Dolomite Grains Research Articles

Dolomite overgrowths suggest a primary origin of cone-in-cone

AbstractA long-debated aspect of cone-in-cone structures is whether the mineral aggregates composing the structure precipitated with their conical form (primary cone-in-cone), or whether the cones formed after precipitation (secondary cone-in-cone). A calcite deposit from the Cretaceous of Jordan bears all the defining characteristics of the structure. Trace dolomite within the sample supports the primary cone-in-cone hypothesis. The host sediment is a biosiliceous mudstone containing abundant rhombohedral dolomite grains. Dolomite rhombohedra are also distributed throughout the calcite of the cone-in-cone. The rhombohedra within the calcite locally have dolomite overgrowths that are aligned with calcite fibres. Evidence that dolomite co-precipitated with calcite, and did not replace calcite, includes (i) preferential downward extension of dolomite overgrowths, in the presumed growth-direction of the cone-in-cone, from the dolomite grains on which they nucleate, and (ii) planar, vertical borders between dolomite crystals and calcite fibres. Because dolomite overgrows host-sediment rhombohedra and forms part of the cones, it follows that the host-sediment was incorporated into the growing cone-in-cone as the calcite precipitated, and not afterward. The host-sediment was not injected into the cone-in-cone along fractures, as the secondary-origin theory suggests. This finding implies that cone-in-cone in general does not form over multiple stages, and thus has greater potential to preserve the chemical signature of its original precipitation.

Oxygen diffusion and exchange in dolomite rock at 700 °C, 100 MPa

In contact-metamorphic environments dolomite commonly breaks down to calcite + periclase + CO2 as a result of the infiltration of H2O. The transport and exchange of oxygen in dolomite rock during the breakdown reaction were examined experimentally by reacting a cylindrical core of Reed Dolomite with isotopically enriched water having the composition HD18O0.516 O0.5 at 700 °C and 100 MPa for 29 days. Reaction products formed along grain boundaries, fractures, and on the outside surface of the core. Some dolomite grains became enriched in Fe as a result of replacement of the host dolomite. Extensive voids are found in the grain boundaries as a result of the ~25% loss in solid volume during reaction. There are also pores, ~1 μm in diameter, in the dolomite, notably in the vicinity of the replaced dolomite. The distribution of 18O in the dolomite and reaction products was used as a tracer of the transport and exchange of O during reaction. Electron probe microanalysis (EPMA) and secondary ion mass spectrometry (SIMS) analyses showed pervasive infiltration of fluid along grain boundaries and fractures, growth and isotopic exchange with products of reaction, and diffusion of 18O into dolomite grains. The fluid infiltrated efficiently along grain boundaries to the dolomite grain surface. The host dolomite shows an enrichment in 18O along grain boundaries, indicating a diffusive exchange with the fluid. An estimate of the diffusion coefficient of oxygen in dolomite, determined from ion probe spot analyses, gives D ≈ 1 × 10−12 mm2/s. This value is comparable to the oxygen diffusion coefficient for calcite in an H2O-rich fluid. Mass balance of O in the experiment (including dolomite–fluid exchange, the amounts of neomorphic reaction products, and the fluid components) indicates that the reaction products have a 18O concentration only about half that of the fluid. Ion probe spot analyses of calcite from both the center and the edge of the core have the fraction F = 18O/(18O+16O) of 0.14 ± 0.1, whereas the value calculated for the fluid is 0.31. The measured F values of calcite are intermediate between the initial F values of starting water and dolomite, indicating that the reaction products record a mix of both dolomite- and fluid-derived oxygen. The products reached about 45% of isotopic equilibrium, similar to the extent of the mineral–fluid reaction. The Fe-rich, replacement dolomite near the core edge has an elevated value of F = 0.02 ± 0.002, 10 times the value of F ≈ 0.002 for unreacted dolomite, but less than the value for the calcite. The distribution of 18O in the minerals indicates that the breakdown and replacement reactions were faster than O diffusion in dolomite.

