Calcite Microstructures Research Articles

Thermal-induced crystallographic transformation in shells of Mytilus galloprovincialis Lamarck, 1819

Biominerals in hard parts are widely used as paleoenvironmental archives, employing proxies such as elemental and isotopic composition, microstructure, and crystallography. However, the effective selection and application of these proxies in fossil materials depends on their preservational potential. Here we evaluated the preservational potential of these proxies through controlled experiments on bivalves, one of the commonly used fossils in paleoenvironmental reconstructions. We utilized cultured Mytilus galloprovincialis specimens at five controlled temperatures to investigate the impact of elevated temperatures on various proxies extracted from both aragonitic and calcitic components of the same shell, including isotopic composition, microstructure and crystallography. Elevated temperatures first induced changes in the oxygen isotope composition (in both aragonite and calcite), followed by modification in aragonitic parts, such as changes in the aragonite phase, textures, nacre microstructure, and grain size of aragonite crystals. Despite aragonite's susceptibility to temperature, the microstructure and texture of calcite remained largely unaltered, demonstrating significantly higher resilience. Our findings emphasize the preservational potential of different shell proxies under heat exposure, ranging from oxygen isotopes to microstructure and texture. A ranking of preservation potential provides a practical guide for selecting well-preserved specimens, as textures and microstructures are not always reliable indicators. In distinguishing aragonite from calcite in fossils, our study pioneers a new avenue by proposing the analysis of high-angle boundary (HAB) content, including twin boundaries, as a reference for altered aragonite.

Calcite recrystallization and its impact on speleothem geochemistry

Speleothems are among the most important archives for past climatic and environmental change. Calcite recrystallization can modify the authigenic structure and geochemical composition of the speleothems and affect the reliability of calcite stalagmites as repositories of authigenic geochemical proxies of past climates and environments. The criteria for distinguishing primary from secondary speleothem calcite, and the conditions (open or semi-closed) of speleothem calcite recrystallization remain poorly understood. Thus, in this study, we investigated the fabric, geochemical composition, and recrystallization dynamics of a partially recrystallized calcite stalagmite (DDH-Z-2) from Didonghe Cave in Shaanxi Province, China, through petrographic observations, fluorescence microscopy, and geochemical analyses (stable isotopes, trace elements, UTh isotopes). We found that: (1) in the DDH-Z-2 stalagmite, open elongated columnar calcite recrystallized into compact elongated columnar calcite. Particulate organic matter and fulvic and humic acids were removed during recrystallization, while aromatic compounds were preserved and became incorporated into the secondary calcite; (2) calcite recrystallization was affected by multiple factors, including external fluid chemistry, primary calcite microstructure, and organic matter; (3) calcite recrystallization occurred under open, fluid-buffered conditions for alteration of the stable isotopes (δ18O and δ13C) and trace elements (Mg, Sr, U). The effect of external fluid composition on trace element (Mg, Sr) composition of secondary calcite varied across the stages of calcite recrystallization. Caution should, therefore, be exercised when using geochemical proxies in stalagmites composed of inclusion-free elongated columnar calcite: such calcite is likely to be recrystallized, and thus record the composition of reactive fluids at the time of recrystallization. Regarding the geochemical system of speleothem diagenesis, the contribution of the parent material and the sources of reactive fluids are key factors to consider.

Electron backscatter diffraction analysis unveils foraminiferal calcite microstructure and processes of diagenetic alteration

Abstract. Electron backscatter diffraction (EBSD) analysis enables a unique perspective of the internal microstructure of foraminiferal calcite. Specifically, EBSD provides crystallographic data from within the test, highlighting the highly organised “mesocrystal” structure of crystallographically aligned domains throughout the test, formed by sequential deposits of microgranular calcite. We compared EBSD maps across the test walls of both poorly preserved and well-preserved specimens of the planktonic foraminifera species Globigerinoides ruber and Morozovella crater. The EBSD maps, paired with information about intra-test distributions of Mg/Ca ratios, allowed us to examine the effects of different diagenetic processes on the foraminifera test. In poorly preserved specimens EBSD data show extensive reorganisation of the biogenic crystal microstructure, indicating differing phases of dissolution, re-precipitation and overgrowth. The specimens with the greatest degree of microstructural reorganisation also show an absence of higher concentration magnesium bands, which are typical features of well-preserved specimens. These findings provide important insights into the extent of post-depositional changes, in both microstructure and geochemical signals that must be considered when utilising foraminifera to generate proxy archive data.

