Calcite Microcrystals Research Articles

The implications of laboratory 222Rn flux measurements to the radioactivity in groundwaters: the case of a karstic limestone aquifer

Laboratory time-scale experiments were conducted on Carboniferous Limestone gravels from the Mendip Hills area, England, with the purpose of evaluating the release of 222Rn to the water phase. The specific surface areas of the samples were 4.14 and 1.69 cm 2 g −1 , which provided, respectively, values of 50.6 and 12.7 pCi for the released Rn. These results allowed the calculation of the emanation coefficient of this rock matrix with respect to the release of Rn, where completely different values corresponding to 23% and 6% were found, suggesting that the extent to which grain boundaries or imperfections in aggregates of micro-crystals of calcite intersect the particle surface certainly affects the Rn release. They also permitted the evaluation of models for the generation of Rn in rocks and transfer to water, in order to interpret the radioactivity due to this gas in groundwaters from the karstic aquifer of the Mendip Hills area, where the calculated activities in groundwater based on the values of 23% and 6% for the emanation coefficient were about 51 and 15 times higher than actually measured in groundwater. Therefore, the emanation coefficient in nature is considerably smaller than in the lab experiment, and another factor k (0 < k < 1) may be introduced into the equations related to the modelling, with the aim of adjusting the theoretical-practical results.

Application of recording and processing of energy‐filtered image sequences for the elemental mapping of biological specimens: Imaging‐Spectrum

SummaryComputerized energy‐filtered transmission electron microscope (EFTEM) permits the recording and the processing of energy‐filtered images, allowing a part of an electron energy‐loss spectrum for each picture element to be obtained. This method, called ‘Imaging‐Spectrum’, uses a Zeiss CEM902 coupled to several image analysis systems. The actual configuration records sequences of 48 images, 256 × 256 pixels, in steps of the energy loss, ΔE. Processing these sequences results in part of a core‐loss EELS‐spectrum for each pixel. This approach produces elemental maps with a short processing time. We have implemented three kinds of background calculation for the image subtraction. The influence of the irradiation dose and of the energy selecting slit width on the quality of the spectra is investigated. The method is applied to the analysis of some biological specimens (pericellular coat behaviour during adhesion between macrophages and red blood cells and location of calcite microcrystals in dental pulp cells). The Imaging‐Spectrum method appears to be suitable for the analysis of large areas.

CHEMICAL AND MINERALOGICAL CHARACTERIZATION OF POTTERY FROM MANISES (VALENCIA, SPAIN)