Kinetics of dolomite dissolution in a packed bed by acidified desalinated water

A post treatment process commonly applied in water desalination practice serves to introduce calcium and carbonate alkalinity to the water in order to protect the distribution system from corrosion damage. Addition of magnesium ions also is currently under consideration. Dolomite is a double salt of CaCO3 and MgCO3 and its dissolution by acidified desalinated water is of interest as this could serve to provide calcium, magnesium and carbonate alkalinity in a single re-mineralization operation. A critical literature review highlights the limitations of published data on dolomite dissolution and in particular their inadequacy for predicting dissolution in industrial equipment. The present study examines the kinetics of re-mineralization of acidified desalinated water in continuous flow over a packed bed of dolomite grains. A kinetic model based on mass transfer control is presented and shown to be in excellent agreement with experimental data. Results of this study indicate that dolomite dissolution is a viable process for re-mineralization of desalinated water.

Dolomite microstructures between 390° and 700 °C: Indications for deformation mechanisms and grain size evolution

Dolomitic marble on the island of Naxos was deformed at variable temperatures ranging from 390 °C to >700 °C. Microstructural investigations indicate two end-member of deformation mechanisms: (1) Diffusion creep processes associated with small grain sizes and weak or no CPO (crystallographic preferred orientation), whereas (2) dislocation creep processes are related with larger grain sizes and strong CPO. The change between these mechanisms depends on grain size and temperature. Therefore, sample with dislocation and diffusion creep microstructures and CPO occur at intermediate temperatures in relative pure dolomite samples. The measured dolomite grain size ranges from 3 to 940 μm. Grain sizes at Tmax >450 °C show an Arrhenius type evolution reflecting the stabilized grain size in deformed and relative pure dolomite. The stabilized grain size is five times smaller than that of calcite at the same temperature and shows the same Arrhenius-type evolution. In addition, the effect of second phase particle influences the grain size evolution, comparable with calcite. Calcite/dolomite mixtures are also characterized by the same difference in grain size, but recrystallization mechanism including chemical recrystallization induced by deformation may contribute to apparent non-temperature equilibrated Mg-content in calcite.

Alteration of Bakken reservoir rock during CO2-based fracturing—An autoclave reaction experiment

This study was conducted to document and assess the effects of fluid–rock interactions when CO2 is used to create the fractures necessary to produce hydrocarbons from low-permeability rocks. The primary objectives are to (1) identify and understand the geochemical reactions of CO2-based fracturing, and (2) assess potential changes in porosity and permeability of formation rock. Autoclave experiments were conducted at reservoir conditions exposing middle Bakken core fragments to CO2-saturated synthetic formation brine and to supercritical CO2 (sc-CO2) only. Ion-milled core samples were examined before and after the reaction experiments using scanning electron microscopy (SEM), which enabled us to image the reaction surface in extreme detail and unambiguously identify mineral dissolution and precipitation.The most significant change in the reacted samples exposed to the CO2-saturated brine is dissolution of the carbonate minerals, particularly calcite, which shows severe corrosion. Dolomite grains were corroded to a lesser degree. Quartz and feldspars remained intact, and some pyrite framboids underwent slight dissolution. Additionally, a small amount of calcite precipitation took place, as indicated by numerous small calcite crystals formed at the reaction surface and in the pores. The changes of aqueous chemical composition are consistent with the petrographic observations with an increase in Ca and Mg and associated minor elements, and a very slight increase in Fe and sulfate.When exposed to sc-CO2 only, changes observed include etching of the calcite grain surface and precipitation of salt crystals (halite and anhydrite) due to evaporation of residual pore water into the sc-CO2 phase. Dolomite and feldspars remained intact, and pyrite grains were slightly altered. Mercury intrusion capillary pressure (MICP) tests on reacted and unreacted samples show an increase in porosity when an aqueous phase is present but no overall porosity change with sc-CO2. The results also show an increase in permeability for brine-reacted samples.

