 2017). Other models suggest a dissolution-reprecipitation reaction of calcite to dolomite (Rivers 2023). High-resolution micro-spectroscopy techniques (such as transmission electron microscopy, TEM; and scanning transmission X-ray microscopy, STXM) can be used to determine chemical changes in crystals nucleating in a matrix, however to date very little studies have focused on observing dolomite mineralization at the nano-scale. The present study investigates microbial mats collected from hypersaline salt flats in the Persian gulf at micro- to nano-meter scales using high-spatial and -energy resolution TEM (Thermo Scientific Talos 200X at the Canadian Centre for Electron Microscopy) and STXM (PolLux Beamline at the Swiss Light Source at Paul Scherrer Institut), specifically to see changes in carbonate mineralization due to interactions with organic matter. C, Ca and O elemental maps of carbonate crystals were obtained with EDXS (energy-dispersive X-ray spectroscopy) in TEM. These crystals were also indexed by SAED (selected area electron diffraction in TEM). Fine spectral signatures (near-edge X-ray absorption fine structures, or NEXAFS) at the C K-edge (280-290 eV) and Ca L2,3-edge (344-356 eV) in STXM were used to determine the chemical identity of carbonate minerals and surrounding organic matter of the microbial mats. The results of the study show that dolomite nucleates in close association with the organic matter of the mats, where degradation is highest (defined in our adjacent study as the increase in C:N ratio). In TEM, polycrystalline dolomite is seen mineralizing in the matrix of the microbial mat organic material (Fig. 1). In STXM, the identity of the carbonate mineral changes from calcite on the outside to dolomite on the inside of the microbial mat particle (Fig. 2). In addition, our microsensor observations of elevated H2S concentrations, surface oxygenation from oxygenic phototrophy, high reduction potential, high organic carbon, high Mg:Ca ratio and high organic matter degradation (by C:N ratio) in each of the studied microbial mats confirms that the ideal dolomite mineralization conditions according to models of the dolomite problem are present in each case.

Lacustrine-evaporitic microbial dolomite from a Plio-Pleistocene succession recovered by the SG-1 borehole in the Qaidam Basin, NE Tibetan Plateau

The Plio-Pleistocene evaporitic lacustrine succession in the Qaidam Basin, which was recovered by drill core SG-1, provides the material for understanding the maturation of dolomite since dolomite is present in the core from 7.6 to 870 m depth, spanning a time interval of 0.1–2.8 Ma. Twenty-two samples were collected at different burial depths. The samples were examined by thin-section petrography and SEM for textures, and analyzed for mineralogy (XRD), trace elements and carbon and oxygen isotopes. The main mineral compositions of each sample are dolomite, quartz, halite and gypsum. Dolomite is mostly scattered with spheroidal to ellipsoidal shapes, several microns in diameter, partially encapsulated in gypsum. Microbial micropores and calcified microorganisms are observed on and within the dolomite crystals and in the thin sections. The degree of ordering of the dolomite gradually increases with burial depth, which basically conforms to the first-order reaction function. The crystal constants of dolomite decrease with burial depth, indicating recrystallisation with increasing overburden. The trace elements of the dolomites are significantly different from those of hydrothermal dolomites, but close to lacustrine microbial carbonate. It is concluded that the dolomites in core SG-1 are mainly authigenic, with microbial processes a likely facilitator of dolomite precipitation. The dolomite formed and then evolved in crystal structure during burial, gradually approaching ordered stochiometric dolomite. This study gives clues to link microbial dolomite to the massive ideal dolomite rock encountered in the geological record.

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.