Frictional strength, electrical conductivity, and microstructure of calcite–graphite mixtures sheared at a subseismic slip rate

Calcite is a common mineral found in carbonate-bearing fault zones. Crystalline graphite (Gr) can be generated within calcite-bearing fault zones, which may contribute to their low strength and high electrical conductivity. To investigate the mixed effects of Gr on calcite gouges, a dedicated assembly was employed in a rotary-shear friction experiment, which enabled simultaneous measurement of friction and electrical conductivity. The experiments were conducted on synthetic dry calcite–Gr mixtures with 0–50 wt% Gr under a fixed normal stress of approximately 2 MPa at approximately 20 °C. A constant equivalent slip rate of 1 mm/s was imposed, with displacements ranging from 0.8 to 4.6 m. To resolve the microstructure evolution, four additional displacement-controlled experiments were performed on the 6 wt% Gr-bearing specimens under the same conditions. Following high peak friction, all Gr-bearing mixtures experienced dramatic slip weakening. The steady-state electrical conductivity of the specimens increased by over eight orders of magnitude when the Gr content exceeded 6 wt%. Microstructures indicated that the calcite particles initially formed a strong framework with distributed Gr, followed by the gradual development of conductive strain-localization zones (SLZs) and Gr-lubricated slip surfaces with increasing displacement. Even the mixtures with low-Gr content (< 2.4 vol%) developed smooth slip surfaces, sustaining low-level electrical conductivities throughout the experiments. For specimens with high electrical conductivities (Gr > 5.3 vol%), abundant Gr flakes were present in the SLZs, forming an interconnected structure. Combined with our companion study on quartz-Gr mixtures, we propose that the conductivity and frictional strength trends with displacement for Gr-bearing mixtures are not always consistent. Mineral crystal properties (e.g., hardness and crystal structure) may influence the frictional properties of these mixtures significantly, while the level of electrical conductivity of a fault zone cannot be used to infer its frictional strength.

Twinned calcite as an indicator of high differential stresses and low shock pressure conditions during impact cratering

AbstractShock‐related calcite twins are characterized in calcite‐bearing metagranite cataclasites within crystalline megablocks of the Ries impact structure, Germany, as well as in cores from the FBN1973 research drilling. The calcite likely originates from pre‐impact veins within the Variscan metagranites and gneisses, while the cataclasis is due to the Miocene impact. Quartz in the metagranite components does not contain planar deformation features, indicating low shock pressures (&lt;7 GPa). Calcite, however, shows a high density (&gt;1/μm) of twins with widths &lt;100 nm. Different types of twins (e‐, f‐, and r‐twins) crosscutting each other can occur in one grain. Interaction of r‐ and f‐twins results in a‐type domains characterized by a misorientation relative to the host with a misorientation angle of 35°–40° and a misorientation axis parallel to an a‐axis. Such a‐type domains have not been recorded from deformed rocks in nature before. The high twin density and activation of different twin systems in one grain require high differential stresses (on the order of 1 GPa). Twinning of calcite at high differential stresses is consistent with deformation during impact cratering at relatively low shock pressure conditions. The twinned calcite microstructure can serve as a valuable low shock barometer.

