Variation In Mg Content Research Articles

Effect of Mg content on the dislocation characteristics and discontinuous dynamic recrystallization during the hot deformation of Al-Mg alloy

The effect of Mg content on the dynamic recrystallization behavior during the hot deformation of Al-Mg alloys were investigated. Three different Mg contents of Al-6Mg, Al-8Mg, and Al-9Mg were hot-torsioned under the deformation temperature of 723 K and a strain rate of 0.1 s−1. Using and electron backscatter diffraction, the dislocation characteristics during the hot deformation of each alloy were evaluated. The results of X-ray line profile analysis showed that the strain anisotropy parameter and dislocation arrangement parameter decreased when the strain increased, and when the fraction of edge dislocation and screening of dislocation field (interaction between dislocations) increased. However, the variation in Mg content affected the strain dependence of the dislocation parameters, which means that the Mg content in the Al-Mg alloy affected dislocation characteristics and dynamic recrystallization. In addition, the fraction of low-angle boundaries was accelerated when the strain increased at lower Mg content, while the formation of serrated grain boundaries was remarkably observed. Consequently, considering the absence of twinning, at all strains, higher Mg content increased the dynamic recrystallization rate and induced the flow softening at relatively low strains.

Effect of magnesium doping on thermoelectric and magnetic properties of copper chromite ceramic samples

We investigated thermoelectric and magnetic properties of polycrystalline CuCr 1-x Mg x O 2 (x = 0; 0.002; 0.008; 0.015; 0.030) ceramic samples, synthesized by a special chemical homogenization method (nitrate synthesis). Temperature dependencies of magnetization were measured in magnetic field H = 1000 G. Transition temperature do not noticeably shift with variation of Mg content. Seebeck coefficient S increases with temperature for all investigated samples. At room temperature S decreases from nearly 700 mV/K in pristine material to about 250 mV/K for the copper chromite with 3at.% Mg. Heat conductivity k of all investigated samples is close to the heat conductivity of undoped CuCrO 2 crystals. Resistivity of investigated samples decreases several orders of magnitude with increase of Mg content up to 3%. Temperature dependence of resistivity and Seebeck coefficient was interpreted by hopping conductivity in the presence of the Coulomb (parabolic) gap in the density of states.

Strengthening mechanism of age-hardenable Al–xMg–3Zn alloys

ABSTRACTThe addition of 3 wt-% Zn to the traditional Al–Mg alloy doubled its strength. To understand the strengthening mechanism, the enhancement in strength with the variation in Mg content was studied. The grain boundary and solid solution strengthening decrease with the reduction in the Mg content; however, their contributions to the yield strength are insignificant. The contribution of precipitation strengthening to the yield strength is more than 80%; however, due to the unchanged precipitate characteristics, the strengthening effect changes slightly with the variation in the Mg content. The reduction in strength is primarily due to Mg solid solution strengthening. The variation in the ductility of Al–Mg–Zn alloys was studied by fracture analysis.

Testing carbonate chemostratigraphy across differentiated ancient shallow-platform environments (Early Kimmeridgian, S Iberia)

Shallow-platform settings with marked differences in paleoplatform bottom physiography influence the degree of connection with oceanic waters and overall circulation patterns, even when sharing the same palaeoclimatic conditions. Two Kimmeridgian shallow-marine settings have been explored to test the sensitivity and reliability of carbonate chemostratigraphy to detect such differences. An integrated overview of the obtained elemental trends depicted four major facies, shared along specific stratigraphic intervals of both depositional records. Diagenesis obliterated original geochemical signals only throughout the siliciclastics-rich interval, corresponding to the most landward setting. For the remaining facies, elemental features could be attributed to the differential action of forcing mechanisms operating along the south-Iberian paleomargin during Kimmeridgian times. The highest degree of continental influence can be recognized by a strong relationship between Fe and Mn for the most proximal setting, which fades out along the mixed carbonate-fine siliciclastic rhythmic deposition in more open settings. A characteristic geochemical signature of progressively more positive δ13C values and significantly higher Sr content is identified for the interval dominated by biogenic sponge buildups. Such a local response is related to local forcing by upwelling in the surroundings of a coral fringe. The geochemical signature of a hydrothermal origin can be clearly differentiated from the influence of mere terrigenous pulses. Accordingly, the decoupling of Fe and Mn along marginal settings is the clue to detecting major events of palaeogeographic restructuring. Observed temporal variations in Mg content along both studied sections are attributed to tectonic activity influencing nearshore/coastal water masses. By integrating chemostratigraphic information and complementary evidence, the palaeoenvironmental mechanisms promoting differentiated sedimentary records along ancient subtropical, shallow, coastal settings can be disentangled.

