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[A 120] and [A 240] are devoid of nesquehonite. The spectra also show a low intensity sharp band at 1092 cm 1 (Fig. 4D and E). A similar feature has been described from natural dypingite (Frost et al., 2009). The width of broad Author's personal copy Phase transitions in the system MgO–CO2–H2O 7 Fig. 4. FT-Raman spectra of samples: (A) [AS0]; (B) [A20]; (C) [A80]; (D) [A120]; (E) [A240]; (F) [S240]. [N] denotes a nesquehonite band assignment; [HM] a hydromagnesite band assignment; [Dy-t] a dypingite-type mineral band assignment. The black diamond present in Mg 5(CO 3) 4(OH) 2 XH 2O-rich assemblages marks the area of broad band absorption in the ca 1000–1090 cm 1 region. See text for details. low intensity scattering on the shoulder of the high intensity 1120 cm 1 band is narrower in [A240] than in [A120]. Spectra of precipitates formed under static conditions complement XRD data, showing a variety of bands
assigned to nesquehonite and/or hydromagnesite (e.g., Fig. 4F). No increase in the (1117/1099 cm 1 ) intensity ratio with increasing heating time is evident, suggesting no change in the abundance of nesquehonite relative to hydromagnesite with increasing time. Nevertheless, the intensity ratio is elevated relative to the [A20], [A50] and [A80] spectra, consistent with greater nesquehonite generation in the agitated setting. In the H 2O–OH region, overlapping bands at ca 3156, 3312 and 3437 cm 1 are consistent with water stretching vibrations for synthetic [Mg(HCO3,OH) 2H2O] (Hales et al., 2008). There is no evidence for broad band scattering in the ca 950–1090 cm 1 region, suggesting that this spectral attribute is singular to the dypingite-type phases generated in the agitated setting. 4.4. FT-mid infrared The [AS0] spectrum (Fig. 5A) shows a series of bands assigned to the [CO 2 3 ] anion in hydromagnesite (e.g., Lanas and Alvarez, 2004): 1117 cm 1 (symmetrical stretching vibration); 793, 852 and 885 cm 1 (bending vibrations), 1420 and 1477 cm 1 (anti-symmetric stretching vibrations). Weak absorption at ca 1660 cm 1 (H2O bending vibration) is also compatible with hydromagnesite (e.g., White, 1974). The spectrum also shows a broad concave spectral feature at ca 1000 cm 1 , which occurs in some hydromagnesite and dypingite spectra (e.g., Raade, 1970; White, 1971). This feature has previously been assigned to deformation modes of Mg–OH units (Frost et al., 2008) and solid phase incorporation of bicarbonate (e.g., Zhang et al., 2006). In the H2O–OH region, [AS0] is comparable with published spectra of hydromagnesite (e.g., White, 1971), with absorption at ca 3038 and 3444 cm 1 (H2O stretching vibrations) and super-imposed sharp ([OH ] stretching vibration) bands at 3510 and 3650 cm 1 . Samples [A20] and [A50] show bands at 1098 and 852 cm 1 (e.g., Fig. 5B), consistent with symmetric stretching, and bending of the [CO 2 3 ] anion for nesquehonite (e.g., White, 1974; Lanas and Alvarez, 2004; Zhang et al., 2006; Ferrini et al., 2009). Weak absorption at 888 cm 1 is also evident, consistent with XRD and FT-Raman evidence for hydromagnesite, produced at 25 °C in subordinate concentrations to nesquehonite. The spectra also show bands at ca 1420, 1471, and 1519 cm 1 in keeping with the antisymmetric stretching mode of [CO 2 3 ] for nesquehonite, superimposed on anti-symmetric stretching vibrations of hydromagnesite. Resolution of the individual anti-symmetric stretching modes of nesquehonite also varies in separate studies (e.g., Coleyshaw et al., 2003; Kloprogge et al., 2003; Zhang et al., 2006). Moderate absorption at ca 1648 cm 1 is assigned to the superimposed OH bending mode of H2O for nesquehonite and hydromagnesite (White, 1971). In