Carbonation dynamics of hydrated alite revealed by electron microscopy

CO2 sequestration in concrete is crucial to relieve the environmental burden, while the carbonation mechanisms remain unclear. Here, we explored the carbonation dynamics of alite hydrates through electron microscopy. We discovered that calcite is the main phase of carbonate crystals throughout the entire carbonation period in the alite system. The shape evolution of calcite crystals was captured: spindle carbonates initially formed on C-S-H substrate and then transformed into rhombohedrons; Intermediate states such as polyhedral particles and layered rhomboids were also observed. Based on our quantitative calculations, the growth rate of calcite particles was determined approximately 0.2 μm/day, which may be affected by the relative concentration between calcium ions and CO2 source. A direct relation between the microstructure and mechanical properties of calcite was uncovered using atomic force microscopy. Furthermore, we found that the morphology development of calcite crystals during carbonation may be explained by the surface energy variation of different facets. This work suggests a unique approach to track carbonation kinetics and provides new opportunities to unveil underlying carbonation mechanisms at the nanoscale.

Observational and theoretical evidence for frictional-viscous flow at shallow crustal levels

Along the Hikurangi Subduction Margin, accretionary prism uplift has exposed the Hungaroa fault zone, an inactive thrust developed within the Middle to Late Eocene Wanstead Formation. Within the ~33 m-wide fault core, deformation of the smectitic, calcareous mudstone matrix produced a penetrative foliation that locally wraps around clasts. Deformation occurred at temperatures constrained by syntectonic calcite vein clumped isotope thermometry, which yielded a narrow range of Δ 47 values between 0.445 ± 0.024‰ and 0.482 ± 0.013‰, corresponding to a mean calcite precipitation temperature of 82 −12 +13 °C. Optical and scanning electron microscopy analyses reveal that calcite underwent: dissolution along stylolites and clast, vein, and microlithon margins; precipitation in foliation-parallel and foliation-perpendicular extension veins; and precipitation in hybrid veins and strain fringes. Maximum differential stress estimates obtained from calcite twin densities (44.1 ± 13.9 to 96.6 ± 20.8 MPa) are consistent with those sustainable by a cohesionless fault at ~3 km depth with a friction coefficient in the range measured for two calcareous mudstones (μ = 0.38 to 0.50) and a micrite clast (μ = 0.61 and 0.64). Marlstone clasts within the foliated calcareous mudstone matrix contain mutually cross-cutting shear fractures and extension veins with crack-seal textures, providing evidence for temporal fluctuations in shear strength resulting from pore fluid overpressure transients. At strain rates imposed during laboratory experiments, frictional sliding involves granular flow processes. Yet, calcite microstructures indicate that diffusive mass transfer played an important role in accommodating deformation. We model the fault zone rheology assuming diffusion-controlled frictional-viscous flow, with deformation at strain rates γ ˙ ≤ 10 −9 s −1 able to have taken place at very low shear stresses ( τ < 10 MPa) given sufficiently short diffusion distances ( d < 0.1 mm), even in the absence of pore fluid overpressures. However, if grain-scale and fracture-scale processes change the diffusion distance, fault zones deforming via frictional-viscous flow can exhibit temporally variable strain rates. Thus, our results suggest that the shallow (up-dip) limit of the seismogenic zone is not a simple function of temperature in fault zones governed by a frictional-viscous flow rheology. • Displacement occurred via distributed deformation across a 33 m-wide fault core. • Fault core rocks experienced spatiotemporal variations in stress and strength. • Frictional-viscous flow can explain the weakness of shallow crustal thrust faults. • Frictional-viscous flow can occur at temporally variable strain rates.

Development and passive exhumation of high-pressure shear zones (Blueschist Unit, Syros): Insights from quartz and columnar calcite microstructures

Quartz and columnar calcite microstructures in two major high-pressure (HP) shear zones (Syros Island, Greece), which carry meta-igneous rocks over calcite marbles, were studied to investigate the deformation conditions associated with their development and exhumation. Quartz-rich bands within the hanging-wall of the shear zones record the deformation during the operation of the shear zones under HP conditions at slightly decreasing temperatures (from 500 °C to 450 °C). At HP conditions, the shear zones are associated with (E)SE-directed shearing under plane strain conditions, as well as with a temporal increase in strain rate of one order of magnitude (from ∼10−12 to ∼10−11). Well-preserved columnar calcite within marbles of the footwall indicates that ductile shearing along the shear zones should have abruptly ceased during the early stages of exhumation, passing to a period of relative quiescence within the stability field of aragonite at temperatures ∼430–450 °C. Progressive exhumation and cooling below 400 °C are associated with weak and distributed (E)NE-directed shearing with strain rate slower than ∼10−14. Our results from north(east) Syros indicate a change in transport direction from (W)NW-(E)SE at the deep subduction levels to (W)SW-(E)NE during exhumation. This change is linked to a change in deformation, from localized to weak and distributed revealing that the Blueschist Unit in north(east) Syros was exhumed as a nearly rigid body.