Episodic carbonate precipitation in the CM chondrite ALH 84049: An ion microprobe analysis of O and C isotopes

We have determined the O and C isotope compositions of dolomite grains and the C isotope compositions of calcite grains in the highly altered CM1 chondrite, ALH 84049, using Secondary Ion Mass Spectrometry (SIMS). Chemically-zoned dolomite constitutes 0.8 volume percent (vol%) of the sample and calcite 0.9vol%. Thirteen separate dolomite grains have δ13C values that range from 37 to 60 (±2) ‰, δ18O values from 25 to 32 (±3) ‰, and δ17O values from 10 to 16 (±3) ‰ (VSMOW). Intragrain δ13C values in dolomite vary up to 10‰. The δ13C values of three calcite grains are distinct from those of dolomite and range from 10 to 13 (±2) ‰ (PDB). Calcite and dolomite appear to record different precipitation episodes. Carbon isotope values of both dolomite and calcite in this single sample encompass much of the reported range for CM chondrites; our results imply that bulk carbonate C and O isotope analyses may oversimplify the history of carbonate precipitation. Multiple generations of carbonates with variable isotope compositions exist in ALH 84049 and, perhaps, in many CM chondrites. This work shows that one should exercise caution when using a clumped isotope approach to determine the original temperature and the isotopic compositions of water for CM chondrite carbonates. Less altered CM meteorites with more-homogeneous C isotope compositions, however, may be suitable for bulk-carbonate analyses, but detailed carbonate petrologic and isotopic characterization of individual samples is advised.

Transport and retention of engineered Al2O3, TiO2, and SiO2 nanoparticles through various sedimentary rocks.

Engineered aluminum oxide (Al2O3), titanium dioxide (TiO2), and silicon dioxide (SiO2) nanoparticles (NPs) are utilized in a broad range of applications; causing noticeable quantities of these materials to be released into the environment. Issues of how and where these particles are distributed into the subsurface aquatic environment remain as major challenges for those in environmental engineering. In this study, transport and retention of Al2O3, TiO2, and SiO2 NPs through various saturated porous media were investigated. Vertical columns were packed with quartz-sand, limestone, and dolomite grains. The NPs were introduced as a pulse suspended in aqueous solutions and breakthrough curves in the column outlet were generated using an ultraviolet-visible spectrophotometer. It was found that Al2O3 and TiO2 NPs are easily transported through limestone and dolomite porous media whereas NPs recoveries were achieved two times higher than those found in the quartz-sand. The highest and lowest SiO2-NPs recoveries were also achieved from the quartz-sand and limestone columns, respectively. The experimental results closely replicated the general trends predicted by the filtration and DLVO calculations. Overall, NPs mobility through a porous medium was found to be strongly dependent on NP surface charge, NP suspension stability against deposition, and porous medium surface charge and roughness.

Influence of stress and strain on dolomite rim growth: a comparative study

Triaxial compression and torsion experiments were performed to investigate the influence of non-isostatic stress and strain on dolomite reaction rim growth using orientated natural calcite and magnesite single crystals at a temperature of 750 °C, 400 MPa confining pressure, stresses between 7 and 38 MPa, and test durations up to 171 h. Reaction products were composed of a polycrystalline magnesio-calcite layer, palisade-shaped dolomite, and granular dolomite grains. In all experiments, inelastic deformation was partitioned into calcite and reaction products, while magnesite remained undeformed. Calcite deformed by twinning and dislocation creep, where the activation of additional glide systems at high stress allowed high strain. Depending on grain size, magnesio-calcite deformed by diffusion creep and/or grain boundary sliding, twinning, and dislocation creep. Dolomite deformed mainly by diffusion creep, assisted by enhanced dislocation activity allowing Ca enrichment in the granular rim. A weak crystallographic preferred orientation of the reaction products was observed. In triaxial compression, dolomite rim growth was diffusion-controlled and showed no influence of axial stresses up to 38 MPa on the reaction kinetics. At high strain (>0.1), the magnesio-calcite layer is wider suggesting faster growth kinetics. This may be related to additional diffusion pathways provided by enhanced dislocation activity. At very high strain (>0.3–0.6), twisted samples showed a gradual decrease in layer thickness of dolomite and magnesio-calcite with increasing strain (-rate).