Controls of temperature, alkalinity and calcium carbonate reactant on the evolution of dolomite and magnesite stoichiometry and dolomite cation ordering degree - An experimental approach

Natural and synthetic dolomites exhibit large scatter of structural and compositional characteristics with respect to variations in their stoichiometry and cation ordering degree (COD), but the physicochemical factors controlling dolomitization reactions are still poorly understood and by far not quantified. In the present study, dolomite was synthesized in batch reactors at temperatures of 150°C, 180°C and 220°C by reacting calcite or aragonite with an artificial solution containing MgCl2 and NaHCO3 over a 360day period. The obtained results on the evolution of the stoichiometry of dolomite and magnesite and on the COD of dolomite indicate that the formation of stoichiometric and well ordered dolomite and of stoichiometric magnesite follow a ripening process and proceed through dissolution and re-precipitation of a sequence of intermediate phases, such as Ca-magnesite, huntite, very-high-Mg-calcite (VHMC) and disordered dolomite. The initial occurrence and the metastability of the precursor phases were dependent particularly on reaction temperature and to a lesser extent on the CaCO3 source used. Temperature is the major rate-limiting parameter of the evolution of dolomite and magnesite stoichiometry and dolomite COD, corroborating results from prior experiments and natural deposits. From our data, we determined kinetic rates for the stoichiometric ripening of magnesite and dolomite and for the dolomite COD for the first time using a first-order reaction kinetic approach. The kinetics increase with temperature following the linear Arrhenius equation, in turn allowing for activation energies of the respective processes to be determined (44.0, 57.4 and 9.3kJ·mol−1, respectively). Interestingly we found the reaction rate for magnesite stoichiometric ripening is approximately one third of that for dolomite regardless of the temperature, carbonate alkalinity or CaCO3 phase used for reaction. Furthermore, we see that the activation energies required for the formation of ideal dolomite decreases in sequence from the precipitation of an initial VHMC phase (∼133.5kJ·mol−1; Arvidson and Mackenzie, 1999), followed by stoichiometric ripening (57.4kJ·mol−1) to cation ordering (9.3kJ·mol−1; for perfect ordering).Extrapolation of the obtained dolomitization rates indicates that about 1.4 and 6.8Myr are required to approach ideal dolomite at 50 and 25°C, respectively, explaining the large absence of ordered dolomite in modern sedimentary successions. The present dataset provides new insights into dolomitization pathways and rates and contributes to a better understanding of the wide range of stoichiometries and COD values of dolomite and its evolution versus magnesite observed in the geological record and in laboratory studies.

A Comparison of Nanometer-Scale Growth and Dissolution Features on Natural and Synthetic Dolomite Crystals: Implications for the Origin of Dolomite

Abstract This paper describes experiments in which mechanisms of crystal growth are inferred from the surface nanotopography of natural and synthetic dolomite using atomic force and scanning electron microscopy. Synthetic dolomite was formed by replacement of calcite at 218°C in Mg–Ca–Cl solutions. The first products to form during dolomite synthesis experiments, as detected with X-ray diffraction (XRD), are invariably poorly ordered (nonideal) dolomite. As the reaction proceeds, the degree of cation order increases, yet the products remain relatively disordered. Upon examination, the surfaces of nonideal dolomite are covered with mounds 10–200 nm wide and 1–20 nm high. Following depletion of the calcite reactant, dolomite becomes relatively well ordered and stoichiometric (ideal). The surfaces of ideal dolomite are covered with broad, flat layers separated by steps that measure tenths to tens of nanometers high. Comparison of natural and synthetic dolomite surfaces after etching in 0.5% H2SO4 for 10–20 seconds indicates that high-temperature synthetic and low-temperature natural nonideal dolomite surfaces are covered with mounds identical to the growth features found on unetched nonideal synthetic dolomite. In contrast, synthetic and low-temperature ideal dolomite surfaces, when etched, are characterized by flat layers with deep, euhedral etch pits. These results suggest that despite the wide range in formation conditions, natural and synthetic dolomites form by the same growth mechanisms. Furthermore, the two different surface nanotopographies described here are consistent with a model in which nonideal dolomite forms by precipitation of amorphous mounds that quickly crystallize to nonideal dolomite while ideal dolomite later replaces nonideal dolomite and grows by spiral growth.

