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(XRD) data was reported by the authors. Dihydrate or monohydrate magnesium carbonates have also been reported during the solvent-mediated [N ! HM] transition (e.g., Hopkinson et al., 2008). Complexity in this part of the system is compounded by similarities between the thermal behaviour of nesquehonite and hydromagnesite (e.g., Beck, 1950; Kazakov et al., 1959; Lanas and Alvarez, 2004), and evidence for the presence of bicarbonate in some synthesised and natural samples of nesquehonite. These attributes have led some investigators to assign a basic magnesium carbonate formula to nesquehonite: [Mg(OH, HCO3) 2H2O] (e.g., Wells, 1915; Beck, 1950; Kazakov et al., 1959; Hales et al., 2008). Hence, either nesquehonite ranges in composition, or there is a structural isomer of the mineral (Hales et al., 2008). 3. EXPERIMENTS 3.1. Experimental materials and methods Two experiments were conducted. In both cases, the initial solution was prepared by adding 3.1 g of pulverised brucite [Mg(OH)2] to 350 ml of distilled water at 25 °C and sparged with pure CO 2 at 1 atm for 19 h until the pH stabilized at 6.79. At this point, the CO2 flow was stopped and the temperature increased from 25 to 58 °C (within 10 min). In the first experiment, the system was held at that temperature for 240 min (static experiment). In the second experiment, the solution was subject to the same heating conditions but it was sonicated (agitated experiment). The rate of [H + ] decrease at 58 °C accompanying CO 2 degassing from pure water previously sparged with CO2 at 25 °C, in the absence of dissolved magnesium, yielded pseudo-first order rate constants for the decrease in [H + ] of k = 0.0225 min 1 when the solution is sonicated and k = 0.0046 min 1 when degassing is exclusively thermally- Author's personal copy Phase transitions in the system MgO–CO2–H2O 3 Fig. 1. The hydrated magnesium carbonate minerals in the system CO2–MgO–H2O, adapted from Canterford et al. (1984). Specified values of X in synthesised MgCO 3 XH 2O phases include 1.3 and 0.3 H 2O(Zhang et al., 2006). The chemical formula of dypingite may show 5 or 6 H 2O molecules (Xiong and Lord, 2008). The International Mineralogical Association – Commission on New Minerals and Mineral Names (IMA/ CNMNC) status of giorgosite is questionable. The status of nesquehonite is grandfathered: i.e. the original description preceded the establishment of the CNMNC in 1959, listed formula [MgCO3 3H2O]. The unnamed mineral [Mg5(CO3)4(OH)2 8H2O] is not listed. driven. The rate equation is expressed as: pHt pH0 = kt/2.303. Previous studies show that electric conductance provides a real-time semi-quantitative method for monitoring carbonate mineral precipitation events during CO2 degassing from aqueous solutions (e.g., Zeppenfeld, 2006). In this study, it is assumed that the electric conductance is largely due to [Mg 2+ ], [HCO3 ] and [H + ]. Electro-conductivity measurements were taken using a Jenway 4010 conductivity meter. Temperature was measured with a Fisher Scientific platinum sensor (Pt-100O) thermometer (±0.1 °C), while pH readings were taken with a Mettler Toledo pH meter (±0.01 pH). The conductivity probe was calibrated with a standard salt solution supplied by Hanna Instruments, 6.44 parts per thousand KCl at 25 °C, Lot. No B399, calibrated as 12.88mS, zero was reverse-osmosis water. The pH probe was calibrated against pH-10 (borate), pH-7 (phosphate) and pH-4 (phthalate) NIST-traceable buffers purchased from Fisher Scientific. The temperature correction for both probes was linear, electric conductivity measurements were corrected to the 25 °C reference point using a 2.1% per °C coefficient. Readings were acquired from mid-water depth in both experiments. The precision and detection limit were ±0.2 mS and ca 0.6 mS respectively. The magnesium concentration in the solutions was measured by atomic absorption spectroscopy (AAS) using a Perkin Elmer AAnalyst 200 instrument. Small aliquots (2 ml) of the experimental solutions were withdrawn from mid-water depths, filtered through a cellulose acetate membrane (0.2 lm pore size) and diluted 1:100 with water containing 1 ml of spectroscopic grade nitric acid. This solution was further diluted as appropriate so that the final magnesium concentration fell within the calibration range of (0.2–1.2 ppm) of the instrument. The limit of detection was ca 0.02 ppm. The experiments were repeated several times and halted at different times for solid phase sample collection and to
verify the reproducibility of conductance and pH readings. In the case of the static experiment, precipitation was marked first by deposition of solids at the base of the vessel and then progressive development of a surface film of acicular crystals. The two precipitate types were homogenised during sampling. In the agitated experiment, precipitates remained suspended in the parent solution. After filtration through a 0.2 lm filter, powders were air dried at 25 °C for three days, at constant humidity, then lightly ground prior to analysis. Samples were labelled with the prefix [A x] for the agitated experiment and [S x] for the static experiment, where x = time in minutes at which the sample was taken after the onset of heating. Samples labelled [AS0] were collected subsequent to CO 2 sparging, and prior to heating and sonication. 