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Frost, Ray L. and Hales, Matthew C. and Martens, Wayde N. (2008) Thermo-Raman spectroscopy of synthetic Nesquehonite – implication for the geosequestration of greenhouse gases. Journal of Raman Spectroscopy, 39(9). pp. 1141-1149.

Thermo-Raman spectroscopy of synthetic Nesquehonite – implication for the geosequestration of greenhouse gases

Matthew C. Hales, Ray L. Frost• and Wayde N. Martens

Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001, Australia.

Abstract

Pure nesquehonite (MgCO3.3H2O)/Mg(HCO3)(OH).2H2O was synthesised and characterised by a combination of thermo-Raman spectroscopy and thermogravimetry with evolved gas analysis. Thermo-Raman spectroscopy shows an -1 -1 2- intense band at 1098 cm which shifts to 1105 cm at 450°C, assigned to the ν1 CO3 -1 symmetric stretching mode. Two bands at 1419 and 1509 cm assigned to the ν3 antisymmetric stretching mode shift to 1434 and 1504 cm-1 at 175°C. Two new peaks at 1385 and 1405 cm-1 observed at temperatures higher than 175°C are assigned to the - antisymmetric stretching modes of the (HCO3) units. Throughout all the ThermoRaman spectra a band at 3550 cm-1 attributed to the stretching vibration of OH units. Raman bands at 3124, 3295, and 3423 are assigned to water stretching vibrations. The intensity of these bands is lost by 175°C. The Raman spectra were in harmony with the thermal analysis data. This research has defined the thermal stability of one of the hydrous carbonates namely nesquehonite. ThermoRaman spectroscopy enables the thermal stability of the mineral nesquehonite to be defined and further the changes in the formula of nesquehonite with temperature change can be defined. Indeed Raman spectroscopy enables the formula of nesquehonite to be better defined as Mg(OH)(HCO3).2H2O.

Keywords: Nesquehonite, Hydromagnesite, Thermogravimetric analysis, Hot-stage Raman Spectroscopy

Introduction

The name Nesquehonite comes from the location of a Coal mine at Nesquehoning, Pennsylvania, USA where the mineral was first found. Nesquehonite exhibits the crystal structure of monoclinic-prismatic. Nesquehonite has the symmetry point group of P21/n. 1 There is some argument in the literature as to the true empirical formula of nesquehonite. Nesquehonite belongs to a group of secondary carbonate minerals known as hydroxy-carbonates to which other mineral variations belong such as hydrozincite. It is commonly quoted as either being (MgCO3.3H2O) or Mg(HCO3)(OH).2H2O. The aim of this study is to determine the formula of the mineral. At a unit cell level, the mineral appears to be a layer structure as seen with other hydroxy-carbonate minerals. Magnesium ions are contained in an octahedrally coordinated site which exhibits distortion. The water of hydration is understood to be located within the layers of the greater structure, according to the molecular models as displayed in Figures 1a and 1b

• Author for correspondence ([email protected])

1 Nesquehonite is found in nature as either prismatic crystals with a slender prism shape or radial crystals which radiate from a nucleation centre without producing stellar forms. Large single crystals of nesquehonite were grown during the synthesis of the mineral in this study. The physical properties of the crystal structure and twining seen in nesquehonite have been discussed in depth by various authors 2-5 but since this is not the focus in this study will not be discussed further at this point even though the empirical chemical formula is being investigated.

Geo-sequestration is a method where by various greenhouse gases such as carbon dioxide (CO2) can be trapped either physically or chemically in systems other than that of the atmosphere in order to prevent the detrimental effects on global warming that greenhouse gases have. The feasibility for various carbonate minerals to provide long term stable CO2 storage options has been explored by various authors 6-11 . Some common methods involved pumping liquefied CO2 into fishers located underground were oil/ gas deposits once existed. One of the main problems with this suggestion is that if there is a rupture of the storage site due to man-made or seismic activity the results could be disastrous.

A well known chemical test referred to as the “limewater” test is the simplest example of how CO2 be trapped as a relatively stable mineral CaCO3. The reaction for the limewater test is as follows: n+ n+ CO 2 (aq) + M (OH)n (aq) + H 2O(l) ⎯⎯→ M CO3(s) + H 2O(l)

CO2 reacts with a metal hydroxide solution in water and forms an insoluble carbonate precipitate. It was found in the literature that alkaline earth metal carbonate hydroxy hydrates are very useful in sequestering CO2 to form stable minerals. Currently there are trials proceeding on the feasibility of pumping CO2 into “mineralising solutions” below the surface to see if vast, stable storage systems can be created. The idea of “mineralising solutions” is not new, it can also be seen in action around the world in the aquatic environment. Oceans, lakes and streams chemically uptake CO2 and form various carbonate minerals in order to control the pH of the water system. In fact there is a great concern that if the atmospheric partial pressure of CO2 increases too much, acidification of the oceans will occur 12, as the concentrations of various metal cations, such as Na+, Ca2+ and Mg2+ in solution decrease.

