Precipitation Of Brucite Research Articles

Magnesium content within the skeletal architecture of the coral Montastraea faveolata: locations of brucite precipitation and implications to fine-scale data fluctuations

Small portions of coral cores were analyzed using a high-resolution laser ablation inductively coupled plasma mass spectrometer (LA ICP-MS) to determine the geochemical signatures within and among specific skeletal structures in the large framework coral, Montastraea faveolata. Vertical transects were sampled along three parallel skeletal structures: endothecal (septal flank), corallite wall, and exothecal (costal flank) areas. The results demonstrate that trace element levels varied among the three structures. Magnesium (Mg) varied among adjacent structures and was most abundant within the exothecal portion of the skeleton. Scanning electron microscopy (SEM) revealed the presence of hexagonal crystals forming thick discs, pairs or doublets of individual crystals, and rosettes in several samples. High Mg within these crystals was confirmed with energy dispersive spectroscopy (EDS), infrared spectrometry, and LA ICP-MS. The chemical composition is consistent with the mineral brucite [Mg(OH2)]. These crystals are located exclusively in the exothecal area of the skeleton, are often associated with green endolithic algae, and are commonly associated with increased Mg levels found in the adjacent corallite walls. Although scattered throughout the exothecal, the brucite crystals are concentrated within green bands where levels of Mg increase substantially relative to other portions of the skeleton. The presence and locations of high-Mg crystals may explain the fine-scale fluctuations in Mg data researchers have been questioning for years.

Transport-controlled kinetics of dissolution and precipitation in the sediments under alkaline and saline conditions

Over 1.6 million liters of radioactive, high-temperature, Al-rich, alkaline and saline high-level waste (HLW) fluids were accidentally discharged from tank leaks onto the sediments at the Hanford Site, Washington. In order to better understand processes that might occur during the migration of HLW through sediments and to estimate their extents, we studied the effects of Al-rich, alkaline and saline solutions on soil mineral dissolution and precipitation during reactive transport. Metal- and glass-free systems were used to conduct miscible-displacement experiments at 50 °C under CO 2 and O 2 free conditions. Results showed significant release of Si, K, Al, Fe, Ca, Mg, and Ba into the aqueous phase. The transport-controlled release of these elements was time dependent as evidenced by its extent varying with the fluid residence time. Silica initial dissolution rates (6.08 × 10 -11 and 5.38 × 10 -13 mol m -2 s -1) increased with base concentration, decreased with Al concentration, and decreased with fluid residence time. Aluminum precipitation rates varied in the range from 0.44 to 1.07 × 10 -6 mol s -1 and were faster in these column experiments than in previous batch studies. The initial rate constant of Al precipitation reaction was 0.07 h -1 (half-life of 9.9 h at about 3 PV); it increased up to 0.137 h -1 (half-life of 5.1 h at about 20 PV). The precipitates identified with SEM and suggested from the modeling results were mainly NO 3-cancrinite. SEM analyses also indicated the formation of sodalite when Al was not present in the leaching solution. In addition, results from modeling suggested the precipitation of brucite, goethite and gibbsite; the latter may precipitate in the presence of high Al concentrations. Aqueous and solid phase transformations caused by base-induced dissolution and subsequent secondary phases precipitation should be important determinants of the fate of contaminants and radionuclides in the vadose zone under alkaline and saline conditions.

Experimental study of brucite dissolution and precipitation in aqueous solutions: surface speciation and chemical affinity control

Dissolution and precipitation rates of brucite (Mg(OH) 2) were measured at 25°C in a mixed-flow reactor as a function of pH (2.5 to 12), ionic strength (10 −4 to 3 M), saturation index (−12 < log Ω < 0.4) and aqueous magnesium concentrations (10 −6 to 5·10 −4 M). Brucite surface charge and isoelectric point (pH IEP) were determined by surface titrations in a limited residence time reactor and electrophoretic measurements, respectively. The pH of zero charge and pH IEP were close to 11. A two-pK, one site surface speciation model which assumes a constant capacitance of the electric double layer (5 F/m 2) and lack of dependence on ionic strength predicts the dominance of >MgOH 2 + species at pH < 8 and their progressive replacement by >MgOH° and >MgO − as pH increases to 10–12. Rates are proportional to the square of >MgOH 2 + surface concentration at pH from 2.5 to 12. In accord with surface speciation predictions, dissolution rates do not depend on ionic strength at pH 6.5 to 11. Brucite dissolution and precipitation rates at close to equilibrium conditions obeyed TST-derived rate laws. At constant saturation indices, brucite precipitation rates were proportional to the square of >MgOH 2 + concentration. The following rate equation, consistent with transition state theory, describes brucite dissolution and precipitation kinetics over a wide range of solution composition and chemical affinity: R=k Mg + · {>MgOH 2 +} 2 · (1−Ω 2) where k Mg + is the dissolution rate constant, {> i} is surface species concentration (mol/m 2), and Ω is the solution saturation index with respect to brucite. Measurements of nonsteady state brucite dissolution rates, in response to cycling the pH from 12 to 2 (pH-jump experiments), indicate the important role of surface hydroxylation — that leads to the formation of Mg oxo or –hydroxo complexes — in the formation of dissolution-active sites. Replacement of water molecules by these oxygen donor complexes in the Mg coordination sphere has a labilizing effect on the dynamics of the remaining water molecules and thus increases reaction rates.