Effect of compositional changes on microstructure in additively manufactured aluminum alloy 2139

A matrix of nine experiments was conducted on aluminum alloy 2139 samples made by an electron beam directed energy deposition additive manufacturing process to characterize the effect of magnesium vaporization loss on microstructure and properties. The nine-sample matrix included three different magnesium levels deposited onto material at three different temperatures. The goal was to quantify the variability in the deposited material as a function of allowable magnesium content and the local temperature of the substrate or previous layer. The results show that temperature variations in the substrate up to 100 K have no significant effect on the amount of magnesium loss; however, relatively large variations in Mg content, up to a ±0.25 wt%, can significantly affect the microstructure and mechanical properties. This is due to a change in how the precipitate phase forms, which influences the mechanism by which the alloy is strengthened.

Glycerol hydrogenolysis using a Ni/Ce‐Mg catalyst for improved ethanol and 1,2‐propanediol selectivities

AbstractGlycerol, a byproduct from biodiesel production, is an inexpensive and renewable alternative feedstock for producing valuable chemicals. Hydrogenolysis of glycerol produces ethanol and 1,2‐propanediol (1,2‐PDO) with the help of appropriate catalysts. Commercially available non‐specific Ni‐based catalysts have been reported for use in producing ethanol along with 1,2‐PDO. In this work, a new Ni catalyst on various support materials was developed for the selective production of ethanol and 1,2‐PDO using glycerol hydrogenolysis. The Ni catalyst on CeO2 showed the highest potential for producing both ethanol and 1,2‐PDO. The catalyst with 0.15–0.50 g/g (15–50 wt%) Ni on CeO2 improved glycerol conversion at reaction temperatures of 215–245 °C, however 1,2‐PDO selectivity was unsatisfactory. An improvement in 1,2‐PDO selectivity was attained when Al, Si, Zn, and/or Mg were added as promoters to the Ni catalyst on CeO2, while improvement in the selectivity for ethanol occurred only with the addition of Si or Mg. A variation in Mg content showed that 1,2‐PDO selectivity was favoured at higher Mg content, while ethanol selectivity peaked at 10 % Mg. Systematic investigations found that the catalyst with 0.25 g/g (25 wt%) Ni on a Ce:Mg (4:1 mol:mol) support provided good selectivities for 1,2‐PDO at 68.10 % and ethanol at 9.0 %, respectively.