3.2. Solid phase analysis Fourier-transform mid-infrared (FT-IR), FT-Raman, and scanning electron microscope analyses (SEM) were conducted at the University of Brighton (UK). FT-Raman analyses were undertaken with a 1064 nm Nd-YAG laser source using a Nicolet Nexus FT-Raman module. Sharp bands are accurate to 2 cm 1 . FT-IR analyses were performed using a Nicolet Avatar 320 FT-IR, fitted with a diamond attenuated total reflectance (ATR) accessory. The detection limit for mineral phase(s) in mixed assemblages of basic and hydrate magnesium carbonates is 65%, for FT-Raman and ca 10% for FT-IR. Electron microscopic analyses were conducted with a Jeol JSM6310 scanning electron microscope (SEM). X-ray powder diffraction data (performed at the Natural History Museum, London, UK) were collected using an INEL curved position sensitive detector (PSD). This detector has an output array of 4096 digital channels representing an arc of 120°2h and permits the simultaneous measurement of diffracted X-ray intensities at all angles of 2h across 120° with a static beam-sample-detector geometry. Copper Ka1 radiation was selected from the primary beam using a germanium 111 single-crystal monochromator. Horizontal and vertical slits were used to restrict the beam to 0.14 by 5.0 mm respectively. Powdered samples were mounted on a single-crystal sapphire substrate, and measurements made in reflection geometry with the sample surface (spinning in its own plane) at an angle of 2° to the incident beam. Data collection times were 15 min for each sample. NIST Silicon powder SRM640 and Silver Behenate were used as external 2h calibration standards and the 2h linearization of the detector was performed using a leastsquares cubic spline function. The detection limit for mineral phase(s) in mixed assemblages of basic and hydrate magnesium carbonates is ca 5% modal abundance. 4. RESULTS 4.1. Electrical conductivity, AAS and pH Small amounts of precipitate formed during sparging of CO2 at 25 °C (samples [AS0]). AAS analysis indicated a corresponding reduction in [Mg 2+ ] from 0.15 to 0.138 M Author's personal copy 4 L. Hopkinson et al. / Geochimica et Cosmochimica Acta 76 (2012) 1–13 during the nineteen hours of CO 2 sparging. The onset of heating in both experiments produced electrical conductance data which divides into temporal domains. The first 16 min are defined by a steep decline in electrical conductivity in both experiments (Fig. 2A), consistent with supersaturation, solid phase nucleation, and subsequent growth (e.g., Söhnel and Mullin, 1978; Zeppenfeld, 2006). In the agitated setting, between ca 16 and 40 min, there is a second large decrease in electric conductivity, consistent with a second pulse of mineral formation. The [Mg 2+ ] is reduced to 0.074 M at 25 min, increasing to 0.083 M at 35 min, thereafter declining to 0.076 M after 120 min (Fig. 2A). In the static environment, within the same time frame, a modest decrease in conductivity and [Mg 2+ ] is observed (Fig. 2A), consistent with slow growth of crystals in solution as supersaturation was depleted (e.g., Söhnel and Mullin, 1978). Within the first ten minutes of the experiments, the pH increases at a faster rate in the agitated environment than the static environment (Fig. 2B), consistent with accelerated CO 2 loss in the former. Between 15 and 20 min, the pH in the agitated environment undergoes an abrupt reduction from 7.93 to 7.86, reflecting CO2 production resulting from carbonate mineral formation. Between 20 and 120 min, the pH in the agitated setting shows little variation, measured at pH = 7.98 at the 120-min mark. In contrast, the pH in the static environment undergoes a protracted gradual increase, achieving parity with the agitated environment readings after ca 35 min and pH = 8.09 120 min into the experiment (Fig. 2B). Monitoring of the static and agitated experiments continued intermittently for up to 240 min. No appreciable changes in pH or electric conductance, relative to the 120-min readings occurred in this time frame. 4.2. X-ray diffraction [AS 0] precipitates are identified as hydromagnesite, [Mg5(CO3)4(OH)2 4H2O] (Fig. 3A). Samples [A20], [A50], and [A80] are dominated by nesquehonite with subordinate hydromagnesite and traces of a dypingite-like mineral phase(s) for which the closest match is [Mg 5(CO 3) 4(OH) 2 5H2O] (Fig. 3B and C). Note that discrimination between [MgCO3 3H2O] and [Mg(HCO3,OH) 2H2O] is not possible by XRD. Sample [A 120] is dominated by hydromagnesite, includes a dypingite-like phase(s), but is devoid of nesquehonite. The diffraction pattern shows two low-angle peaks not present in dypingite (Fig. 3C). Only the reference diffraction pattern of the unnamed dypingite-like phase, [Mg5(CO3)4(OH)2 8H2O] (Suzuki and Ito, 1973) contains these peaks, of which, the peak at 2.7° (33 A ˚ d-spacing) is the main distinguishing feature relative to the 5H2O version of dypingite and hydromagnesite (Fig. 3). Given that Suzuki and Ito (1973) present a water determination, the experimental data is in keeping with the unnamed mineral. The multi-minerallic make up of the precipitates, combined with extensive overlap of diffraction patterns between dypingite and the unnamed mineral dictates that the simultaneous presence of dypingite is not precluded. Sample [A240] gives a diffraction pattern consistent with an additional dypingite-like mineral phase with subordinate
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