Raman spectroscopy has proven very useful for the study of minerals 13-15. Indeed Raman spectroscopy has proven most useful for the study of diagentically related minerals as often occurs with carbonate minerals 16-20. Some previous studies have been undertaken by the authors using Raman spectroscopy to study complex secondary minerals formed by crystallisation from concentrated sulphate solutions 13- 35. Previous thermal analysis studies on the decomposition of nesquehonite have been limited and have never been completed with the use of evolved gas analysis or thermal Raman Spectroscopy to plot decomposition spectroscopically 36-41.

The aim of this paper is to present Raman and thermoanalytical studies of nesquehonite and to discuss the spectra from a structural point of view. The paper is a part of systematic studies of vibrational spectra of minerals of secondary origin in the oxide supergene zone and their synthetic analogs.

2 Experimental

Synthesis of nesquehonite

It was found that nesquehonite was preferentially precipitated from solution - when equimolar amounts of 0.5M Mg(NO3)2 and 0.5M 100% HCO3 solutions were mixed drop wise over a period of 10 minutes at controlled reaction temperature of 45oC. At this point it is interesting to note that the precipitate dissolved forming a clear solution. The liquor was left in the centrifuge tube for a week and a new precipitate was found to have formed producing large crystals as seen in the optical microscope images in Figure 2.

X-ray diffraction

XRD analyses were performed on a PANalytical X’Pert PRO® X-ray diffractometer (radius: 240.0 mm). Incident X-ray radiation was produced from a line focused PW3373/10 Cu X-ray tube, operating at 45 kV and 35 mA. The incident beam passed through a 0.04 rad, Soller slit, a ½ ° divergence slit, a 15 mm fixed mask and a 1 ° fixed anti scatter slit. After interaction with the sample, the diffracted beam was detected by an X’Celerator RTMS detector fitted to a graphite post-diffraction monochrometer. The detector was set in scanning mode, with an active length of 2.022 mm. Samples were analysed utilising Bragg-Brentano geometry over a range of 3 – 75 ° 2θ with a step size of 0.02 ° 2θ, with each step measured for 200 seconds.

Thermal analysis

Thermal decomposition of the nesquehonite was carried out in a TA® Instruments incorporated high-resolution thermo-gravimetric analyzer (series Q500) in a flowing nitrogen atmosphere (90cm3/min). Approximately 30 mg of sample underwent thermal analysis, with a heating rate of 5°C/min, resolution of 6, to 1000°C. With the quasi-isothermal, quasi-isobaric heating program of the instrument the furnace temperature was regulated precisely to provide a uniform rate of decomposition in the main decomposition stage. The TGA instrument was coupled to a Balzers (Pfeiffer) mass spectrometer for gas analysis. Only water vapour, carbon dioxide and oxygen were analyzed.

Thermal Raman Spectroscopy

The crystals of nesquehonite were placed and oriented on the stage of an Olympus BHSM microscope, equipped with 10x and 50x objectives and part of a Renishaw 1000 Raman microscope system, which also includes a monochromator, a filter system and a Charge Coupled Device (CCD). Raman spectra were excited by a HeNe laser (633 nm) at a resolution of 2 cm-1 in the range between 100 and 4000 cm-1. Details of the technique have been published by the authors 42-45. Spectra at elevated temperatures were obtained using a Linkam thermal stage (Scientific Instruments Ltd., Waterford Surrey, England). Spectra were taken at 25oC intervals up to a temperature of 200oC in order replicate the acquisition of data in the TGA-MS plots. Intervals of 50oC were used where there was no evident change in mass or the mass derivative. Acquisition intervals of 25oC were resumed from 350-425oC in

3 order to record changes in the spectra as the sample began to thermally decompose. Spectral Manipulation such as baseline adjustment, smoothing and normalisation was performed using GRAMS® software package (Galactic Industries Corporation Salem, NH, USA)

Results and Discussion

Synthesis of nesquehonite

Synthetic nesquehonite used in this thermal stability study was synthesised by a wet chemical precipitation method similar to that published by Kloprogge et al. into the synthesis of nesquehonite 46. Other synthesis methods have been used 47-50 but are similar. The literature contains a number of methods for the synthesis of pure nesquehonite 5,46,50,51. Various authors 5,50,52-54 have discussed the effects of temperature and partial pressure of carbon dioxide pCO2 on the stability of the synthetic nesquehonite.

It is hypothesised that the initial precipitate was MgHCO3 although subsequent experiments failed to trap the initial precipitate for analysis. The following reactions are envisaged.