Magnesium in Crop Production, Food Quality and Human Health

Howmagnesium (Mg) supply in soils contributes tomaintenance of good yields under various stressful environmental conditions, especially under soil acidity and aluminum toxicity was one of the key themes of the 2nd International Symposium on Magnesium in Crop Production, Food Quality and Human Health held in Sao Paulo, Brazil, in November 2014. A role of Mg in human nutrition, changes in Mg supply in the global food chain, and variation in Mg content in plant-based foods were other important topics of the symposium that was organized by the Institute of Applied Plant Nutrition (IAPN, Georg August University, Gottingen, Germany). The University of Sao Paulo, Sabanci University (Turkey), Center for Magnesium Education & Research (USA), and the Brazilian office of the International Plant Nutrition Institute (IPNI) were the co-organizers of the Symposium. “The current mineral nutritional situation of agricultural soils and major crop plants in Brazil was the important reason for us to organize the second symposium on magnesium in Brazil”, said Prof. Dr. Klaus Dittert, scientific director of the Institute of Applied Plant Nutrition in his opening speech. Indeed, Symposium presentations showed that crops growing in common Brazilian soils such as Oxisols and Ultisols respond substantially to fertilisers containing Mg and micronutrients especially Zn (Dechen et al. 2015). These soils are acidic, with high concentration of aluminum and low levels of Ca and Mg. The Mg-containing liming materials used to increase soil pH do not adequately meet the crop Mg demand. Therefore, soluble Mg fertilisers are important in such acidic soils to improve Mg nutrition and protect roots from Al toxicity. Besides high Al concentration, high K:Mg or Ca:Mg ratios can also interfere with root Mg uptake. All these factors emphasize importance of Mg fertilisation programs (Senbayram et al. 2015). One of critical physiological roles ofMg in plants is regulation of assimilate partitioning into sink organs such as roots and seeds (Cakmak andKirkby 2008). It was shown during the Symposium that Mg deficiency affects wheat yield by decreasing individual seed weight rather than seed number per spike. Magnesium has a role in protecting roots from oxidative stress caused by toxicity of heavy metals and Al (Rengel et al. 2015). Limited transport of photoassimilates from shoots to roots and decreased H-ATPase activity, needed for release of organic acid anions from roots to inactivate Al in the rhizosphere, makeMg-deficient plants highly susceptible to Al toxicity. Concentration and content ofMg in various edible plant tissues andorganswerediscussedat theSymposiumwithrespect tohuman and animal nutrition.Different plant species grown under the same conditions differ substantially in their shoot Mg concentrations (White et al. 2015). For example, species of the order Poales have consistently lower shoot Mg concentrations than Caryophyllales. It was suggested that such differences have important implications for the nutritional value of animal diets. An important natural variation inMg uptake and tissue concentrations was documented in Arabidopsis thaliana (Xiao et al. 2015) and can be used for characterization of physiological and genetic mechanisms affecting Mg homeostasis in plants, which has implications for biofortification of food crops with Mg. Biofortification of food crops with Mg to improve human nutrition is becoming a popular topic because of increasing number of reports indicating reduced dietary intake of Mg, especially in western countries. However, there are also reports (based on food supply and composition) suggesting a minimal risk of deficient dietary Mg intake in many countries (Kumssa et al. 2015). Consumption of diets with high Ca:Mg ratio was discussed at the Symposium as an important factor contributing to impaired human Mg nutrition, especially in USA. High intracellular Ca:Mg ratio in body may be an important factor in chronic disorders such as cardiovascular disease, metabolic syndrome and type-2 diabetes (Rosanoff et al. 2015). Moreover, reduced dietary intake of Mg may result in diverse physiological alterations, especially in heart, brain, kidneys and vasculature, leading to neurodegenerative diseases, immunodeficiency, stroke, and cancer (Nielsen 2015). These findings highlight importance of adequate content of Mg in food crops to meet the human demand for Mg. Hence, future research on linking Mg nutrition of crops, pastures and animals with the human diet, health and well-being is a high priority. The organizers thank K+S Kali GmbH for its substantial support, and Crop & Pasture Science for publishing this magnesium special issue.

Characteristics of Heat Treated Al7Si-Mg-Zn - 5 wt.% SiC Squeeze Casted Composite with Variation of Mg Content for Tactical Vehicle Application

Composite as main materials for ballistic applications has been developed in order to reduce density which leads to lower fuel consumption and faster mobilization. Composite is required to own high hardness and high impact strength for good ballistic performance. Particulate composites Al-7Si-Mg-Zn reinforced by SiC is designed for ballistic applications due to its light weight and high hardness. Whilst the high hardness showed brittle properties, heat treatment process is applied to this composite to reduce it. This research aims to study the effect of magnesium as alloying element to composite Al-7Si-Mg-Zn reinforced by SiC particulate which applied to precipitation hardening. Composites Al-7Si-Zn-SiC with 2, 4 and 6 wt. % Mg is solution treated at 500 oC for 1 hour, followed by ageing at 200 oC. The characterization was carried out by hardness testing, microstructure observations, SEM and EDX observations, impact testing and fractographic observations. Results showed that Mg does not affect hardness of composite by precipitation hardening. Composite with 2, 4, 6 wt. % Mg had 63.83, 62.27, 62.48 HRB on its peak hardness. Mg did not become precipitate in matrix Al-7Si-Mg-Zn because of its low diffusivity in aluminium. Mg worked as wetting agent that reduces interface tension between aluminium matrix and SiC particles in order for composite to own better interface bonding. Therefore impact testing showed significant increase of impact strength with the increase of Mg content. Composite with 2, 4, 6 wt. % Mg had 2075, 3006, 3257 J/mm2 impact strength respectively