Surface alteration of dolomite in dedolomitization reaction in alkaline media

The phases precipitated on the dolomite surface and the influence of alkalis on the properties of this surface were characterized. Experiments consisted of the immersion of single crystals of dolomite in saturated portlandite solutions with different alkalinity, temperature, and silica content. Most calcite forms in the solution as {104} rhombohedra. Brucite crystallizes as platelets, piles of platelets, sponge-like and {001} truncated ditrigonal pyramids attached to the dolomite surface. The morphology of brucite crystals depends on the pH of the solution. Apparent dissolution rate constants for dolomite at 75 °C are one order of magnitude higher than at room temperature. Initially, calcite grows on the dolomite surface with the same structural orientation {104}. In a second stage, when calcite has reached equilibrium with the solution, the new-formed {104} calcite surface starts playing an important role on the precipitation of brucite, while calcite continues crystallizing in the bulk solution. The calcite surface, negatively charged, can adsorb specifically metal cations. Mg 2+, liberated by the dissolution of dolomite, would move attached to the surface, until they are trapped on these negatively charged surfaces, while OH − supplied by the alkaline solution favors the nucleation and growth of brucite. The attachment of Mg 2+ to the surface controls the place where brucite grows.

Potentiometric measurements of the first hydrolysis quotient of magnesium(II) to 250°C and 5 molal ionic strength (NaCl)

The first molal hydrolysis quotient, Q1.1, of Mg2+ was measured potentiometrically from 1 to 250°C at ionic strengths of 0.11, 0.31, 1.01, and 5.0 mol-kg-1 in an aqueous NaCl medium using a hydrogen-electrode, concentration cell. Only hydrolysis of the first four percent of the magnesium in solution could be followed before precipitation of brucite, Mg(OH)2(cr), occurred. The log Q1.1 values were fitted as a function of temperature and ionic strength using four adjustable parameters. The resulting constants are compared with the limited existing low temperature data. At infinite dilution and 25°C the following quantities are reported: logK1.1 = -11.68±0.05, †Hso = 70.1±1.2 kJ-mol-1, †So = 11±4 J-K-1-mol-1, and †Cpo = 0 J-K-1-mor-1. At each ionic strength, including the values extrapolated to infinite dilution, the heat capacity change for the hydrolysis reaction was zero,i.e., logQ1.1 was found to be a linear function of the reciprocal temperature in Kelvin, at least over the measured range of l-250°C. The hydrolysis constants at infinite dilution were modeled to 550°C and two kbar pressure with a function incorporating solvent density using published results obtained at these extreme conditions.

Solution-mineral equilibria in the system MgO-SiO 2-H 2O-MgCl 2 at 500°C and 1 kbar

The quench pH of MgCl 2 solutions equilibrated at 500°C and 1 kbar pressure with talc + quartz is between 0.9 and 1.6, that of solutions equilibrated with forsterite + talc 4.2 to 7.7, and that of solutions equilibrated with brucite + forsterite between 6 and 9. The addition of NaCl and CaCl 2 does not affect the quench pH measurably. The low quench pH of solutions equilibrated with talc + quartz is due to the ionization of HCl O during the quench. The quench pH of solutions equilibrated with forsterite + talc is apparently controlled by the reaction of Mg-OH complexes with HCl O during cooling, that of brucite + forsterite solutions by the precipitation of magnesium hydroxychlorides and/or brucite during cooling. At 500°C and 1 kbar the solutions are neutral to slightly acid. The low quench pH of solutions equilibrated with talc + quartz shows that these can give rise to extensive proton metasomatism with wall rocks during cooling. This is not the case for solutions equilibrated at 500°C and 1 kbar with forsterite + talc or brucite + forsterite, and explains the lack of proton metasomatism around ultrabasic bodies. The SiO 2 concentration in solutions equilibrated with forsterite + talc is sufficient to lead to the precipitation of quartz on cooling. This may explain the occurrence of quartz with some ultramafic bodies. Mg 2(OH) 3Cl appears to be a stable phase in equilibrium with brucite + forsterite in runs containing more than 0.1 mole MgCl 2/kg of solution. Much of the chlorine in altered ultramafic bodies may be present as a component of this and/or other magnesium hydroxychlorides.