Mediaeval Manises pottery has been very much studied [1] and there are many,articles about this important product made in Spain and exported to·a great number of countries in Europe [2, 3], but there is almost no published information available to help formulate a suitable approach to analysis, conservation and restoration. This type of med.iaeval pottery appeared after the Christian conquest of Valencia in 1238, in villages near the city. It took its inspiration from Islamic pottery of the eleventh century, but there is no direct derivation [4]. The products of Valencia ' s mud6jar clay are green/mulberry, blue and lustreware. Our study is centered on the blue ceramic, found in Manises, because it is the least known as a link between the other two. This type forms the largest group and has a long history: its production has continued unbroken since the fourteenth century. The present communication shows the preliminary results obtained from sev~ral pieces of pottery . Although the purpose of this work is the chemical and mineralogical characterization of the pottery, the study includes a morphological examination which provides information about the problems associated with both the consolidation of the glaze and··the adhesion of the glaze to the pottery surface. A small fragment (a few mm2) was detached from the surface of each piece of pottery, using a scalpel. The fragment, including both glaze and ceramic body, was embedded in cold-setting epoxy resin, cured a~d polished in order to obtain a cross-section. Finally the samples were sputter-coated with carbon for scanning electron microscopy. X-ray microprobe analysis was carried out in a scanning ISI-DS 130 Kevex microscope fitted with a Kevex (EDAX) energy-dispersive X-ray spectrometer, at 20-30kV with a counting time between 110 and 300 seconds. X-ray diffraction is the most efficient method for petrological characterization of mineral species. A small quantity (a few mg) of the body and glaze was ground with an agate pestle and mortar and mounted in the diffractometer chamber. X-ray diffraction analysis was carried out in a D-500 Siemens X-ray diffractometer operating at 40kV and 30mA. The pieces studied include each of the different typologies of Manises pottery, based on stylistic criteria related to vessel form, colour and decoration. All the pieces studied have a white ceramic glaze applied to the pottery surface and decorated with cobalt blue pigment. .A first examination with light microscopy and scanning electron microscopy provided information about the morphology of both body and glaz~. The body showed a fine silty clay matrix with high porosity. Several microscopical irregularly shaped grains of filler (chamotte) could be seen and a few microcrystals of clay minerals quartz, calcite and haematite were also identified. .An irregular glaze-body interface was revealed. A higher concentration of metals responsible for the colour in the interface region of the glaze was measured by means of electron probe microanalysis; this result suggests that an 'under-glaze' technique could have. been used. By applying the combined SEM/EDAX technique the composition of eight major and minor elements could be determined: The mean values of concentration expressed as oxides were: Si02 54.6%, Al203 18.0%, K20 3.14%, Ti02 0.61%, Fe2036.10%, CaO 15.6%, MgO ~.85% and MnO <0.08%. The samples analyzed were all similar in composition. This suggests that these pieces were made with raw materials from the same local source. X-ray diffraction analysis identified kaolinitesmectite, montmorillonite and vermiculite as the mineral clays that form the ceramic body and plagioclase (anorthite), calcite, haematite and quartz as accessory minerals. The X-ray diffraction results obtained from the glaze were not very co~clusive but confirmed the microanalytical results. The presence in the body of calcite (which is decomposed to calcium oxide and carbon dioxide at 880°C), as well as the presence of quartz and lead oxide as major components· of the glaze, allowed us to estimate the firing temperature employed in the manufacture of these ceramics as not higher than 950-1059°C. On the other hand, the presence ofhaematite in the body indicated that an oxidation firing process was involved. An adequate supply of air was probably supplied by open kilns, influenced by the preceding Islamic peri~d [5]. The authors wish to acknowledge the helpful contributions of D. Jose Perez Camps, director of the Manises Pottery Museum. Authors' address: Departamento de Conservacion y Restauracion de Bierzes.Culturales, Facultad de Bellas Artes, Universidad Politecnica de Valencia, Camino de Vera ~4, 46022 Valencia, ~am. .

Progressive Changes in the Structure of Hardened C3S Cement Pastes due to Carbonation

The structures of partially carbonated hardened C3S cement pastes have been investigated by a combination of 29Si magic angle spinning nuclear magnetic resonance spectroscopy and analytical transmission electron microscopy, supported by X‐ray diffraction and thermogravimetric analysis. Progressive changes in structure are reported for thin slices for a paste carbonated in pure CO2 for times from 1 to 16 h, and the results are compared with those for a paste carbonated for 2 months in air. C‐S‐H gel of reduced Ca:Si ratio and increased silicate polymerization was formed during the early stages of carbonation. The morphology of the original C‐S‐H was, in the main, retained. A cross‐linked silica‐rich gel formed at later times in paste carbonated in CO2 but not up to the time of 2 months in air. Calcium carbonate took the form of microcrystals of vaterite and calcite which formed dense masses between gel fibrils and around partially reacted CH crystals, possibly accounting for the observed slowing in the rate of reaction of CH with time.

Odontoblast-like cytodifferentiation of human dental pulp cells in vitro in the presence of a calcium hydroxide-containing cement

The cement produced microcrystals of calcite by reaction with culture medium supplemented with calf serum. Human dental pulp cells seeded on such a substrate preferentially adhered and aggregated around the microcrystals. Immunofluorescence and immunogold labelling revealed a high affinity of serum fibronectin molecules for the calcite crystals. At 4 weeks in culture, the cells had various features of differentiated odontoblasts, notably nuclear polarization, typical appearance of the Golgi apparatus, synthesis of type I collagen and absence of type III, and apical accumulation of actin and vimentin. These cells also elaborated a collagenous extracellular matrix which did not mineralize.