Characteristics of Heat Treated Al7Si-Mg-Zn - 5 wt.% SiC Squeeze Casted Composite with Variation of Mg Content for Tactical Vehicle Application

Composite as main materials for ballistic applications has been developed in order to reduce density which leads to lower fuel consumption and faster mobilization. Composite is required to own high hardness and high impact strength for good ballistic performance. Particulate composites Al-7Si-Mg-Zn reinforced by SiC is designed for ballistic applications due to its light weight and high hardness. Whilst the high hardness showed brittle properties, heat treatment process is applied to this composite to reduce it. This research aims to study the effect of magnesium as alloying element to composite Al-7Si-Mg-Zn reinforced by SiC particulate which applied to precipitation hardening. Composites Al-7Si-Zn-SiC with 2, 4 and 6 wt. % Mg is solution treated at 500 oC for 1 hour, followed by ageing at 200 oC. The characterization was carried out by hardness testing, microstructure observations, SEM and EDX observations, impact testing and fractographic observations. Results showed that Mg does not affect hardness of composite by precipitation hardening. Composite with 2, 4, 6 wt. % Mg had 63.83, 62.27, 62.48 HRB on its peak hardness. Mg did not become precipitate in matrix Al-7Si-Mg-Zn because of its low diffusivity in aluminium. Mg worked as wetting agent that reduces interface tension between aluminium matrix and SiC particles in order for composite to own better interface bonding. Therefore impact testing showed significant increase of impact strength with the increase of Mg content. Composite with 2, 4, 6 wt. % Mg had 2075, 3006, 3257 J/mm2 impact strength respectively

Magnesium in crop production, food quality and human health

Howmagnesium (Mg) supply in soils contributes tomaintenance of good yields under various stressful environmental conditions, especially under soil acidity and aluminum toxicity was one of the key themes of the 2nd International Symposium on Magnesium in Crop Production, Food Quality and Human Health held in Sao Paulo, Brazil, in November 2014. A role of Mg in human nutrition, changes in Mg supply in the global food chain, and variation in Mg content in plant-based foods were other important topics of the symposium that was organized by the Institute of Applied Plant Nutrition (IAPN, Georg August University, Gottingen, Germany). The University of Sao Paulo, Sabanci University (Turkey), Center for Magnesium Education & Research (USA), and the Brazilian office of the International Plant Nutrition Institute (IPNI) were the co-organizers of the Symposium. “The current mineral nutritional situation of agricultural soils and major crop plants in Brazil was the important reason for us to organize the second symposium on magnesium in Brazil”, said Prof. Dr. Klaus Dittert, scientific director of the Institute of Applied Plant Nutrition in his opening speech. Indeed, Symposium presentations showed that crops growing in common Brazilian soils such as Oxisols and Ultisols respond substantially to fertilisers containing Mg and micronutrients especially Zn (Dechen et al. 2015). These soils are acidic, with high concentration of aluminum and low levels of Ca and Mg. The Mg-containing liming materials used to increase soil pH do not adequately meet the crop Mg demand. Therefore, soluble Mg fertilisers are important in such acidic soils to improve Mg nutrition and protect roots from Al toxicity. Besides high Al concentration, high K:Mg or Ca:Mg ratios can also interfere with root Mg uptake. All these factors emphasize importance of Mg fertilisation programs (Senbayram et al. 2015). One of critical physiological roles ofMg in plants is regulation of assimilate partitioning into sink organs such as roots and seeds (Cakmak andKirkby 2008). It was shown during the Symposium that Mg deficiency affects wheat yield by decreasing individual seed weight rather than seed number per spike. Magnesium has a role in protecting roots from oxidative stress caused by toxicity of heavy metals and Al (Rengel et al. 2015). Limited transport of photoassimilates from shoots to roots and decreased H-ATPase activity, needed for release of organic acid anions from roots to inactivate Al in the rhizosphere, makeMg-deficient plants highly susceptible to Al toxicity. Concentration and content ofMg in various edible plant tissues andorganswerediscussedat theSymposiumwithrespect tohuman and animal nutrition.Different plant species grown under the same conditions differ substantially in their shoot Mg concentrations (White et al. 2015). For example, species of the order Poales have consistently lower shoot Mg concentrations than Caryophyllales. It was suggested that such differences have important implications for the nutritional value of animal diets. An important natural variation inMg uptake and tissue concentrations was documented in Arabidopsis thaliana (Xiao et al. 2015) and can be used for characterization of physiological and genetic mechanisms affecting Mg homeostasis in plants, which has implications for biofortification of food crops with Mg. Biofortification of food crops with Mg to improve human nutrition is becoming a popular topic because of increasing number of reports indicating reduced dietary intake of Mg, especially in western countries. However, there are also reports (based on food supply and composition) suggesting a minimal risk of deficient dietary Mg intake in many countries (Kumssa et al. 2015). Consumption of diets with high Ca:Mg ratio was discussed at the Symposium as an important factor contributing to impaired human Mg nutrition, especially in USA. High intracellular Ca:Mg ratio in body may be an important factor in chronic disorders such as cardiovascular disease, metabolic syndrome and type-2 diabetes (Rosanoff et al. 2015). Moreover, reduced dietary intake of Mg may result in diverse physiological alterations, especially in heart, brain, kidneys and vasculature, leading to neurodegenerative diseases, immunodeficiency, stroke, and cancer (Nielsen 2015). These findings highlight importance of adequate content of Mg in food crops to meet the human demand for Mg. Hence, future research on linking Mg nutrition of crops, pastures and animals with the human diet, health and well-being is a high priority. The organizers thank K+S Kali GmbH for its substantial support, and Crop & Pasture Science for publishing this magnesium special issue.

