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Author's personal copy Phase transitions in the system MgO–CO 2–H 2O during CO 2 degassing of Mg-bearing solutions Laurence Hopkinson a,⇑ , Petra Kristova b,1 , Ken Rutt b,2 , Gordon Cressey c,3 a School of Environment and Technology, University of Brighton, Cockcroft Building, Lewes Road, Brighton BN2 4GJ, United Kingdom b School of Pharmacy and Biomolecular Sciences, University of Brighton, Cockcroft Building, Lewes Road, Brighton BN2 4GJ, United Kingdom c Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom Abstract Received 5 January 2011; accepted in revised form 12 October 2011; available online 18 October 2011 This study documents the paragenesis of magnesium carbonates formed during degassing of CO2 from a 0.15 M Mg 2+ aqueous solution. The starting solutions were prepared by CO2 sparging of a brucite suspension at 25 °C for 19 h, followed by rapid heating to 58 °C. One experiment was performed in an agitated environment, promoted by sonication. In the second, CO2 degassing was exclusively thermally-driven (static environment). Electric conductance, pH, and temperature of the experimental solutions were measured, whereas Mg 2+ was determined by atomic absorption spectroscopy. Precipitates were analysed by X-ray diffraction, Fourier transform (FT) mid-infrared, FT-Raman, and scanning electron microscopy. Hydromagnesite [Mg5(CO3)4(OH)2 4H2O] precipitated at 25 °C was followed by nesquehonite [Mg(HCO3,OH) 2H2O] upon heating to 58 °C. The yield of the latter mineral was greater in the agitated solution. After 120 min, accelerated CO2 degassing resulted in the loss of nesquehonite at the expense of an assemblage consisting of an unnamed mineral phase: [Mg5(CO3)4(OH)2 8H2O] and hydromagnesite. After 240 min, dypingite [Mg5(CO3)4(OH)2 5H2O (or less H2O)] appears with hydromagnesite. The unnamed mineral shows greater disorder than dypingite, which in turn shows greater disorder than hydromagnesite. In the static environment, there is no evidence for nesquehonite loss or the generation of [Mg5(CO3)4(OH)2 X- H 2O] phases over the same timeframe. Hence, results indicate that the transformation of nesquehonite to hydromagnesite displays mixed diffusion and reaction-limited control and proceeds through the production of metastable intermediates, and is interpreted according to the Ostwald step rule. Nevertheless, variations in the chemistry of nesquehonite, combined with the established tendency of the mineral to desiccate, implies that its transformation to hydromagnesite is unlikely to follow a single simple sequential reaction pathway. Ó 2011 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 76 (2012) 1–13 For several important groups of minerals, including carbonates, mineral paragenesis frequently results in ⇑ Corresponding author. Tel.: +44 (0) 1273 642239; fax: +44 (0) 1273 642285. E-mail addresses: l.hopkinson@brighton.ac.uk (L. Hopkinson), p.kristova@brighton.ac.uk (P. Kristova), k.rutt@brighton.ac.uk (K. Rutt), g.cressey@nhm.ac.uk (G. Cressey). 1 Tel.: +44 (0) 1273 642065; fax: +44 (0) 1273 642285. 2 Tel.: +44 (0) 1273 642076; fax: +44 (0) 1273 642285. 3 Tel.: +44 (0) 20 7942 5711; fax: +44 (0) 20 7942 5537. 0016-7037/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2011.10.023 www.elsevier.com/locate/gca assemblages that do not conform to predictions based on equilibrium thermodynamics (e.g., Morse and Casey, 1988). In the system MgO–CO2–H2O at 0–60 °C, the stable phases in equilibrium with aqueous solutions containing Mg ions and carbon dioxide are brucite [Mg(OH) 2] at very low partial pressures of CO2, and magnesite [MgCO3], the most stable magnesium carbonate under Earth surface conditions (e.g., Königsberger et al., 1999; Hänchen et al., 2008). Nevertheless, Mg-carbonate precipitation is strongly kinetically-controlled (e.g., Königsberger et al., 1999). Inhibition of magnesite formation has been ascribed to the high hydration energy of Mg 2+ (e.g., Hänchen et al., 2008). Instead of magnesite, a variety of magnesium hydrate and
asic carbonates (i.e. containing hydroxyl groups) form, of which hydromagnesite [Mg5(CO3)4(OH)2 4H2O (or 5H2O)] is the only stable mineral at atmospheric CO2 pressures within the temperature interval typical of most surface environments (Langmuir, 1965). Nevertheless, a number of hydromagnesite-like phases, which differ in unit cell parameters and in some cases, numbers of waters of crystallization, also exist (e.g., Raade, 1970; Davies and Bubela, 1973; Canterford et al., 1984). The relationship between these mineral phases and hydromagnesite as well as the causative factors behind their genesis in preference to hydromagnesite are uncertain, although it seems that the existence of hydromagnesite-like phases is, at least in part, accountable for discrepancies in solubility data for hydromagnesite reported in the literature (Königsberger et al., 1999). The mineral nesquehonite, reported as either [MgCO 3 3H 2O] or [Mg(HCO 3, OH) 2H 2O], forms at ca 10–50 °C and higher than atmospheric PCO2 (e.g., Wells, 1915; Beck, 1950; Kazakov et al., 1959; White, 1971; Stephan and MacGillavry, 1972; Coleyshaw et al., 2003; Hales et al., 2008; Cheng and Li, 2010). The mineral is unstable at near surface ambient conditions. In experimental settings, nesquehonite transforms to hydromagnesite [N ! HM] in an aqueous medium at 52–65 °C, depending on the