Rock pulverization and localization of a strike-slip fault zone in dolomite rocks (Salzach–Ennstal–Mariazell–Puchberg fault, Austria)

Detailed investigations of dolomite fault rocks, formed at shallow crustal depths along the Salzach–Ennstal–Mariazell–Puchberg (SEMP) fault system in the Northern Calcareous Alps, revealed new insights into cataclasite formation. The examined Miocene, sinistral strike-slip faults reveal grain size reduction of dolomite host rocks by tensile microfracturing at a large range of scales, producing rock fragments of centimetre to micrometre sizes. In situ fracturing leads to grain size reduction down to grain sizes <25 μm, producing mosaic breccias and fault rocks which have previously been described as “initial/embryonic” and “intermediate” cataclasites. At all scales, grain fragments display little to no rotation and no or minor evidence of shear deformation. The observed microstructures are similar to those previously described in studies on pulverized rocks. Microstructural investigations of cataclasites and mosaic breccias revealed aggregations of small dolomite grains (<50 μm) that accumulated on top of large fragments or as infillings of V-shaped voids between larger grains and show constant polarity throughout the investigated samples. Fabrics indicate deposition in formerly open pore space and subsequent polyphase cementation. The newly described tectonic geopetal fabrics (geopetal-particle-aggregates, GPA) prove that these faults temporarily passed through a stage of extremely high porosity/permeability prior to partial cementation.

The Effect of Dolomite Additive on Cement Hydration

The effect of dolomite on alite hydration was investigated in order to elucidate the effect of dolomite addition in cement hydration. The rate of heat evolution both in cement – dolomite and alite – dolomite system was taken as a starting point. Subsequently the chemical shrinkage, conductivity of liquid phase and rheological parameters of pastes were characterized. The observations of microstructure were carried out under SEM and the hydration degree of alite was determined by XRD. The accelerating effect of additive was proved. At low percentage dolomite plays a role of cement replacement; at higher dosage – the „dilution” effect can be observed. However, increasing dolomite content is accompanied by higher amount of hydration products, as a results of crystallization on the fine dolomite grains and better absorption of water. The hydration degree of alite increases as well.

The mechanical and microstructural behaviour of calcite-dolomite composites: An experimental investigation

The styles and mechanisms of deformation associated with many variably dolomitized limestone shear systems are strongly controlled by strain partitioning between dolomite and calcite. Here, we present experimental results from the deformation of four composite materials designed to address the role of dolomite on the strength of limestone. Composites were synthesized by hot isostatic pressing mixtures of dolomite (Dm) and calcite powders (% Dm: 25%-Dm, 35%-Dm, 51%-Dm, and 75%-Dm). In all composites, calcite is finer grained than dolomite. The synthesized materials were deformed in torsion at constant strain rate (3 × 10−4 and 1 × 10−4 s−1), high effective pressure (262 MPa), and high temperature (750 °C) to variable finite shear strains. Mechanical data show an increase in yield strength with increasing dolomite content. Composites with <75% dolomite (the remaining being calcite), accommodate significant shear strain at much lower shear stresses than pure dolomite but have significantly higher yield strengths than anticipated for 100% calcite. The microstructure of the fine-grained calcite suggests grain boundary sliding, accommodated by diffusion creep and dislocation glide. At low dolomite concentrations (i.e. 25%), the presence of coarse-grained dolomite in a micritic calcite matrix has a profound effect on the strength of composite materials as dolomite grains inhibit the superplastic flow of calcite aggregates. In high (>50%) dolomite content samples, the addition of 25% fine-grained calcite significantly weakens dolomite, such that strain can be partially localized along narrow ribbons of fine-grained calcite. Deformation of dolomite grains by shear fracture is observed; there is no intracrystalline deformation in dolomite irrespective of its relative abundance and finite shear strain.