Multi-isotopic and trace element evidence against different formation pathways for oyster microstructures

Shells of oysters (Ostreidae) are predominantly composed of foliated and chalky calcite microstructures. The formation process of the more porous chalky structure is subject to debate, with some studies suggesting that it is not formed directly by the oyster but rather through microbial mineralization within the shell. Here, this hypothesis is tested in modern shells of the Pacific oyster (Crassostrea gigas) from coastal regions in France and the Netherlands. We combine measurements of stable carbon, oxygen, nitrogen, sulfur, and clumped isotope ratios with high-resolution spatially resolved element (Na, Mg, Cl, S, Mn and Sr) data and microscopic observations of chalky and foliated microstructures in the oyster shells. Our results show no isotopic differences between the different microstructures, arguing against formation of the chalky calcite by microorganisms. However, we observe a small difference in the oxygen isotope ratio (0.32‰) and clumped isotope composition (0.017‰) between the microstructures, which is likely caused by sampling biases due to seasonal differences in growth rate and the short timespan over which the chalky microstructure forms. We therefore recommend sampling profiles through the foliated microstructure to control for strong seasonal variability recorded in the shell which can bias environmental reconstructions. High-resolution (25–50 µm) Na, Mg, Cl, S, Mn and Sr profiles yield empirical distribution coefficients between seawater and shell calcite for these elements. Significant differences in element concentrations and distribution coefficients were confirmed between the two microstructures, likely reflecting differences in mineralization rates or inclusion of non-lattice-bound elements. Only Mg/Ca ratios in the foliated microstructure vary predictably with growth seasonality, and we show that these can be used to establish accurate oyster shell chronologies. The observed effect of mineralization rate on element incorporation into oyster shells should be considered while developing potential element proxies for paleoclimate reconstructions. Trace element proxies in oyster shells should be interpreted with caution, especially when element chemical properties were measured in different microstructures.

Depositional environment and tectonic backdrop of meta-carbonates in the Eastern Himalayan ophiolites, India: insights from calcite microstructures, whole-rock elements and stable isotopes

Depositional environment and tectonic backdrop of meta-carbonates in the Eastern Himalayan ophiolites, India: insights from calcite microstructures, whole-rock elements and stable isotopes

Calcite microstructure, geochemistry and chronology within the fault zone reveal the paleoseismic records of the Yadong rift, southern Tibet

