A previously published hydrothermal brine-river water mixing model driven by ocean crust production suggests that the molar Mg/Ca ratio of seawater (mMg/Ca(sw)) has varied significantly (approximately 1.0-5.2) over Precambrian time, resulting in six intervals of aragonite-favouring seas (mMg/Ca(sw) > 2) and five intervals of calcite-favouring seas (mMg/Ca(sw) < 2) since the Late Archaean. To ev...

Solutions equilibrated with soils under laboratory conditions often exhibit Ca and CO^~ activities that exceed calcite saturation. This supersaturation has been attributed to various possibilities, none of which have been fully able to explain the phenomenon. In this study we report on the attainment of calcite supersaturation in calcite + water + organic matter systems under controlled CO2 con...

was dissolved from the soil; Le., equilibrium was ap­ Soils in the laboratory were reacted with agricultural drainage­ and distilled waters at various carbon dioxide (COl) levels. The soil water solutions yielded different Ion activity product (lAP) values than that of calcite, depending on experimental technique and whether calcite "equilibrium" is approached from supersaturation or under­ sat...

We examined the processes of calcite nucleation and crystal growth to determine the dominant mechanism for precipitation of calcite in the presence of dissolved organic carbon (DOC). Our experiments, conducted at 25°C and at a Ca:HCO, molar ratio of 1:2, were performed with and without calcite seeds. Scanning electron micrograph observations showed that the particle size of calcite crystals in ...

Several mechanisms have been proposed to explain the interactions between proteins and mineral surfaces, among them a combination of electrostatic, stereochemical interactions and molecular recognition between the protein and the crystal surface. To identify the mechanisms of interaction in the lysozyme– calcium carbonate model system, the effect of this protein on the precipitation kinetics an...

Effects of carbonation temperature and compacting pressure on basic properties of calcite block were studied using Ca(OH)2 compact made with 0.2-2.0 MPa and their carbonation at 200-800ºC for 1 h. Microporous calcite was obtained only when carbonated at 600ºC using Ca(OH)2 compact made with 0.2 MPa even though thermogravimetry analysis showed that calcite powder was stable up to 920ºC under CO2...

The enzyme urease drives the hydrolysis of urea leading to the release of ammonium ions and bicarbonate; in the presence of calcium, the rise in pH leads to increased calcium carbonate saturation and the subsequent precipitation of calcite. Although such alkalinizing ureolysis is widespread in nature, most studies have focussed on bacteria (i.e. indigenous communities or urease-active Sporosarc...

Calcite microfossils are widely used to study climate and oceanography in Earth's geological past. Coccoliths, readily preserved calcite plates produced by a group of single-celled surface-ocean dwelling algae called coccolithophores, have formed a significant fraction of marine sediments since the Late Triassic. However, unlike the shells of foraminifera, their zooplankton counterparts, coccol...

In this study we present microstructural data from hydrothermal veins in the sedimentary cover and the igneous basement recovered from Hole U1414A, Integrated Ocean Drilling Program (IODP) Expedition 344 (Costa Rica Seismogenesis Project), to constrain deformation mechanism operating in the subducting Cocos Plate. Cathodoluminescence studies, mechanical e-twin piezometry and electron backscatte...