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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] 


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

(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- 


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 



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

hydromagnesite. The dypingite-like structure appears to 

have a smaller cell volume than the unnamed mineral and 

the 5H2O version of dypingite, as all peaks are shifted to 

smaller d-spacings (Fig. 3E). Hereafter, excluding hydromagnesite, 

[Mg5(CO3)4(OH)2 XH2O] minerals where 

X = 11, 8, 6, 5 or less are collectively referred to as dypingite-type 

mineral phases. The full-width at half-maximum 

values (FWHM) of the dypingite-type minerals are wider 

than hydromagnesite, suggesting greater long-range disorder 

in the dypingite-type minerals. Hydromagnesite shows 

similar FWHM throughout, even though the peaks have 

different relative intensities, as monitored by the peak at 

9.6 degrees. The static experiment precipitates show 

nesquehonite-rich hydromagnesite bearing assemblages. 

There is no evidence of a shift in the abundance of nesquehonite 

relative to hydromagnesite with time, or the appearance 

of dypingite-type phases. 



Fig. 2. Experimental results for: (A) electrical conductance measurements (agitated environment symbol: , static environment symbol: 

). AAS analyses of [Mg 2+ ]tot/(M) concentrations (static environment symbol: , agitated environment symbol: ); (B) pH measurements, 

(agitated environment symbol: , static environment symbol: ). Agitated environment solid phase samples are: [AS0], [A20], [A50], [A80], 

[A 120] and [A 240]. Static environment samples are: [S 60], [S 120], [S 180], and [S 240]. 

4.3. FT-Raman 

All FT-Raman spectra contained significant noise, particularly 

those acquired from the agitated experiment. This 

relates to the acknowledged difficulty in analysing disordered 

magnesium hydrate and hydroxyl carbonates (e.g., 

White, 1974; Frost et al., 2009), compounded by the rapid 

synthesis under our experimental conditions. The [AS0] 

spectra (Fig. 4A) show a high intensity band at 1117 cm 1 , 

consistent with the v1 internal mode of [CO 2 

3 ] for hydromagnesite 

(e.g., Edwards et al., 2005) and a range of low 

intensity bands assigned to hydromagnesite. In the 2500– 

3800 cm 1 (H2O–OH) region, [AS0] shows an asymmetric 

band at ca 3660 cm 1 . The spectra of [AS0] samples collected 

after 5, 8, and 19 h of CO2 sparging are identical, suggesting 

that hydromagnesite precipitated directly from the 

parent solution.

The [A20] and [A50] spectra are very similar, showing a 

high intensity band at 1098 cm 1 , with a shoulder at 

1117 cm 1 , plus a variety of low intensity bands assigned 

to nesquehonite and or hydromagnesite (e.g., Fig. 4B). 

Hence, results of the FT-Raman analysis support XRD evidence 

for a transition from hydromagnesite to nesquehonite 

formation. The spectra show broad bands centred at ca 

1480 and 1780 cm 1 . Bands at similar frequencies have 

been assigned to the anti-symmetric stretching of [CO 2 

3 ] 

and the water bending mode for nesquehonite (Hales 

et al., 2008). In the H2O–OH region, the asymmetric feature 

between ca 3000–3500 cm 1 contains poorly resolved overlapping 

bands at ca 3440, 3310 and 3156 cm 1 , the disposition 

of which are broadly compatible with synthetic 

[Mg(HCO3,OH) 2H2O] (e.g., Hales et al., 2008). A subsample 

of [A20] was analysed periodically over five days 



Fig. 3. X-ray diffraction pattern of samples: (A) [AS 0]; (B) [A 20]; (C) [A 80]; (D) [A 120]; (E) [A 240]. The star marked on the stick diagram of 

[Mg5(CO3)4(OH)2 8H2O], is the main peak enabling discrimination of the mineral phase from dypingite and hydromagnesite. 

of drying. The 1098/1117 cm 1 intensity ratio was uniform, 

suggesting that the loss of nesquehonite at the expense of 

[Mg5(CO3) 4(OH) 2 XH2O] phases was solely solventmediated. 