断裂带中的方解石脉作为流体的直接记录，保存了丰富的断层活动信息。亚东裂谷是藏南地区南北向裂谷系中一条具有区域规模的大型裂谷，自北向南分别为热隆、涅如堆以及帕里次级裂谷。本文在涅如堆地堑南、北部和帕里地堑北部裂谷断层选取三个采样点，对沿断层发育的方解石脉进行研究，揭示了脉体与地震活动的关系、流体来源以及古地震发生时间。显微结构观察显示，方解石脉主要具有块状和拉长块状结构，分别代表了同震快速沉淀以及震后的同构造期沉淀。涅如堆地堑北部方解石脉具有Eu元素正异常以及高⁸⁷Sr/⁸⁶Sr比值的热液特征，异常轻的碳氧同位素值显示出地幔流体和大气水贡献。涅如堆地堑南部和帕里地堑北部方解石脉具有Ce和Eu负异常以及Y正异常的低温特征，与区域内灰岩相近的δ¹³C和⁸⁷Sr/⁸⁶Sr值反映了围岩来源，偏负的δ¹⁸O表明同样存在大气水贡献。方解石的地球化学特征反映了裂谷断层切割深度大，在断层活动期间能够沟通不同深度域的流体。断层亚东裂谷涅如堆段方解石脉铀系测年结果为距今679±129ka，帕里段北部结果为距今544±26ka、444±15ka和408±13ka，由于以上样品均具有同震或同构造期的内部结构特征，因此测年结果代表了地震活动时间。结合涅如堆段发育的断层三角面高差，计算出裂谷断层中更新世以来的E-W向伸展速率为0.17~0.24mm/yr，小于晚更新世以来0.8±0.3mm/yr的伸展速率，以及GPS观测的伸展速率6mm/yr。即中更新世以来，亚东裂谷涅如堆段断层活动速率逐渐增大。该地区断裂带中广泛发育的方解石脉能够为基岩区的断层活动和古地震研究提供宝贵的资料。;Calcite veins within fault zones, the direct records of fluids circulation, preserve a wealth of faulting activity information. The Yadong rift is a regional-scale structure in southern Tibet, part of the series of NS-trending South Tibetan rifts. From north to south, the Yadong rift is bounded to the east by the Relong, Nierudui, and Pagri normal faults. In this paper, we study the calcite veins from three sites along the Nierudui and Pagri faults in order to decipher their relationships with the seismicity, fluid source and paleoearthquake timing in this area. Microstructure observations show that calcite veins have blocky and elongated blocky textures, which can represent co-seismic rapid precipitation and post-seismic syntectonic precipitation, respectively. Calcite veins in the northern Nierudui graben show a positive Eu element anomaly and a high ⁸⁷Sr/⁸⁶Sr value. The abnormally negative δ¹³C and δ¹⁸O values show the contribution of mantle fluid and meteoric water, respectively. Calcite veins in the southern Nierudui graben and northern Pagri graben show negative Ce and Eu, and positive Y, which may indicate a low-temperature tectonic setting. δ¹³C and ⁸⁷Sr/⁸⁶Sr values similar to that of the regional limestone indicates the source of the host rock. The geochemical characteristics of the calcite veins may imply that the fault cuts deep across the crust, which facilitates fluids circulation at different depths during faulting activity. Results from uranium-series dating of calcite veins are 679±129ka in Nierudui, and 544±26ka, 444±15ka, and 408±13ka in the northern Pagri. Based on the vein microstructures, we suggest that the veins are coseismic or syntectonic structures, and as a result, U-Th dating results thus represent the timing of seismic activity. Combined with vertical offsets along the fault in Nierudui, we calculate an E-W extension rate of 0.17~0.24mm/yr since ~700ka, which is significantly smaller than the 0.8±0.3mm/yr since ~20ka, and the 6mm/yr observed by GPS. Therefore, we suggest that since the Middle Pelistocene, the rate of faulting activity along the Nierudui graben of the Yadong rift has gradually increased. The widespread calcite veins within the fault zones can provide valuable data for the study of faulting activity and paleoearthquakes timing in bedrock.

Hard to Soft: Biogenic Absorbent Sponge-like Material from Waste Mussel Shells

Summary Mussel shells are a source of renewable CaCO3 and can be obtained from aquaculture waste streams. Valorization of waste shells relieves strain on landfills and aligns with the goals of a circular economy. Here, we present the preparation of an absorbent CaCO3 material, soft calcite (SC), prepared from waste blue mussel shells under mild conditions. Calcite is the only polymorph present in SC, despite the presence of aragonite in the shell. SC has a nest-like morphology, unlike the highly ordered calcite in the shell. SC is formed by the reassembly of calcite prisms and may be held together by the organic matrix “glue” from the original shell as evidenced by NMR spectroscopy. This inorganic sponge adsorbs dyes from aqueous solution and absorbs crude oil from seawater with good recyclability. Our results demonstrate how a natural structure can be modified in a sustainable fashion to yield a higher-value material.

Surfactin as a Green Agent Controlling the Growth of Porous Calcite Microstructures.