the H2O–OH region, [A20] and [A50] show a sharp band at 3555 cm 1 and over-lapping bands at ca 3430, 3246 and 3122 cm 1 (Fig 5b). The spectra are dissimilar to [MgCO3 XH2O] phases (Zhang et al., 2006) in which no sharp band absorption occurs at 3555 cm 1 . The four bands do, however, broadly coincide with Raman active bands of synthesised [Mg(HCO3,OH) 2H2O], measured at 3124, 3295, 3423 and 3550 cm 1 of which the first three Author's personal copy 8 L. Hopkinson et al. / Geochimica et Cosmochimica Acta 76 (2012) 1–13 are assigned to water stretching vibrations and the latter to the stretching mode of OH units (Hales et al., 2008). The samples also show broad low intensity band(s) in the ca 2500 cm 1 region which partially overlaps with the characteristic frequency range (2200–2500 cm 1 ) for the bicarbonate ion (White, 1971). Samples [A80] and [A120] contain bands which coincide with the internal modes of the [CO 2 3 ] anion for hydromagnesite and dypingite (Fig. 5C and D). Also evident is an increase in 888 cm 1 absorption intensity relative to [A20] and [A 50], consistent with a reduction in the ratio of nesquehonite to hydromagnesite and dypingite-type phases with increasing heating time. Given that the dominant (or sole) dypingite-type phase identified by XRD in [A 120] is [Mg5(CO3)4(OH)2 8H2O], it seems that the unnamed mineral, like dypingite, shows short-range order akin to hydromagnesite, with respect to the internal modes of the [CO 2 3 ] anion. Sample [A 240], which shows XRD evidence for a second dypingite-type phase, similarly possesses short-range order of the carbonate anion akin to hydromagnesite. In the H 2O–OH region, samples [A 80], [A 120] and [A 240] show the progressive development of broad absorption at ca 2950 and 3430 cm 1 , assigned to water stretching bands, with super-imposed sharp bands at 3510 and 3650 cm 1 , comparable in frequency with hydromagnesite and some dypingite spectra, in which the bands are assigned to stretching vibrations of OH units (e.g., White, 1974; Frost et al., 2008). Samples [A80], [A120] and [A240] show variably resolved broad, low-intensity absorption centered at ca 1000 cm 1 (Fig. 5C–E). The feature coincides with two bands (at 948 and 1012 cm 1 ) identified in dypingite and assigned to deformation modes of Mg–OH units (Frost et al., 2008). The feature is also present in some published spectra of hydromagnesite (e.g., White, 1971). The spectra of precipitates formed under static conditions show a range of bands assigned to nesquehonite and/or hydromagnesite (e.g., Fig. 5F), with the latter mineral contributing more strongly to the spectra, than in samples [A20], [A50], [A80] recovered from the agitated experiment. In the H 2O–OH region of samples formed under static conditions, spectra are similar to nesquehoniterich spectra of samples generated in the agitated environment. Weak absorption at 3655 cm 1 is assigned to OH units in hydromagnesite. 4.5. SEM Samples [AS0] consist of platy (ca 1–2 lm length) hydromagnesite crystals, organised in agglomerates (Fig. 6A). Samples [A 20] and [A 50] are dominated by columnar nesquehonite crystals, ca 5–30 lm in the longest dimension (Fig. 6B). Also present are subordinate quantities of hydromagnesite agglomerates. Sample [A 80] consists of nesquehonite rods, showing etch pits and overgrowths of platy basic carbonates, producing a ‘house of cards’ texture (Fig. 6C). The texture is believed to be related to a dissolution–recrystallization self-assembly growth mechanism, in which unstable, dissolving nesquehonite micro-rods function as templates for hydromagnesite, which in turn act as nucleation points for further hydromagnesite platelets
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