Effect of Mg Content on Age Hardening of an Extruded Al-Si-Mg-Mn Alloy

The main task of this study was to investigate the effect of Mg content on hardness of extruded Al-Si-Mg-Mn alloy after solution and artificial ageing heat-treatments. The Al-Si-Mg-Mn alloys with the variation of Mg content were fabricated through casting, homogenization and extrusion processes. The extruded samples were heat-treated – T6 temper. Thermal analysis with metallurgical examination was performed in order to optimize the condition of solution heat-treatment. Then, microstructure change was discussed by using optical microscopy and SEM-EDS. And the corresponding Vickers hardness was measured as a function of ageing time. It was found that the Vickers hardness slightly increased due to improved ageing efficiency and increased hard phase as the Mg content increased.

Effect of annealing temperature on the structural and optical properties of Zn 1− xMg xO particles prepared by oxalate precursor

Zn 1− x Mg x O particles were prepared using zinc and magnesium oxalate precursor by co-precipitated method. The lattice constants of Zn 1− x Mg x O proved that the interstitial Mg formed at 500 °C and Mg replaced Zn in ZnO tetrahedral coordination at 800 °C. Compared with the ZnO, the absorbing band edge of the Zn 1− x Mg x O displayed blue shifts. The room temperature photoluminescence was similar to ZnO and variation of Mg content did not change the shape or peak position of the emission spectra markedly when it was annealed at 500 °C. However, its blue emission band disappeared, and a relatively strong green light emission at 498 nm appeared after annealed at 800 °C. The photoluminescence intensity ratios I (green)/ I (UV) of Zn 1− x Mg x O varied with Mg content and the green light emission peak shifted from 498 nm to 472 nm when Mg content increased from 0 to 2.0 at.%.