An observational and thermodynamic investigation of carbonate partial melting

Melting experiments available in the literature show that carbonates and pelites melt at similar conditions in the crust. While partial melting of pelitic rocks is common and well-documented, reports of partial melting in carbonates are rare and ambiguous, mainly because of intensive recrystallization and the resulting lack of criteria for unequivocal identification of melting. Here we present microstructural, textural, and geochemical evidence for partial melting of calcareous dolomite marbles in the contact aureole of the Tertiary Adamello Batholith. Petrographic observations and X-ray micro-computed tomography (X-ray μCT) show that calcite crystallized either in cm- to dm-scale melt pockets, or as an interstitial phase forming an interconnected network between dolomite grains. Calcite–dolomite thermometry yields a temperature of at least 670 °C, which is well above the minimum melting temperature of ∼600 °C reported for the CaO–MgO–CO2–H2O system. Rare-earth element (REE) partition coefficients (KDcc/do) range between 9–35 for adjacent calcite–dolomite pairs. These KD values are 3–10 times higher than equilibrium values between dolomite and calcite reported in the literature. They suggest partitioning of incompatible elements into a melt phase. The δ18O and δ13C isotopic values of calcite and dolomite support this interpretation. Crystallographic orientations measured by electron backscattered diffraction (EBSD) show a clustering of c-axes for dolomite and interstitial calcite normal to the foliation plane, a typical feature for compressional deformation, whereas calcite crystallized in pockets shows a strong clustering of c-axes parallel to the pocket walls, suggesting that it crystallized after deformation had stopped. All this together suggests the formation of partial melts in these carbonates.A Schreinemaker analysis of the experimental data for a CO2–H2O fluid-saturated system indeed predicts formation of calcite-rich melt between 650–880 °C, in agreement with our observations of partial melting. The presence of partial melts in crustal carbonates has important physical and chemical implications, including a drastic drop in rock viscosity and significant change in the dynamics and distribution of fluids within both the contact aureole and the intrusive body.

Influence of dolomite microcrystals on the technological properties of Santa Cruz de Mudela clays used for building ceramics

This study is focused on the anomalously high plasticity of Santa Cruz de Mudela clays, traditionally used for the industrial production of bricks and roof tiles. Such a high plasticity makes necessary to add degreaser material to the ceramic paste despite they have very high carbonate content. These materials have been characterised both mineralogically and technologically to their optimum industrial utilisation. They are mainly composed of phyllosilicates (illite, kaolinite, smectite and palygorskite) and quartz with high dolomite content that is a contradiction to the high plasticity. Images of scanning electronic microscopy (SEM) reveal dolomite crystals with grain sizes of 2μm, explaining the high plasticity of these materials. Very small dolomite grains could improve the distribution during mixing process as well as the reaction between both CaO and MgO produced from clay minerals during the firing process.

Aragonite, breunnerite, calcite and dolomite in the CM carbonaceous chondrites: High fidelity recorders of progressive parent body aqueous alteration

Carbonate minerals in CM carbonaceous chondrite meteorites, along with the silicates and sulphides with which they are intergrown, provide a detailed record of the nature and evolution of parent body porosity and permeability, and the chemical composition, temperature and longevity of aqueous solutions. Fourteen meteorites were studied that range in petrologic subtype from mildly aqueously altered CM2.5 to completely hydrated CM2.0. All of them contain calcite, whereas aragonite occurs only in the CM2.5–CM2.2 meteorites and dolomite in the CM2.2–CM2.0. All of the aragonite crystals, and most of the calcite and dolomite grains, formed during early stages of parent body aqueous alteration by cementation of pores produced by the melting of tens of micrometre size particles of H2O-rich ice. Aragonite was the first carbonate to precipitate in the CM2.5 to CM2.2 meteorites, and grew from magnesium-rich solutions. In the least altered of these meteorites the aragonite crystals formed in clusters owing to physical restriction of aqueous fluids within the low permeability matrix. The strong correlation between the petrologic subtype of a meteorite, the abundance of its aragonite crystals and the proportion of them that have preserved crystal faces, is because aragonite was dissolved in the more altered meteorites on account of their higher permeability, and/or greater longevity of the aqueous solutions. Dolomite and breunnerite formed instead of aragonite in some of the CM2.1 and CM2.2 meteorites owing to higher parent body temperatures. The pore spaces that remained after precipitation of aragonite, dolomite and breunnerite cements were occluded by calcite. Following completion of cementation, the carbonates were partially replaced by phyllosilicates and sulphides. Calcite in the CM2.5–CM2.2 meteorites was replaced by Fe-rich serpentine and tochilinite, followed by Mg-rich serpentine. In the CM2.1 and CM2.0 meteorites dolomite, breunnerite and calcite were replaced by Fe-rich serpentine and Fe–Ni sulphide, again followed by Mg-rich serpentine. The difference between meteorites in the mineralogy of their replacive sulphides may again reflect greater temperatures in the parent body regions from where the more highly altered CMs were derived. This transition from Fe-rich to Mg-rich carbonate replacement products mirrors the chemical evolution of parent body solutions in response to consumption of Fe-rich primary minerals followed by the more resistant Mg-rich anhydrous silicates. Almost all of the CMs examined contain a second generation of calcite that formed after the sulphides and phyllosilicates and by replacement of remaining anhydrous silicates and dolomite (dedolomitization). The Ca and CO2 required for this replacive calcite is likely to have been sourced by dissolution of earlier formed carbonates, and ions may have been transported over metre-plus distances through high permeability conduits that were created by impact fracturing.