The [A80] spectrum shows an elevated (1117/1098 cm 1 ) 

intensity ratio relative to [A20] and [A50](Fig. 4C), consistent 

with the presence of hydromagnesite generated at 25 °C, 

combined with nesquehonite dissolution, and coeval formation 

of dypingite-type phases. The [A80] and [A120] spectra 

show broad low intensity asymmetric scattering in the ca 

1000–1090 cm 1 region which is multi-component in nature 

(Fig. 4D). The lower frequency region coincides with Mg– 

OH deformation vibrations in dypingite (Frost et al., 

2009). Low intensity scattering in the 1070 cm 1 region is 

consistent with the [CO 2 

3 ] symmetric stretching mode 

(Frost et al., 2008). In keeping with XRD analysis, samples

[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 



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 



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 hydromag- 

nesite 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 hydro- 

magnesite, 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

(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- 



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 



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

abundance [Mg(HCO3,OH) 2H2O] is dominant, possibly to 

the exclusion of the tri-hydrate. It seems unlikely that the 

temperature of synthesis plays a pivotal role in determining 

whether basic or hydrate nesquehonite forms, as both variants 

have been synthesised at low temperatures, and both 

occur in natural near-surface ambient temperature settings 

(e.g., Kazakov et al., 1959; Coleyshaw et al., 2003; Hales 

et al., 2008). Accordingly, it is plausible that the pH influences 

the extent to which HCO 3 is incorporated into 

nesquehonite or which isomer of nesquehonite is produced. 

In this respect, it is interesting to note that the synthesis of 

nesquehonite [MgCO3 XH2O] phases by Zhang et al. (2006) 

occurred at pH values of 8.5–12.5. In contrast, in the 

experiment documented here, nesquehonite synthesis was 

achieved at pH < 8, yet the temperatures of synthesis and 

timeframes for mineral formation in the two studies are 

comparable. 

The solubility of dypingite-type phases is not known 

with any degree of certainty. Nevertheless, XRD results 

indicate that the system was supersaturated with dypingite-type 

phase(s) 20 min after the beginning of the heating 

stage, coincident with the onset of very low levels of undersaturation 

in nesquehonite (Fig. 7). The progressive emergence 

of dypingite-type phases is associated with particles 

heterogeneously nucleating on decomposing nesquehonite, 

giving rise to house of cards textures. Heterogeneous nucleation 

may be responsible for Ostwald step rule behaviour, 

as the next most stable phase is often more structurally similar 

to the precursor phase than a thermodynamically more 

stable phase (Morse and Casey, 1988). 

Nesquehonite dissolution and dypingite-type mineral 

formation are more extensive in the agitated environment, 

relative to the static environment. Strong hydrodynamic 

shear forces generated by sonication can increase the rate 

of dissolution of suspended solids by de-agglomeration, 



Fig. 7. Agitated experiment saturation indices. Note, there are no available thermodynamic data for dypingite-type phases. See text for 

details. 

simultaneously accelerating the formation of viable nuclei 

to increase the rate of crystallization of carbonate mineral 

phases (e.g., Kim et al., 2011). No protohydromagnesite 

was identified in this study, either because it was rapidly 

superseded by [Mg5(CO3)4(OH)2 XH2O] phases, or because 

its formation was prohibited by the nature of the nesquehonite 

precursor. This may be due to conditions of synthesis 

or incongruent water loss prior to complete dissolution. 

Samples [AS0] (hydromagnesite), the static environment 

powders, and samples [A80], [A120] and [A240] (dypingitetype 

and hydromagnesite bearing) all show pronounced 

broad band infrared absorption in the ca 1000 cm 1 region, 

assigned in large measure to Mg(OH) deformation modes. 