Meteoric diagenesis, marine diagenesis, and microporosity in Pleistocene and Oligocene limestones, Enewetak Atoll, Marshall Islands

Microporosity occurs in Pleistocene and Oligocene limestones cored on Enewetak Atoll. In cores of Pleistocene limestone (10–80 m deep), mi microporosity is present in aragonitic skeletal grains that underwent partial intrafabric dissolution (chalkification) leaving a porous meshwork of aragonite needles. Microporous aragonite is most abundant in 5–10 m thick intervals below zones of intense diagenesis associated with paleo-freshwater lenses. Extensive microporosity was apparently generated by slow intrafabric dissolution of aragonite in mixing zones below freshwater lenses. Microporous aragonite is an intermediate step in the calcitization or dissolution of aragonite. Most calcitized aragonite (neomorphic calcite) formed by precipitation of sparry calcite in (over) microporous aragonite. Aragonitic fossils were not converted to microporous, microcrystalline calcite. Therefore, data from the Pleistocene of Enewetak suggest that widespread, microporous, microcrystalline calcite does not form directly from pure aragonite, at least not in open meteoric systems or mixing zones. Microporous micrite occurs in Oligocene wackestones 930 m deep deposited in a slope environment. The microporous micrite is characterized by equant and prismatic crystals of low-magnesium calcite (LMC). Equant crystals are anhedral to euhedral and 2–10 μm in diameter. Prismatic calcite crystals are subhedral, 5–10 μm wide, and 20–50 μm long. Stable isotopic and trace element geochemistry suggest that microporous micrite formed by diagenetic alteration of high-magnesium calcite (HMC) micrite in deep seawater undersaturated with respect to HMC but supersaturated with respect to LMC (probably near the sediment-water interface). Mineralogy and crystal morphology of the microporous aragonite is very different from microporous shelf limestones in the Thamama Group (Cretaceous, Persian Gulf; see Moshier, 1989). Equant microcrystals of calcite in the Oligocene micrites are similar to those in microporous parts of the Thamama Group. Widespread microporosity in LMC limestones could form by alteration of HMC micrite in mixing zones or in seawater undersaturated with respect to HMC but supersaturated with respect to LMC.

Calcified filaments - an example of biological influences in the formation of calcrete in South Australia

Scanning electron microscope (SEM) studies of calcareous soils and calcretes from South Australia reveal a fossilized community of soil micro-organisms dominated by filamentous structures preserved in fine detail by calcite. In the various calcrete lithological facies, the filaments form dense mats within channels and voids, and also occur within the matrix where they are intimately associated with micrite. The calcite forming the filaments has a variety of crystal habits: the nature of the microcrystals is specific to each filament but varies significantly between adjacent filaments. In the calcareous soils there are various stages between the primary filaments and the calcite encrusted structures characteristic of the calcretes, suggesting that in vivo biochemical processes dominate the mechanisms of calcification. This hypothesis is supported by the specificity of the habit of calcite microcrystals on each filament. It is suggested that the organisms deposit calcite microcrystals within the mucilaginous sheath or in the cell wall (or both) as a detoxification mechanism in response to their highly calcareous environment. Based on the identification of structures resembling fruiting bodies, at least some of the filaments appear to have been fungal hyphae, which are known to be responsible for stabilizing macroaggregates in soils. Calcified filaments may produce permanently stabilized macroaggregates which provide the locus for further carbonate precipitation, leading to eventual induration of the soil.

Ikaite pseudomorphs in the Zaire deep-sea fan: An intermediate between calcite and porous calcite

Translucent brown aggregates of calcium-carbonate crystals have been found in cores from the Zaire deep-sea fan (west equatorial Africa). The aggregates are well preserved but very friable. Upon storage they become yellowish white and cloudy and release water. Chemical, mineralogical (XRD), petrographical, crystal-morphological, and stable-isotope data demonstrate that the crystals have passed through three phases: (1) an authigenic carbonate phase, probably calcium carbonate, which is represented by the external habit of the present crystals; (2) a translucent brown ikaite phase (CaCO 3 ·6H 2 O), unstable at temperatures above 5 °C; and (3) a phase consisting of calcite microcrystals that are poorly cemented and form a porous mass within the crystal form of the morphologically unchanged first phase. The transformation from the first phase into ikaite was probably a kinetic replacement. The transformation from ikaite into the third phase occurred because of storage at room temperature. The presence of ikaite is indicative of a low-temperature, anaerobic, organic-carbon-rich marine environment. Ikaite is probably the precursor of a great number of porous calcite pseudomorphs, and possibly also of many marine authigenic microcrystalline carbonate nodules.