53Mn‐53Cr dating of aqueously formed carbonates in the CM2 lithology of the Sutter's Mill carbonaceous chondrite

AbstractRadiometric dating of secondary minerals can be used to constrain the timing of aqueous alteration on meteoritic parent bodies. Dolomite is a well‐documented secondary mineral in CM chondrites, and is thought to have formed by precipitation from an aqueous fluid on the CM parent body within several million years of accretion. The petrographic context of crosscutting dolomite veins indicates that aqueous alteration occurred in situ, rather than in the nebular setting. Here, we present 53Mn‐53Cr systematics for dolomite grains in Sutter's Mill section SM51‐1. The Mn‐Cr isotope data show well‐resolved excesses of 53Cr correlated with 55Mn/52Cr ratio, which we interpret as evidence for the in situ decay of radioactive 53Mn. After correcting for the relative sensitivities of Mn and Cr using a synthetic Mn‐ and Cr‐bearing calcite standard, the data yield an isochron with slope corresponding to an initial 53Mn/55Mn ratio of 3.42 ± 0.86 × 10−6. The reported error includes systematic uncertainty from the relative sensitivity factor. When calculated relative to the U‐corrected Pb‐Pb absolute age of the D'Orbigny angrite, Sutter's Mill dolomites give a formation age between 4564.8 and 4562.2 Ma (2.4–5.0 Myr after the birth of the solar system). This age is contemporaneous with previously reported ages for secondary carbonates in CM and CI chondrites. Consistent carbonate precipitation ages between the carbonaceous chondrite groups suggest that aqueous alteration was a common process during the early stages of parent body formation, probably occurring via heating from internal 26Al decay. The high‐precision isochron for Sutter's Mill dolomite indicates that late‐stage processing did not reach temperatures that were high enough to further disturb the Mn‐Cr isochron.

In situ geochemistry of Lower Paleozoic dolomites in the northwestern Tarim basin: Implications for the nature, origin, and evolution of diagenetic fluids

AbstractLower Paleozoic sedimentary rocks in the northwestern Tarim basin were strongly altered by complicated geofluids, which resulted in the occurrence of various diagenetic minerals (e.g., dolomite). Here, in situ major, trace, and rare earth element geochemistry of Lower Ordovician diagenetic dolomite grains as well as petrography were performed to unravel the geochemical features, the nature, and origin of the diagenetic fluids. The results indicate that different geochemical information can be detected within a single sample, even within a single dolomite grain. Five generations of diagenetic dolomite have been identified based on geochemical signatures, resulting from four distinct types of diagenetic fluids: (1) HREE enrichment (PAAS‐normalized), low ΣREE, no Eu anomaly, low Mn, Ba, moderate Fe, and high Sr contents are probably due to early burial dolomitizing fluids; (2) MREE enrichment, high ΣREE, high Mn, Fe, and low Sr content are likely to be associated with Devonian deep‐circulating crustal hydrothermal fluids; (3) flat or LREE enrichment pattern with obviously positive Eu anomaly is inferred to be linked to Permian magmatic hydrothermal fluids; and (4) flat REE pattern, moderate ΣREE, no Eu anomaly, low Mn, Ba, moderate Fe, and high Sr contents are probably due to late burial dolomitizing fluids. The significances of in situ method demonstrated in this study, compared with the whole rock analysis, include not only contamination‐free analysis but also unraveling the internal geochemical variation within a single sample or a mineral grain. Thus, for the geochemical study of complicated diagenetic process, in situ method should be preferentially considered.