An Investigation of Diffusion in Droplets and of Evaporation of Volatile Components into Vacuum

A numerical investigation is performed of the dynamics of diffusion in droplets and of free-molecule evaporation into vacuum of volatile components of magnesium (Mg) and zinc (Zn) from aluminum alloys. The calculations are performed in application to the conditions of my experiments in high-frequency droplet formation under conditions of centrifugal dispersion in vacuum of rotating (overloads of 70–4000) blanks by the melting method under the effect of a concentrated energy source. In view of analogy between mass transfer and heat transfer, available solutions are used in heat transfer under convection cooling of bodies with boundary conditions of the third kind. It is demonstrated that the numerical values of Biot diffusion criteria (Bi) are functions of the properties of alloy components and do not depend on the initial content of volatile components. At the droplet temperature T = 1000 K and radius R = 0.25× 10−3 m, the Biot number is Bi = 14.8 for alloying systems Al-Mg and Bi = 74.4 for Al-Zn. The solution demonstrates that the diffusion resistance within droplets causes a several-fold decrease (compared to the initial value) in the transfer of the mass of volatile component to the droplet surface (with a decrease in the flow of vapors) even for the given short time of droplet flight τ = 0.01 s corresponding to the value of the Fourier criterion Fo = Dτ/R2 = = 1.2×10−3. The final depletion of the composition of droplets of magnesium, which is estimated relative to its permissible content in the alloy according to GOST (State Standard), is about 0.07% for a D16 alloy with a wide range of variation of Mg content from 1.2 to 1.8% by mass, which may be regarded as insignificant. For an AMg6 alloy with a smaller range of variation of Mg content from 5.8 to 6.8%, the depletion is about 0.2%, which may be regarded as appreciable.

Measuring relative grain boundary energies and mobilities in an aluminum foil from triple junction geometry

The effect of Mg content on the dynamic recrystallization behavior during the hot deformation of Al-Mg alloys were investigated. Three different Mg contents of Al-6Mg, Al-8Mg, and Al-9Mg were hot-torsioned under the deformation temperature of 723 K and a strain rate of 0.1 s−1. Using and electron backscatter diffraction, the dislocation characteristics during the hot deformation of each alloy were evaluated. The results of X-ray line profile analysis showed that the strain anisotropy parameter and dislocation arrangement parameter decreased when the strain increased, and when the fraction of edge dislocation and screening of dislocation field (interaction between dislocations) increased. However, the variation in Mg content affected the strain dependence of the dislocation parameters, which means that the Mg content in the Al-Mg alloy affected dislocation characteristics and dynamic recrystallization. In addition, the fraction of low-angle boundaries was accelerated when the strain increased at lower Mg content, while the formation of serrated grain boundaries was remarkably observed. Consequently, considering the absence of twinning, at all strains, higher Mg content increased the dynamic recrystallization rate and induced the flow softening at relatively low strains.

Bacterially Induced Mg-Calcite Formation: Role of Mg2+ in Development of Crystal Morphology

ABSTRACT The production of several morphologies of magnesian calcite crystals by the soil bacterium Myxococcus xanthus is reported for the first time. Mg-calcite crystallization occurred in agar-agar gel culture media in the presence of the live bacteria. The agar-agar used to solidify the nutritive microbial solution acted as a porous system that allowed slow counterdiffusion of cations and anions and of the bacterial metabolites produced. Under these conditions, crystal nucleation and growth occurs, apparently as a consequence of the localized ion supersaturation produced by the microbial metabolites and by microbial supply of heterogeneous nuclei for crystallization. Several morphologies of Mg-calcite typical of those formed under biotic and abiotic conditions developed simultaneously. The crystals produced were not compositionally zoned and showed no significant variation in Mg content, probably as a consequence of the sponge-like character of the precipitates.