This study presents a new, simple way to obtain mesoporous calcite structures via a green method using an eco-friendly surface-active compound, surfactin, as a controlling agent. The effects of synthesis time and surfactin concentration were investigated. The obtained structures were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) coupled with gas mass spectrometry (QMS) analysis. The experimental data showed that surfactin molecules significantly changed the morphology of the calcite crystals, roughening and deforming the surface and creating a greater specific surface area, even at low biosurfactant concentrations (10 ppm). The size of the crystals was reduced, and the zeta potential value of calcium carbonate was more negative when more biosurfactant was added. The XRD data revealed that the biomolecules were incorporated into the crystals and slowed the transformation of vaterite into calcite. It has been shown that as long as vaterite is present in the medium, the calcite surface will be less deformed. The strong influence of surfactin molecules on the crystal growth of calcium carbonate was due to the interaction of surfactin molecules with free calcium ions in the solution as well as the biomolecules adsorption at the formed crystal surface. The role of micelles in crystal growth was examined, and the mechanism of mesoporous calcium carbonate formation was presented.

Comparison on accelerated carbonation of β-C2S, Ca(OH)2, and C4AF: Reaction degree, multi-properties, and products

In-depth understanding regarding the carbonation properties of pure minerals including β-dicalcium silicate (β-C2S), calcium hydroxide (CH), and tetracalcium aluminoferrite (C4AF) was conducted to explore the types, microstructure, crystallinity of products produced during carbonation, thereby developing a tentative microscopic mechanism of the contribution of carbonation to macroscopic properties. An investigation of changes in pH value and Ca leaching of the pure minerals was conducted to explore the effect of dissolution properties to the carbonation. Results showed that the descending order of the ultimate carbonation degrees of minerals was: (i) CH, (ii) β-C2S, (iii) C4AF. The carbonation degrees and rates were not only controlled by the dissolution properties of the minerals, but also the distribution of the produced CaCO3. The order of the contribution of minerals induced by carbonation to the macroscopic mechanical strength was, in descending order: (i) β-C2S, (ii) CH, (iii) C4AF. This difference was the combined effects of the microstructure, crystallinity, and particle size of calcite. Effects of the compact stack and the strong mechanical bond between the calcite particles were observed in carbonated β-C2S samples; In contrast, pronounced defects on the calcite particles were observed in carbonated C4AF samples. Carbonation is an effective way to reduce the density of which mainly contained phases of β-C2S and C4AF, and to store greenhouse gas CO2 permanently. These findings are meaningful for the further in-depth researches of the carbonation mechanism of materials which primarily contain β-C2S, CH and C4AF phases.

Comparative roles between aragonite and calcite calcium carbonate whiskers in the hydration and strength of cement paste

Aragonite CaCO3 whisker (CW) could improve the mechanical properties of cementitious composites. The microstructure of calcite is different from that of aragonite, but only a few studies reported different effects in cementitious composites between incorporating aragonite and calcite. In order to fully use CW as high-performance, low-cost microfiber and clearly understand different mechanisms between aragonite and calcite CW, in this study, the heat treatment method was used to convert aragonite CW to calcite CW, and the effect of calcite and aragonite on the hydration and strength of CW reinforced cement paste (CWRC) was studied to bridge the gaps in this area. The aragonite and calcite CWs exhibited the overall physical dilution effect in Portland cement, significantly decreasing the hydration heat of cement and the total amount of non-evaporable water and Ca(OH)2. Calcite CW presents a more significant nucleation effect than aragonite CW. Compared to aragonite CWRC, the hydration of silicon phase in calcite CWRC is faster and the hydration heat release is higher; however, there is no significant difference in the hydration of aluminum phase. The Ca(OH)2 content in calcite CWRC was more than that in aragonite CWRC. More hydration products formed on the surface of calcite CW than that on aragonite CW, improving the bonding strength between calcite CW and cement matrix. Hence, both the compressive strength and flexural strength of calcite CWRC are higher than those of aragonite CWRC. The rheological test of fresh aragonite and calcite CWRC proved that both aragonite and calcite CWs significantly decreased the flow of cement paste, presenting viscosity effect. Compared to aragonite CW, calcite CW improved the flowability of fresh cement paste, indicating the lubricant effect of calcite CW. Aragonite and calcite CW present chemical effect similar to limestone powders, i.e., aluminum phase in Portland cement interacted with CO32- dissolved from CW to form mono- and semi- carbonate. The Ca(OH)2 orientation indexes of cement decreased by the introduction of CWs. Aragonite and calcite CWs can also be used as microfiber and filler in cementitious composites. These effects improved the micro-structure, thus enhanced the flexural and compressive strength of cement paste.