Temperature Dependence of Mineral Precipitation Rates Along the CaCO3-MgCO3 Join

The large variation in precipitation rate and abundance of mineralscomprising the CaCO3–MgCO3 binary join can be understood in terms of their large differences in activation energy. Following the treatment of Lippmann (1973), activation energy isextrapolated along the join as a linear function of mole percentmagnesium. For the dolomite-type carbonates, the predicted activationenergy is compatible with recent measurements of calcian protodolomitekinetics; cation ordering in ideal dolomite can thus be seen as anadditional contribution to activation energy. Although no activationenergies are available for magnesian calcites, treatment of rate datafor these phases using the formalism of stoichiometric saturationsuggests a possible change in mechanism or rate-limiting step astemperature is decreased from 25 to 5 °C.

Authigenic Dolomite in a Saline Soil in Alberta, Canada

AbstractThe formation of dolomite is poorly understood and highly controversial, since well‐ordered dolomite has never been synthesized under earth surface conditions and natural occurrences of modern dolomite have been reported only rarely from restricted sedimentary environments. It is generally accepted that dolomite does not form in soils. This study investigates the characteristics and origin of dolomite in a sulfatic saline soil in east‐central Alberta, Canada, in which dolomite makes up more than half of the <2‐µm fraction. X‐ray diffraction analysis shows that the dolomite is stoichiometric and ordered, with only slight deviations in unit cell parameters relative to ideal dolomite. The clay‐sized dolomite is enriched in 18O and depleted in 13C relative to detrital dolomite and dolomite present in larger size fractions, and has a radiocarbon age ranging from 1270 to 5270 yr before present, clearly indicating an authigenic origin. The very small dolomite particle size and differences in isotope content from coexisting calcite suggests that the dolomite formed by direct precipitation in the soil rather than by dolomitization of a preexisting calcite.

Dolomite stoichiometry and Ostwald's Step Rule

Nucleation kinetics responsible for Ostwald's Step Rule in the transition of calcite to dolomite at 193°C in 1 M 0.66 Mg to Ca ratio solutions were investigated. Nucleation and crystal growth kinetics were determined from differences in the induction period between isothermal experiments and experiments cycled between 193°C and room temperature. The cycled reactions used 12 h and 48 h heating periods interchanged with 12 h at room temperature. All three experimental designs produced high magnesium calcite (HMC), nonstoichiometric dolomite, and near ideal dolomite. No differences were seen in product size or morphology among designs. The length of the induction period of HMC was the same in all three experimental designs. This indicates that stable nucleation of HMC requires less than 12 h of heating and that the remaining induction period reflects HMC crystal growth. Isothermal and 48 h experiments produced a more stoichiometric dolomite (approx. 46% Mg) after 107 to 109 h of heating. The 12 h series did not detect this phase until 181 h of heating. This indicates that nucleation of the more stoichiometric phase was suppressed. Nucleation theory, these experiments, and the structural and thermodynamic properties of HMC and more stoichiometric dolomite lead us to conclude that the surface free energy of HMC is less than that of more stoichiometric dolomite. Ostwald's Step Rule in these calcite to dolomite transformations is consistent with surface free energy controlled nucleation kinetics.

Dolomite-calcite equilibrium at 220 to 240 °C at saturation vapour pressure: Experimental data

Small amounts of dolomite and calcite were added as reactants to a series of CaCl 2-MgCl 2 solutions with variable Ca:Mg ratios at temperatures of 220°C and 240°C at vapour pressure in sixty experimental runs. Dolomitization of calcite and calcitization of dolomite in these runs indicates that the value of log ( aCa 2+ aMg 2+ ) in solutions at equilibrium with calcite and dolomite ranges from about 0.4 to 0.9 at these temperatures. This is in agreement with previous experimental work at higher and lower temperatures. These experimentally determined equilibrium log ( aCa 2+ aMg 2+ ) values are less than those calculated by thermodynamic based programs, such as SUPCRT and PTA, for equilibrium with ordered dolomite and those which may be calculated from calorimetric data obtained from ordered, ideal dolomite. This may indicate that the experimentally observed calcite-dolomite equilibrium is metastable and that the precipitated dolomites are imperfectly ordered. Precipitation of partially disordered, metastable dolomite of stoichiometric composition may be favoured over that of ideal dolomite at these temperatures for kinetic reasons.