Skeletal carbonate mineralogy of New Zealand bryozoans

On many cool-water carbonate shelves, both modern and ancient, bryozoans are the dominant sediment contributor. Because skeletal carbonate mineralogy can be related to both taxonomic and environmental controls, and has important implications for understanding the geochemistry and diagenesis in carbonate deposits, it is highly relevant to develop a comprehensive database for bryozoan skeletal mineralogy. This X-ray diffraction study of modern New Zealand bryozoans more than doubles the published data available on bryozoan mineralogy and allows for statistical analysis of variability in mineralogy, particularly the hitherto unknown background variation within individuals and within populations. Such a database allows evaluation of differences among populations, taxa, and growth forms. The 412 analyses of 49 bryozoan species fall into four mineralogical categories: (a) 61% are single calcite, which ranges widely from low to high wt% MgCO 3 varieties; (b) 6% (in the genera Cellaria and Macropora) comprise dual calcites, a dominant one low in Mg ( X=1.8 wt% MgCO 3) and the other much higher ( X=9.0 wt% MgCO 3); (c) 27% are mainly calcite with some aragonite; and (d) 6% are mainly aragonite with some calcite. The bimineralic calcite–aragonite skeletons can arise from different processes, including biologically mediated changes in precipitation and diagenetic alteration following death. There is small but significant variation in Mg content in calcite both within colonies (Mg increasing with age) and within populations, for a total of within-species variance of 0.526. We found, however, more variation among species in both calcite/aragonite ratio and Mg content than that within species. Whereas some genera and families are reasonably consistent mineralogically (e.g., low-Mg calcite cyclostomes such as Telopora buski), others are highly variable (e.g., Chaperia acanthina). Among the higher taxa, cyclostomes tend to be entirely or mainly calcite with a low to moderate range of MgCO 3 ( X=2.1 wt%), whereas cheilostomes are far more variable, with anascans ( X=4.1 wt%) having a lower MgCO 3 content than ascophorans ( X=5.6 wt%). All dual-calcite species are anascans in the Calloporoidea. The proportions of aragonite and of Mg in calcite are greater in evolutionarily younger taxa, at least at the higher levels. In relation to growth forms, erect rigid varieties are calcitic except for several robust-branching genera such as Adeonellopsis. Encrusting forms, particularly unilaminar sheets, are highly variable and contain most of the bimineralic calcite–aragonite species. Erect flexible forms are either single or dual calcite, and extremely variable in Mg content. Free-living or vagrant forms are usually aragonitic. In relation to latitude and temperature variation, few species show any consistent or significant mineralogical trends, although Schizoporella unicornis deposits increasing amounts of aragonite at lower (warmer) latitudes. Mecynoecia purpurascens includes more (but still low) Mg in its calcite skeleton with increasing latitude, a reversal of the Mg–temperature relationship noted in several other skeletal carbonate producers.

Variability in ostracod partition coefficients D(Sr) and D(Mg) : Implications for lacustrine palaeoenvironmental reconstructions

Living and sub-recent ostracods were collected from four lakes and four springs, located in Belgium, France and Spain, characterized by low water Mg/Ca ratios (from 0.08 to 0.65). Valves of ostracods belonging to Cyprideis torosa, Candona neglecta, C. marchica and C. candida have been extensively analysed for their magnesium and strontium contents with respect to calcium. Comparisons of these results are made using the same elements from the waters in which the ostracod valves calcified. Our results offer new and somewhat different interpretations with respect to the uptake of Mg and Sr in the calcitic ostracod valves. These findings include the following. (1) The partition coefficient D(Sr) for ostracods belonging to the same genus ( Candona) is not a constant. The observed variability in D(Sr) values could be due to the rate of biological calcification, although more investigations are necessary to assess the influence of this factor; it is possible that some taxa grouped under the genus Candona belong to a different genus. (2) Even at the species level ( Cyprideis torosa), the ostracod D(Sr) cannot be considered to be constant. At low water Mg/Ca ratio (0.65), our D(Sr) value for C. torosa is higher than those determined by others for C. torosa calcified in evolved marine water. (3) The Mg/Ca ratio of the water appears as a major factor influencing D(Mg) for ostracods. For low water Mg/Ca values (<∼2), D(Mg) drastically increases. This phenomenon is also effective for inorganic calcite, implying a non-biologic effect that could be linked to Mg 2+ adsorption. Consequently, large variations in Mg content of fossil ostracod valves from selected lacustrine sequences, with expected low water Mg/Ca (<∼2), can be due to small variations in water Mg/Ca.

Mineral assemblages occurring around hydrocarbon vents in the northern Gulf of Mexico

Magnesian calcites are the most abundant authigenic minerals associated with hydrocarbon vents at 25 sites, in water depths ranging from 100 to 600 m in the Green Canyon area and about 2200 m in the Alaminos Canyon area on the Continental Slope of the northern Gulf of Mexico. The most frequently encountered magnesian calcites have 10–15 mol% MgCO3 and the apparent structural disorder revealed by XRD peak widths increases with Mg substitution. There are no systematic variations in Mg content with respect to water depth or geographic location. The calcite saturation state of the precipitating fluid is primarily determined by the nature of the fluids escaping from the vents, not the ambient seawater.