An assessment of latest Cretaceous &amp;lt;i&amp;gt;Pycnodonte vesicularis&amp;lt;/i&amp;gt; (Lamarck, 1806) shells as records for palaeoseasonality: a multi-proxy investigation

Abstract. In order to assess the potential of the honeycomb oyster Pycnodonte vesicularis for the reconstruction of palaeoseasonality, several specimens recovered from late Maastrichtian strata in the Neuquén Basin (Argentina) were subject to a multi-proxy investigation, involving scanning techniques and trace element and isotopic analysis. Combined CT scanning and light microscopy reveals two calcite microstructures in P. vesicularis shells (vesicular and foliated calcite). Micro-XRF analysis and cathodoluminescence microscopy show that reducing pore fluids were able to migrate through the vesicular portions of the shells (aided by bore holes) and cause recrystallization of the vesicular calcite. This renders the vesicular portions not suitable for palaeoenvironmental reconstruction. In contrast, stable isotope and trace element compositions show that the original chemical composition of the foliated calcite is well-preserved and can be used for the reconstruction of palaeoenvironmental conditions. Stable oxygen and clumped isotope thermometry on carbonate from the dense hinge of the shell yield sea water temperatures of 11°C, while previous TEX86H palaeothermometry yielded much higher temperatures. The difference is ascribed to seasonal bias in the growth of P. vesicularis, causing warm seasons to be underrepresented from the record, while TEX86H palaeothermometry seems to be biased towards warmer surface water temperatures. The multi-proxy approach employed here enables us to differentiate between well-preserved and diagenetically altered portions of the shells and provides an improved methodology for reconstructing palaeoenvironmental conditions in deep time. While establishing a chronology for these shells was complicated by growth cessations and diagenesis, cyclicity in trace elements and stable isotopes allowed for a tentative interpretation of the seasonal cycle in late Maastrichtian palaeoenvironment of the Neuquén Basin. Attempts to independently verify the seasonality in sea water temperature by Mg ∕ Ca ratios of shell calcite are hampered by significant uncertainty due to the lack of proper transfer functions for pycnodontein oysters. Future studies of fossil ostreid bivalves should target dense, foliated calcite rather than sampling bulk or vesicular calcite. Successful application of clumped isotope thermometry on fossil bivalve calcite in this study indicates that temperature seasonality in fossil ostreid bivalves may be constrained by the sequential analysis of well-preserved foliated calcite samples using this method.

Microstructure of calcite deformed by high-pressure torsion: An X-ray line profile study

Calcite aggregates were deformed to high strain using high-pressure torsion and applying confining pressures of 1–6GPa and temperatures between room temperature and 450°C. The run products were characterized by X-ray diffraction, and key microstructural parameters were extracted employing X-ray line profile analysis. The dominant slip system was determined as r{101¯4}⟨2¯021⟩ with edge dislocation character. The resulting dislocation density and the size of the coherently scattering domains (CSD) exhibit a systematic dependence on the P-T conditions of deformation. While high pressure generally impedes recovery through reducing point defect mobility, the picture is complicated by pressure-induced phase transformations in the CaCO3 system. Transition from the calcite stability field to those of the high-pressure polymorphs CaCO3-II, CaCO3-III and CaCO3-IIIb leads to a change of the microstructural evolution with deformation. At 450°C and pressures within the calcite stability field, dislocation densities and CSD sizes saturate at shear strains exceeding 10 in agreement with earlier studies at lower pressures. In the stability field of CaCO3-II, the dislocation density exhibits a more complex behavior. Furthermore, at a given strain and strain rate, the dislocation density increases and the CSD size decreases with increasing pressure within the stability fields of either calcite or of the high-pressure polymorphs. There is, however, a jump from high dislocation densities and small CSDs in the upper pressure region of the calcite stability field to lower dislocation densities and larger CSDs in the low-pressure region of the CaCO3-II stability field. This jump is more pronounced at higher temperatures and less so at room temperature. The pressure influence on the deformation-induced evolution of dislocation densities implies that pressure variations may change the rheology of carbonate rocks. In particular, a weakening is expected to occur at the transition from the calcite to the CaCO3-II stability field, if aragonite does not form.