Formation of dolomite in the Coorong region, South Australia

Cores of unlithified Holocene sediment from shallow ephemeral lakes in the Coorong region S.A., contain dolomite at various stratigraphic intervals. Two distinct dolomite types are recognized using stable carbon and oxygen isotope data, unit cell calculations, transmission electron microscopy (TEM), and bulk composition data. Detailed X-ray diffraction (XRD) analyses show that the dolomite is ordered, but that ordering is variable. Average unit cell parameters indicate that the crystal lattice of one type of dolomite (type A) is expanded in the c o direction (c o = 16.08 A ̊ ) and contracted in the a o direction (a o = 4.799 A ̊ ) relative to ideal dolomite (c 0 = 16.02 A ̊ and a o = 4.812 A ̊ , respectively). Type A dolomite occurs in the centers of the larger basins. The mole fraction of MgCO 3 is as much as 53 ± 1%. The unusual crystal chemistry and (TEM) data indicating a heterogeneous microstructure suggest that this dolomite is generated by rapid precipitation from brines which have highly elevated Mg Ca ratios. Dolomite ordering reflections are present in electron diffraction patterns but are weak. Stable oxygen and carbon isotope values are tightly grouped (ave. δ 18O = +7.6‰, s.d. = 0.6‰; δ 13C = +3.5‰, s.d. = 0.7‰) . The other type of dolomite (type B) is Ca-rich, has lower stable isotope values (ave. δ 18O ≈ +6.4‰, δ 13C ≈ −1.2‰) than type A dolomite, has a more homogeneous microstructure, is expanded in both the a o and c o axes, and is generally better ordered than type A dolomite. These data suggest type B dolomite is precipitated more slowly from less evaporitic brines than type A dolomite.

Atomic resolution microscopy of carbonates. Interpretation of contrast

The atomic resolution microscopy (ARM) at the National Center for Electron Microscopy, Lawrence Berkeley Laboratory, Berkeley, California has been used to image structural features in rhombohedral carbonates. The resolution of the microscope is better than 1.6 A, but beam damage presently limits the resolution of some of our images to slightly better than 2.6 A. More details can be extracted through image processing. We were able to interpret contrast in through focus series of “ideal” dolomite by comparing processed images with multibeam contrast calculations. Fair agreement was obtained for focus and thickness variations both of which display great changes. Even for ideal dolomite, the matching is not straight-forward, due to minor orientation variations, the presence of and amorphous overlayer, and surface roughness induced by ion beam thinning, etc. We also find good agreement for calcian δ-dolomite with a cation distribution model which assumes a periodic substitution of alternating Mg layers by Ca. Some atomic resolution examples are shown for coherent calcite-dolomite intergrowths and δ-dolomite domains in dolomite.

Sedimentology, mineralogy and isotopic analysis of Pellet Lake, Coorong region, South Australia