Nutrient Dynamics in Decomposing Litter in Karri (Eucalyptus Diversicolor F. Muell.) Forests of South-Western Australia

(1) Nutrient content of decomposing litter was determined in karri (Eucalyptus diversicolor) forest, a tall open forest growing in south-western Australia. (2) Changes in nutrient content of five leaf and five non-leaf litters were studied over eighty-two weeks using the litter bag technique. Leaves of karri and the four understorey species Trymalium spathulatum, Bossiaea laidlawiana, Acacia urophylla and Casuarina decussata, karri twigs, bark and fruit and seed pods of B. laidlawiana and A. urophylla were used in the study. (3) Concentration of N, P, K and S in fresh litter were higher in leaf litter from the main understorey plants T. spathulatum, B. laidlawiana and A. urophylla than in leaf litter from karri or C. decussata. (4) During decomposition, concentrations of N, P and Ca increased in most litter components, concentrations of Mg and S changed little and the concentrations of Na, K and Cl decreased markedly. (5) Nutrients in karri leaf litter were released in the order Na> Cl> K> Mg> S > Ca > N > P. With some minor exceptions this order of nutrient mobility was similar for other litter components. (6) During decomposition N and P were imported into many of the litter components from external sources. Net release of N occurred only from litter from the main understorey plants and net release of P occurred only from T. spathulatum and B. laidlawiana leaf litter and karri fruit. (6) A two component model relating amounts of nutrients remaining in decomposing litter to initial lignin: nutrient ratio and exposure period explained from 760% to 880/ of the variation (n = 80) in N, P, K, S and Ca and 640/ of the variation in Mg content of the ten litter components. (7) For each nutrient, results of the model were used to allocate litter fractions to one of three compartments of the decomposition subsystem in which (i) nutrients were mobile and nutrient release occurred soon after decomposition began; (ii) nutrients were less mobile and nutrient immobilization preceded nutrient release; (iii) nutrients were immobilized, being imported from external sources and retained in the litter. For Na, K, Cl, Mg, S and Ca most of the litter components were allocated to the compartment in which nutrients were mobile. For N and P, understorey litters were the major contributors to this compartment and karri twigs and bark were the major contributors to the compartment in which N and P were least mobile. (8) The impact of regular controlled burning on nutrient cycling through the decomposition subsystem is discussed in relation to model predictions mobility of nutrient in the various components of karri forest litter.

Electron Microprobe Study of Mg Distribution in Recent Mg Calcites and Recrystallized Equivalents from the Pleistocene and Tertiary

ABSTRACT The effect of early freshwater diagenesis on Mg distributions in former Mg calcite cements was determined by electron microprobe analysis of two major groups of samples: 1) Mg calcites in Recent limestones not yet exposed to freshwater, and 2) Tertiary-Recent Mg calcites and recrystallized equivalents which have undergone freshwater exposure. Pristine Mg calcites are characterized by a uniform Mg concentration. In contrast, former Mg calcites from the Miocene of Enewetak Atoll, most of which are now radiaxial, prismatic calcites, have a much greater variation in Mg content, and traverses across the calcites reveal definite trends. Mg content of the calcite is inversely correlated with abundance of inclusions. Some of the cloudy growth layers are due to infestation by endolithic organi ms (fungi?). The small variation in Mg content in original Mg calcite cements is a valuable starting point to be aware of when interpreting Mg trends in recrystallized equivalents. Mg content can be related to inclusion abundance in recrystallized, former Mg calcites, but not in pristine Mg calcites. Therefore, Mg trends in altered materials most likely reflect dissolution/reprecipitation processes, not original Mg trends. A likely cause for the low Mg content associated with inclusion-rich growth layers is relatively high rates of dissolution and reprecipitation in these relatively permeable layers. In some cases, the increased permeability is the result of infestation by endoliths. Such organisms may cause the cloudiness observed in some other radiaxial calcites.