Mg(NO3)2 + 2NaHCO3 + H2O ⎯⎯→ MgHCO3 - Mg(HCO3)2 + 3H2O ⎯⎯→ Mg(HCO3)(OH).2H2O + HCO3

Selective precipitation of the mineral by varying the carbonate buffering - 2- 2- (HCO3 /CO3 ) solution was also trialled but as the CO3 ratio increased a mixture of nesquehonite / hydromagnesite was synthesised until single phase hydromagnesite 2- was precipitated at 100% CO3 . At this point it should also be noted that due to the differences in structural formula (i.e. ratio of Mg ions in the crystal lattice) of nesquehonite [Mg(HCO3)(OH).2H2O] and hydromagnesite Mg5(CO3)4(OH)2·4(H2O) the yield of hydromagnesite was significantly lower due to less available Mg2+ ions.

A thermal stability study conducted Lanas et al. 55 discussed the inherent difficulty associated with studying the MgO-CaO-H2O-CO2 system as there are multiple complex minerals which can form during the dolomitization process. Samples produced were characterised for phase specificity using XRD, IR and Raman Spectroscopy.

The X-ray diffraction patterns of the synthesised nesquehonite and its reference pattern are shown in Figure 3. Clearly the mineral synthesised is nesquehonite.

Thermal Analysis of nesquehonite

The thermal analysis of synthetic nesquehonite is shown in Figure 4. The ion current curves for the gas evolution from the thermal decomposition of nesquehonite are shown in Figure 5. The thermo-gravimetric pattern shows a steady three step mass loss starting at 55°C with two steps at 109 and 160°C with mass losses of 14.90, 10.07 and 8.71% respectively. The total mass loss from 55 to 300oC was found to be 34.82%.

A further significant mass loss of 33.81% is observed at 400oC. A further mass loss of approximately 2.5% occurred until a constant mass is obtained. The total mass loss for the analysis is 71.15% which is identical to the predicted 71% theoretical mass loss from nesquehonite according to the following reaction mechanisms:

- H2O Mg(HCO3)(OH).2H2O ⎯⎯→⎯ Mg(HCO3)(OH).H2O …..……………...…..……(1)

- H2O Mg(HCO3)(OH).H2O ⎯⎯→⎯ Mg(HCO3)(OH)…….………………………………(2)

- H2O Mg(HCO3)(OH) ⎯⎯→⎯ MgCO3……………………………..………....…………(3)

-CO2 MgCO3 ⎯⎯→⎯ MgO………………………………………………………….……(4)

The DTG curve initially shows a large sharp peak at 86.5°C, a second broader and significantly less intense broader peak at 123°C and a third very broad peak at 176oC. Finally there is a forth large and broad peak seen at 396.5oC.

The ion current curves for m/Z 16, 17 and 18 are shown in

Figure , these curves show a loss of H2O and OH units at 125 and 185°C and reflect the decomposition seen in the DTG curve at similar temperatures. The ion current curves for m/z = 44 corresponding to the mass of CO2 shows one maxima at 400°C. This value corresponds to the peak in the DTG pattern. The decomposition products were subjected to thin film XRD analysis on low background quartz plates, which gave a positive match for MgO, which agrees with the theoretical and actual mass losses recorded in the thermal analysis experiment.

Thermal Raman Spectra

Nesquehonite forms a complex layered structure is observed with other hydroxy-carbonate minerals forming alternating ribbons of magnesium and carbonate ions along the fibre axis. Magnesium ions are contained in an octahedrally 2- coordinated site which exhibits some signs of lateral distortion in one specific CO3 . 2- This distortion is believed to be due to perturbations of the carbonate (CO3 ) caused by hydrogen bonding to both hydroxyl (OH-) and water of hydration molecules. Molecular models demonstrating the structure of nesquehonite are shown in Figure 1. Water of hydration is understood to be located with in the layers of the greater structure. It should be noted that a slight distortion in one of the co-ordinating carbonates as seen in the model on the left in Figure 1 (off axis).

Carbonates in a range of minerals have been studied by Raman spectroscopy 21,23,27,31,42,56,57. It is important to understand the spectroscopy of the carbonate ion. 2- The free ion, CO3 with D3h symmetry exhibits four normal vibrational modes; a symmetric stretching vibration (ν1), an out-of-plane bend (ν2), a doubly degenerate asymmetric stretch (ν3) and another doubly degenerate bending mode (ν4). The symmetries of these modes are A1´ (R) + A2´´ (IR) + E´ (R, IR) + E´´ (R, IR) and occur at 1063, 879, 1415 and 680 cm-1 respectively. Generally, strong Raman modes