ABSTRACT Gravity cores of Holocene sediments from a shallow ephemeral lake in the Coorong region (Pellet Lake, southeastern coastal Australia) show a mineral assemblage and sequence particular to its hydrology. The mineralogical sequence above an initial dolomitic siliciclastic sand reflects conditions of increasing salinity in the lower portions of the core (i.e. organic‐rich aragonite to magnesite + hydromagnesite + aragonite) followed by a relative decrease in salinity (i.e. magnesite + aragonite + hydromagnesite to aragonite + hydromagnesite) in the upper portions of the core. This sequence is capped by ˜ 0.4 m of micritic dolomite and minor amounts of hydromagnesite, with the relative abundance of dolomite increasing upwards. Three stratigraphically and spatially distinct dolomite units (upper, lower and margin) are recognized using stable carbon and oxygen isotope data, unit cell calculations and MgCO3 mole per cent data of the dolomite.Detailed X‐ray diffraction (XRD) analyses of samples with more than 80% dolomite shows that the dolomite is ordered. Average unit cell parameters, calculated from the XRD patterns, indicate that the upper dolomite unit has crystal lattices expanded in the co direction (co= 16.09 Å) relative to ideal dolomite (co= 16.02 Å) and contracted in the ao direction (ao= 4.796 Å) relative to ideal dolomite (ao= 4.812 Å). The mol fraction of MgCO3 in the upper dolomite shows up to 4.0 ±M 2.0 mole per cent excess Mg in the dolomite crystal lattice (calculated from XRD). This unusual dolomite crystal chemistry is probably generated by rapid precipitation from solutions which have greatly elevated Mg/Ca ratios. Transmission electron microscopy reveals that the upper dolomite has a heterogeneous microstructure which also suggests rapid precipitation from solution. The modulated microstructure found in calcium‐rich dolomite is completely lacking. Dolomite ordering reflections are present in electron diffraction patterns, but are weak.Stable oxygen and carbon isotope values of the upper dolomite are tightly grouped (ave. δ18O ∼+ 7.55%o, δ13C ∼+ 4.10%o), yet show three upward‐lightening oxygen cycles. The oxygen cycles correlate with three upward decreases in the calculated Mg content of the dolomite zone. These cycles may indicate the increased importance of rain‐water dilution of the brine at times when the water in the lake was at its shallowest levels.Analyses of the lower dolomite and the margin dolomite suggest that these units precipitated more slowly from less evaporitic brines than the upper dolomite unit. The lower dolomite is close to stoichiometric, has less evaporitic stable isotope values than the upper dolomite, and has only a slightly expanded co‐axis. The margin dolomite is Ca‐rich, has a more homogeneous microstructure, and has expanded ao and co axes.The abundance of relatively soluble Mg‐bearing phases, such as hydromagnesite and magnesite, may supply additional magnesium for the dolomitization of aragonite and calcite during subsequent diagenesis and burial of the sediment. This process may leave a finely laminated dolomicrite deposit which retains little, if any, evidence of evaporite minerals.

Dolomitization and sulfate reduction in the mixing zone between brine and meteoric water in the newly exposed shores of the Dead Sea

Pleistocene dolomites are associated either with evaporative processes (such as in sabkhas and playas) or with a mixing zone where mixing between two different solutions (e.g. seawater and groundwater) takes place. We report here on the very recent formation of dolomite at shallow depths (6–12 cm) in newly exposed, gypsum rich, sediments on the western shores of the Dead Sea. SEM and EDS analysis, as well as X-ray diffraction analysis indicate that this dolomite is euhedral and close to ideal dolomite in composition. These dolomite crystals differ from anhedral detrital Ca rich dolomites from the Cretaceous section of the watershed found in the whole section. Taking into account the sedimentation rate at the shore vicinity the maximal age of the euhedral dolomite was estimated as less than 60 years. In the three profiles analysed, euhedral dolomite was most abundant at a single layer where the gypsum content was locally lowered due to bacterial reduction of sulfate. The sediments solution at the Dead Sea shore localities represent a mixing zone between Dead Sea residual solution and subsurface freshwater springs. Under such conditions formation of dolomite occurs at highly saline environment, where very high Mg++Ca++ ratios are attained.

Sequential Basal Faults in Devonian Dolomite, Nopah Range, Death Valley Area, California

Dolomite crystals from the Lost Burro Formation (Devonian) in the Nopah Range, eastern California, display basal stacking disorder as evidenced by transmission electron microscopy. Satellites in electron diffraction patterns indicate that stacking of anions and cations is different from that in ideal dolomite. This example conforms to the model of basal defects proposed by Goldsmith and Graf in 1958 to explain nonstoichiometry in dolomite. This dolomite from the Nopah Range was formed by deep burial replacement of micritic limestone, and its peculiar superstructure is tentatively attributed to the late diagenetic conditions during replacement.