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(Hao and Du, 2009). Development of the texture hinges on the balance between the dissolution rate of nesquehonite and the precipitation rate of hydromagnesite, the latter process occurring at a slower rate than nesquehonite dissolution (Hao and Du, 2009). Given that dypingite-type phases appear to grow at the expense of nesquehonite and that [A120] shows pronounced house of cards textural development, in the absence of nesquehonite and the pres- Author's personal copy Phase transitions in the system MgO–CO2–H2O 9 Fig. 5. Mid-infrared spectra of experimental precipitates, for samples: (A) [AS0]; (B) [A20]; (C) [A80]; (D) [A120]; (E) [A240]; (F) [S240]. Between 1900 and 2400 cm 1 the spectra contain ATR related diamond spectra (e.g., Grice et al., 1991). [N] denotes a nesquehonite band assignment; [HM] a hydromagnesite band assignment; [Dy-t] a dypingite-type band assignment. The black diamond present in Mg 5(CO 3) 4(OH) 2 XH 2Orich assemblages marks the area of broad band absorption in the ca 1000–1090 cm 1 region. See text for details. ence of dypingite-type phase(s) (Fig. 6D), it follows that the growth mechanism may be applicable to a range of dypingite-type phases. Static environment samples are dominated by nesquehonite rods, which range from ca 50–200 lm in the longest dimension, in association with small quantities of platy agglomerates of carbonates texturally akin to [AS0]. The samples are devoid of evidence for overgrowth of nesquehonite in the form of platy basic carbonates, or
evidence of partial nesquehonite dissolution, such as etch pits (Fig. 6E and F). 5. DISCUSSION Fig. 7 shows the solution saturation indices for mineral phases relevant to this study, calculated using Geochemists Workbench Ò and data from the agitated experiment. The model indicates that the solution was close to saturation with respect to brucite and supersaturated with respect to magnesite throughout the 120 min of reaction at 58 °C. At room temperature, the solution is assumed to be in equilibrium with hydromagnesite and is undersaturated with nesquehonite. Subsequently, thermally-accelerated CO 2 degassing rapidly promoted the precipitation of nesquehonite [Mg(HCO3,OH) 2H2O]. At this juncture, the parent solution was simultaneously slightly supersaturated with respect to nesquehonite and strongly supersaturated with respect to hydromagnesite. Nevertheless, our experimental Author's personal copy 10 L. Hopkinson et al. / Geochimica et Cosmochimica Acta 76 (2012) 1–13 Fig. 6. Back-scattered images of experimental precipitates. (A) Agglomerate of hydromagnesite crystals from sample [AS0]. (B) Acicular nesquehonite crystals with subordinate hydromagnesite agglomerates Sample [A 20]. (C) Overgrowths of platy carbonates on nesquehonite rod-shaped crystals [A 80]. XRD indicates the presence of nesquehonite, dypingite-type phase(s) and hydromagnesite in the sample. (D) Sample [A120] displaying a well-developed house of cards texture. Sample [A240] is texturally indistinguishable from [A120]. (E) Sample [S120] shows nesquehonite crystals in association with agglomerates of platy crystals texturally comparable to sample [AS0]. (F) Sample [S240] is texturally comparable to [S 120], with no evidence for development of platelets on nesquehonite surfaces. results indicate that nesquehonite was kinetically favoured. This is presumably because metastable reaction products with simpler structures form more rapidly than the more complicated although thermodynamically more stable phase (e.g., Goldsmith, 1953; Morse and Casey, 1988) even though the system was also seeded with hydromagnesite. Hence, growth of hydromagnesite directly from the parent solution most likely continued, albeit at a greatly reduced rate relative to nesquehonite. Results show a greater yield of nesquehonite in the agitated setting and the finer size range of nesquehonite precipitates relative to precipitates obtained from the static environment. The short-lived negative excursion in the pH in the agitated experiment likely results from the enhanced production of CO 2 upon precipitation of the carbonate mineral. The multi-minerallic make up of the precipitates dictates that no unambiguous conclusion concerning the presence of [MgCO 3 3H 2O] can be drawn. Nevertheless, available evidence suggests that, where nesquehonite occurs in
Page 1 and 2: This article appeared in a journal
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Page 13 and 14: nesquehonite in [A 20] are followed

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