reaction time (e.g., Davies and Bubela, 1973; Zhang et al., 2006). This is achieved through one or more dissolution–precipitation step(s), giving rise to a variety of short-lived metastable intermediates, which in some experimental settings include hydromagnesite-like phases (Davies and Bubela, 1973; Hopkinson et al., 2008). Moreover, the direct transformation of nesquehonite to hydromagnesite has also been suggested (Hao and Du, 2009). Hence, nesquehonite acts as a precursor for at least some hydromagnesite and, in some experiments, hydromagnesite-like phases are also generated during the [N ! HM] transition, the stability of the minerals influenced both by temperature and partial pressure of CO2 (Königsberger et al., 1999). It follows that understanding the crystallization behaviour of hydrated magnesium carbonates continues to be a challenge and that factors determining products of the [N ! HM] transition are poorly characterized. To these ends, this study documents the nature of the metastable intermediates, generated in the course of the [N ! HM] transition, during open system degassing of CO2 from heated aqueous solutions. 2. HYDRATED AND BASIC MAGNESIUM CARBONATES A number of magnesium carbonates have been identified in the system MgO–CO2–H2O (Fig. 1). Phases of relevance to this study are [Mg5(CO3)4(OH)2 XH2O] and [MgCO3 X- H 2O] group minerals. The [Mg 5(CO 3) 4(OH) 2] group have four to eleven H 2O molecules: heavy (5H 2O) and light (4H2O) hydromagnesite (e.g., Canterford et al., 1984; Botha and Strydom, 2001), dypingite [Mg5(CO3)4(OH)2 5H 2O (or 6H 2O)] (Raade, 1970), giorgiosite [Mg 5(CO 3) 4 (OH)2 6H2O] (Raade, 1970; Friedel, 1975), an unnamed mineral [Mg5(CO3)4(OH)2 8H2O] (Suzuki and Ito, 1973), and protohydromagnesite [Mg 5(CO 3) 4(OH) 2 11H 2O] Author's personal copy 2 L. Hopkinson et al. / Geochimica et Cosmochimica Acta 76 (2012) 1–13 (Davies and Bubela, 1973; Davies et al., 1977; Canterford et al., 1984). In the MgO–CO2–H2O system, these [Mg5(CO3)4(OH)2 XH2O] mineral phases fall on a line that connects protohydromagnesite with hydromagnesite (Fig. 1). Hydromagnesite is by far the most common naturallyoccurring mineral in the group. It occurs as a weathering product of magnesium-rich rocks, and as deposits in lakes, alkaline wetlands and evaporitic basins (e.g., Stamatakis, 1995; Russell et al., 1999; Power et al., 2009; Last et al., 2010). Accounts of dypingite are far less widespread, although it has been described in similar settings (e.g., Raade, 1970), frequently in association with hydromagnesite (e.g., Power et al., 2007; Last et al., 2010). Rarer still, giorgiosite and the unnamed mineral are described in association with weathered volcanics (Suzuki and Ito, 1973; Friedel, 1975). Dypingite and protohydromagnesite have been synthesised individually during the [N ! HM] transition in separate studies (Davies and Bubela, 1973; Hopkinson et al., 2008). Protohydromagnesite has not been identified in nature. The vibrational spectra of the [Mg5(CO3) 4(OH) 2 XH2O] phases are undocumented except for hydromagnesite and dypingite. The latter mineral shows infrared-active internal modes of the [CO 2 3 ] anion that are indistinguishable from those of hydromagnesite (e.g., Raade, 1970; Canterford et al., 1984). This implies that the two minerals share very similar or identical short-range order, yet, distinct longrange crystal order. More recent spectroscopic studies of dypingite show that variations in the intensity of water stretching bands occurs, indicating that differences in the chemical formula exist, and that [CO 2 3 ] units are variably distorted (Frost et al., 2008, 2009). Heating dypingite results in its conversion to hydromagnesite (Raade, 1970; Botha and Strydom, 2001). Known naturally-occurring [MgCO3 XH2O] minerals are: landsfordite, nesquehonite and barringtonite (Fig. 1). Landsfordite and nesquehonite are typically found in caves, or associated with weathered surfaces of magnesium-rich rocks, where temperatures are low and CO2 partial pressures are often more than ten times atmospheric (Langmuir, 1965). At a CO2 partial pressure of 1 atmosphere, landsfordite transforms to nesquehonite at 10 ± 2 °C. Replacement is spontaneous and irreversible (Hill et al., 1982). Nesquehonite also occurs as evaporative films on magnesium-rich alkaline wetland waters (Power et al., 2007) and in association with dypingite and hydromagnesite in lake shore-line and shallow water carbonate hard grounds (Last et al., 2010). Barringtonite has been identified in association with nesquehonite in precipitates derived from weathered basalts (Nashar, 1965). Heating nesquehonite at ca 100 °C yields an unidentified MgCO3 2H2O phase (Ballirano et al., 2010). Magnesium monohydrate carbonate has also been synthesised by desiccating nesquehonite (Menzel and Brückner, 1930). A range of [MgCO3 XH2O] phases (X < 3) have also been synthesised, in which variations in the structural water content were attributed to varying conditions of synthesis (Zhang et al., 2006). The phases show mid-infrared bands matching the internal modes of the ] anion in nesquehonite, but no X-ray diffraction [CO 2 3
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Page 5 and 6: verify the reproducibility of condu
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Page 13 and 14: nesquehonite in [A 20] are followed

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