The hydromagnesite spectra [AS0] and static environment 

spectra are devoid of the corresponding Raman active 

band(s), although the bands are clearly resolved in the 

FT-Raman spectra of the three precipitates formed in the 

agitated environment. These attributes are consistent with 

reduced symmetry and therefore greater disorder in the 

dypingite-type phases relative to hydromagnesite and, thus, 

the greater ease of crystallization of these phases relative to 

hydromagnesite. The greater numbers of waters of crystallization 

of [Mg5(CO3) 4(OH) 2 8H2O] relative to hydromagnesite 

evidently imparts distinct unit cell parameters, 

simultaneously affecting Mg(OH) deformation modes, yet 

retaining essentially uniform short-range order of the 

[CO 2 

3 ] anion with respect to hydromagnesite. The reduction 

in FT-Raman intensity of scattering in the ca 

1000 cm 1 region in [A240] relative to [A80] and [A120] is 

in keeping with decreasing disorder of dypingite-type 

phases with increasing heating (reaction) time, whereas 

the smaller d-spacing of [A240] relative to [A120] is attributed 

to cell shrinkage with decreasing waters of crystallization. 

Experimental results indicate that, with increasing reaction 

time, small amounts of dypingite in association with

nesquehonite in [A 20] are followed by significant quantities 

of [Mg5(CO3)4(OH)2 8H2O] and, in turn, progressively 

superseded by dypingite-type minerals, with waters of crystallization 

of five or less, accompanied by the loss of resolvable 

nesquehonite. These attributes are consistent with the 

solvent-mediated [N ! HM] transition proceeding through 

the generation of multiple dypingite-type intermediates, in 

which the rates of formation and transformation of metastable 

intermediate to metastable intermediate changes with 

time. The overall decrease in solid phase disorder and water 

content with increasing time is in keeping with the generation 

of dypingite-type phases providing a mechanism which 

serves to minimize entropy production. Thereby, the 

[N ! HM] transition obeys the Ostwald (1897) step rule 

for successive reactions (e.g., van Santen, 1984). 

6. CONCLUSIONS 

The solvent-mediated [N ! HM] transition is shown to 

be mixed diffusion and reaction controlled, with dypingitetype 

phases initially developing from the dissolution products 

of nesquehonite. The temporal existence of different 

dypingite-type phase dominated assemblages is consistent 

with the transition involving a suite of reactions for which 

the sequence of reaction intermediates is controlled by the 

reaction rates, partly accounting for the variety of hydromagnesite-like 

[Mg 5(CO 3) 4(OH) 2 XH 2O] phases, reported 

in separate studies of the [N ! HM] transition (e.g., Davies 

and Bubela, 1973). The system was highly supersaturated 

and seeded with hydromagnesite, throughout the period 

of nesquehonite formation and transformation. Hence, it 

is likely that hydromagnesite continued to form directly 

from the parent solution and indirectly via the [N ! HM] 

transition. Accepting this, it follows that parallel reaction 

pathways existed, with the occurrence of nesquehonite 

and dypingite-type phases controlled by the rates of the direct 

and indirect pathways to hydromagnesite. 

Given that dypingite with five or less waters of crystallization 

supersedes the metastable intermediate with eight 

waters of crystallization, it is possible that dypingite-type 

phases lie progressively further from the free energy of hydromagnesite 

with increasing compositional approach to 

nesquehonite. Nevertheless, nesquehonite varies in composition, 

be it imparted by conditions of synthesis (e.g., Zhang 

et al., 2006) or variable desiccation (e.g., Menzel and 

Brückner, 1930). Further, the degree of congruency of 

nesquehonite dissolution is uncertain. Therefore, the overall 

[N ! HM] transition may not be a simple series of reactions, 

in which all possible dypingite-type intermediates 

are generated. 

ACKNOWLEDGEMENTS 

Carbon Connections are thanked for financial support, Grant 

0038. Dr. A. Mucci is thanked for his editorial handling of this 

work, which has led to its significant improvement. Two anonymous 

reviewers and Dr. M. Prieto are likewise thanked for their 

significant positive contributions to this study. Dr. Martin Smith 

is also gratefully thanked for his involvement. 



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