Microstructural evolution during strain localization in dolomite aggregates

Dolomite aggregates deformed by dislocation creep over a wide range of conditions (T = 700–1000 °C, effective pressure of 900 MPa, strain rates of 10−7 – 10−4/s) strain weaken by up to 75% of the peak differential stress. Microstructural study of samples shortened to different finite strains beyond the peak differential stress shows that strain becomes highly localized within shear zones by high-temperature creep processes, with no contribution of brittle cracking. At low strains (8%), dolomite deforms homogeneously by recrystallization-accommodated dislocation creep. At progressively higher sample strains, deformation is localized into narrow shear zones made up of very fine (∼3 μm) recrystallized grains and relict porphyroclasts (20–100 μm). Finely-recrystallized dolomite grains in the shear zones are largely dislocation free and localized shear is facilitated by diffusion creep. In contrast, original dolomite grains and porphyroclasts in shear zones have high dislocation densities and do not deform after shear zone formation. Calculated strain rates in the shear zones are two to three orders of magnitude faster than the imposed bulk strain rate of the samples and these strain rates are consistent with predictions of the diffusion creep flow law for fine-grained dolomite.

Reaction kinetics of dolomite rim growth

Reaction rims of dolomite (CaMg[CO3]2) were produced by solid-state reactions at the contacts of oriented calcite (CaCO3) and magnesite (MgCO3) single crystals at 400 MPa pressure, 750–850 °C temperature, and 3–146 h annealing time to determine the reaction kinetics. The dolomite reaction rims show two different microstructural domains. Elongated palisades of dolomite grew perpendicular into the MgCO3 interface with length ranging from about 6 to 41 µm. At the same time, a 5–71 µm wide rim of equiaxed granular dolomite grew at the contact with CaCO3. Platinum markers showed that the original interface is located at the boundary between the granular and palisade-forming dolomite. In addition to dolomite, a 12–80 µm thick magnesio-calcite layer formed between the dolomite reaction rims and the calcite single crystals. All reaction products show at least an axiotactic crystallographic relationship with respect to calcite reactant, while full topotaxy to calcite prevails within the granular dolomite and magnesio-calcite. Dolomite grains frequently exhibit growth twins characterized by a rotation of 180° around one of the \([11\bar{2}0]\) equivalent axis. From mass balance considerations, it is inferred that the reaction rim of dolomite grew by counter diffusion of MgO and CaO. Assuming an Arrhenius-type temperature dependence, activation energies for diffusion of CaO and MgO are E a (CaO) = 192 ± 54 kJ/mol and E a (MgO) = 198 ± 44 kJ/mol, respectively.

Biomineralization in the Sea Hare Aplysia punctata Initiated by Nano-Dolomite

X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), high resolution transmission electron microscopy (HRTEM), environmental scanning electron microscopy (ESEM) and energy dispersive X-ray analysis (EDS) were used in study of starting biomineralization processes in embryos of the sea hare species Aplysia punctata. 10 days old embryos appeared amorphous according to XRD patterns. TEM of the same sample showed that first grains of nanocrystalline dolomite began to form in the amorphous area. The identification of dolomite was confirmed according to TEM dark field images and SAED, as well as by HRTEM. In further development stages of the embryos very faint aragonite rings became visible by SAED. It was shown that the biomineralization process in A. punctata started by formation of the dolomite nanograins which served as centres of crystallization for further aragonite deposition in the larval shell. The creation of unusual intermediate crystalline phase of nano-dolomite in A. punctata embryos is of equal interest for biologist and ecologist as an evolutionary ancestral trait of molluskan biomineralization, as well as for materials scientists, as a promising template in potential bioengineering application and design of appropriate biomimetic routes that could lead to the development of new implantable biomaterials. The discussion of the present results is based on recent knowledge on general biomineralization in mollusks.

Analysis of "standard" (Lipica) lemestone tablets and their weathering by carbonate staining and SEM imaging, a case study on the Vis island, Croatia

This paper focuses on the evolution and patterns of microscale weathering forms and dissolution rates of “standard” (Lipica) limestone tablets. Analysis of carbonate weathering using combination of methods (quantitative analysis by the weight loss of "standard" tablets, and qualitative analysis of the weathered surfaces by stained acetate peels and SEM imaging) showed that dissolution takes place not only at the surface of limestone tablets, but also along voids and cavities in limestone tablets which makes total weathering surface larger than the area of the tablet surface. Dissolution is more pronounced on the micritic calcite surfaces (due to different dissolution kinetics of carbonate minerals), resulting in lowering of the surface (calcite matrix) which causes gradual unburial and removal of authigenic dolomite grains.