Effects of limestone petrography and calcite microstructure on OPC clinker raw meals burnability

Limestone represents the main raw material for ordinary Portland cement clinker production. In this study eight natural limestones from different geological environments were chosen to prepare raw meals for clinker manufacturing, aiming to define a parameter controlling the burnability. First, limestones were characterized by X-Ray Fluorescence, X-Ray Powder Diffraction and Optical Microscopy to assess their suitability for clinker production and their petrographic features. The average domains size and the microstrain of calcite were also determined by X-Ray Powder Diffraction line profile analysis. Then, each limestone was admixed with clay minerals to achieve the adequate chemical composition for clinker production. Raw meals were thermally threated at seven different temperatures, from 1000 to 1450 °C, to evaluate their behaviour on heating by ex situ X-Ray Powder Diffraction and to observe the final clinker morphology by Scanning Electron Microscopy. Results indicate the calcite microstrain is a reliable parameter to predict the burnability of the raw meals, in terms of calcium silicates growth and lime consumption. In particular, mixtures prepared starting from high-strained calcite exhibit a better burnability. Later, when the melt appears this correlation vanishes; however differences in the early burnability still reflect on the final clinker composition and texture.

Functionalization of biomineral reinforcement in crustacean cuticle: Calcite orientation in the partes incisivae of the mandibles of Porcellio scaber and the supralittoral species Tylos europaeus (Oniscidea, Isopoda).

In arthropods the cuticle forms an exoskeleton with its physical and chemical properties adapted to functions of distinct skeletal elements. The cuticle of the partes incisivae (PI) in mandibles of terrestrial isopods is a composite of chitin-protein fibrils/fibres and minerals. It consists of an unmineralized tip, a middle region with organic fibrils reinforced mainly with amorphous calcium phosphate and a base region mineralized with amorphous calcium carbonate and calcite. In this study we extend our work on the structure and material properties of the incisive cuticle employing electron backscatter diffraction (EBSD), and investigate calcite orientation patterns in the PI of two terrestrial isopod species from different habitats. We trace small-scale differences in texture sharpness and calcite microstructure, and compare calcite organization and orientation patterns in the PI with those in the tergites of the same isopod species. We observe that in the PI calcite orientation, the degree of crystal alignment, and mode of crystalline domain assemblage is highly varied within short length scales. This contrasts to calcite organization in the tergite cuticle, where calcite has only one specific texture pattern. Such a large range in the variation of calcite organization has not been observed in other carbonate biological hard tissues, such as shells and teeth, where one specific texture and microstructure prevails. Thus, the investigated isopod species are able to control crystallization of the amorphous carbonate precursor in a differential way, most probably related to the function of the individual skeletal element and the animals' behavior.

Crystallography of calcite in pearls

Mineralization in pearls is used as a ‘study model’ to understand basic mechanisms of calcium carbonate biomineralization. In addition to nacre formation, vaterite and calcite can be found in pearls, with the occurrence of calcite being the least studied. In this study, two calcite microstructures, prisms and “pseudo-prisms”, are described. Electron backscatter diffraction (EBSD) analyses show that there is no unique crystallographic orientation for a single calcite microstructure and, instead, each is composed of microcrystals with different orientations. Furthermore, general crystallographic orientations are found for prisms, with c-axis parallel to the long axis, and “pseudo-prisms” with the c-axis perpendicular to their elongation. Pseudo-prisms tend to be grouped in crystallographic domains. Overall, the formation of calcite structures in pearls may not be compared to biomineral structures in shells, which are better defined in terms of crystal habit and present a more constrained crystallographic control, in addition to having a strong correlation between crystallization and the formation of organic envelopes.