Transmission Electron Microscopy and Microanalytical Studies of Ion‐Beam‐Thinned Sections of Tricalcium Silicate Paste

Mature tricalcium silicate cement pastes were examined in ion‐beam‐thinned sections. The microstructure consisted of very homogeneous amorphous hydrate gel surrounding residual unhydrated cores of tricalcium silicate, these regions being linked by a fibrillar outer hydration product. Carbonation had produced microcrystals of calcite within the outer product only. X‐ray microanalysis gave Ca: Si ratios for the hydrate regions which showed considerable deviations but were‐on average in fairly good agreement with indirectly determined Ca: Si ratios of pastes of the same material. There was no significant difference in composition between inner and outer hydration products.

Morphologie et cristallogenèse de microcristaux supergènes de calcite en aiguilles

The morphological study by scanning electron microscopy (S.E.M.) and crystallographical characterization by transmission electron microscopy (T.E.M.) of needle-shaped calcite microcrystals allowed to put in evidence a jagged needle habit that could correspond to a rapid and differentiated crystal growth, followed by dissolution phenomena.

Encroûtements calcaires dans les altérations des marnes éocènes de la falaise de Thiès (Sénégal). Organisation morphologique et minéralogie

The calcareous duricrusts of the weathering zone of Eocene marls from the cliff of Thiès (Senegal) were subject to a very detailed study by macromorphological, micromorphological (microscope and stereoscan) and mineralogical means. The authors show on field that vertical and lateral sequences of calcareous accumulations are similar to those described in mediterranean countries. However, the detailed study of the mechanism of calcareous accumulation (calcitization) emphasizes that the latter takes place principally due to the development of calcite microcrystals (crystalliplasma). This calcitization is maximum to the top of profiles and to the lowest part of sequences ; this accumulation is accompanied by an epigenesis of quartz, dolomite and clay by calcite. To this geochemical environment of calcitization is opposed a decalcitization environment at the bottom of profiles and at the upper part of sequences. This decalcitization at the bottom of profiles is mild and accompanied by neoformation of quartz, dolomite and attapulgite. In these two environments, there are inverse and reversible epigeneses between calcite on the one hand, and the other constituents on the other hand. Decalcitization and calcitization are related to structures ranging from plane to concentric, then massive.

Spiral Growth of Calcitostracum

SPIRAL growth hills built up of many tabular calcite microcrystals can be seen in definite regions of the growing surface of the left shell valve of the calcitostracum of Anomia lischkei under a reflexion microscope (Fig. 1). The spiral growth hill of calcitostracum contrasts interestingly with that of nacre1; the former is composed of many microcrystals of calcite distributed radially outwards2, whereas the latter is formed by many microcrystals of aragonite in nearly parallel orientation3,4 as Laue photographs and microscopes show.

The crystalline structure of calcium carbonate in the avian egg shell: An electron microscope study

The structure of the crystalline portion of the avian egg shell is described, as revealed by the electron microscopy of surface replicas. This structure differs from the microcrystalline configuration generally found in substances of biological origin. The shell consists, namely, of very large dendrites or spherulitic crystals of calcite, closely packed together. Their transverse fracture surface exhibits an interesting, crystallographically controlled cleavage pattern, very similar to that found in certain metal alloys. This cleavage pattern points to a distinct orientation of the crystals with one of their crystallographic directions parallel to the surfae of the shell. At the inside of the main portion of the shell, small, randomly oriented microcrystals of calcite occur, forming the protruding knobs of the so-called mammilla layer.