5 -1 appear around 1100 cm due to the symmetric stretching vibration (ν1), of the carbonate groups, while intense IR and weak Raman peaks near 1400 cm-1 are due to -1 the antisymmetric stretch (ν3). Infrared modes near 800 cm are derived from the out- -1 of-plane bend (ν2). Infrared and Raman modes around 700 cm region are due to the 2- in-plane bending mode (ν4). This mode is doubly degenerate for undistorted CO3 groups. As the carbonate groups become distorted from regular planar symmetry, this mode splits into two components. Infrared and Raman spectroscopy provide sensitive 2- test for structural distortion of CO3 . Farmer stated that the overall results expected from this phenomenon is a progression from normally sharp bands, distinctive to carbonate proceed into more broad and shifted bands resulting in diffuse spectra 1. However, even though the effects of hydrogen bonding are apparent in vibrational spectroscopy of hydrated magnesium carbonates, it is not expected to greatly affect 2- the internal modes of the CO3 in nesquehonite, so sharp bands are still expected.

Nesquehonite exhibits a space group of P21/n with a factor group of C2h. 2- White states the expected fundamental internal modes of the CO3 in nesquehonite -1 2- -1 are as follows, v1 1097cm symmetric stretching of the CO3 ion, v2 852 cm and v3 -1 2- 1518, 1469, 1415 cm antisymmetric stretching of the CO3 ion respectively. A -1 58 water bending mode is expected at 1640 cm . Data for the v4 mode of nesquehonite was not forthcoming in the treatise by Farmer 1. At this point it should be noted that Raman spectra of nesquehonite and other selected hydrated and hydroxyl magnesium carbonates, were not reported in Farmer’s study probably due to the disordered nature of the fine powdered samples.

The Raman spectra of nesquehonite in the 600 to 1200 cm-1 region as a function of temperature are shown in Figure 6. As expected the symmetric stretching 2- -1 mode of the CO3 at 1098cm is very intense with respect to the other peaks in the Raman spectra. The intensity of the peak remains very intense even as the temperature increases. However, there is a definite broadening at the base of the peak with temperature increase. Further there is a (blue) shift in the peak position from 1098 cm-1 at 50 °C to 1105 cm-1 at 450 °C. The intensity of the peak decreases significantly after 350oC and has all but disappeared at 425oC and no intensity remains by 450oC. This observation is in harmony with both the TGA and MS data (as reported above). The Raman band which might be expected to be observed at -1 around 870 cm (ν2 in-plane bending mode) is not observed in the thermoRaman -1 spectra but two bands at 765 and 699 cm assigned to the ν4 out of plane bending mode are readily observed in the thermo-Raman spectra even though the spectra are of a low intensity. The intensity of these bands approaches zero by 300°C.

Vibrational spectroscopic studies of carbonate minerals have been undertaken over an extended period of time 59-61. Adler and Kerr showed that differences in the infrared absorption spectra of carbonates were a function of cation size 59. Raman studies also have been forthcoming but not recently 62-64. Farmer reported the vibrational wavenumbers of calcite structured minerals (table 12. IX page 239) 65. In -1 this table the band positions for smithsonite were listed as 1093 cm (ν1 symmetric -1 -1 stretching mode), 1412 and 1440 cm (ν3 antisymmetric stretching mode), 870 cm -1 (ν2 in-plane bending mode), 733 and 743 cm (ν4 out of plane bending mode) with lattice modes at 307, 200, 165 cm-1. Some variation of peak positions is found in the literature.

6 The Raman spectra of nesquehonite in the 1200 to 1800 cm-1 region as a function of temperature are shown in Figure 7. The two bands at 1419 and 1509 cm-1 are assigned to the ν3 antisymmetric stretching mode. The first band shifts to 1434 cm-1 at 250°C and the second band is observed at 1504 at this temperature. White 58 reported a third band at 1470cm-1. This band is not observed in our spectra. Significant changes are observed in the spectra at 175°C. According to our calculations from the TG patterns, three moles of water are lost in two steps. Thus the dehydration steps may be written as Mg(HCO3)(OH).2(H2O) → Mg(HCO3)(OH).(H2O) + H20 at 157°C and Mg(HCO3)(OH).(H2O) → Mg(HCO3)(OH) + H20 at 180°C According to the DTA patterns of Beck 41 (page 995 of this reference), water of crystallisation is lost in two steps. The reaction begins at 140°C and is complete by 300°C. The dehydration steps as reported is in close agreement with the values of Beck 41. Thus after 175°C in the Raman spectra, the compound being studied is -1 Mg(HCO3)(OH). Thus the appearance of new peaks at 1385 and 1405 cm are - attributed to the antisymmetric stretching modes of the (HCO3) units. The intensity of these bands approaches zero by 425°C.

Further the band at 1700 cm-1 is assigned to the water bending mode (Figure 7). The band shifts to 1688 cm-1 at 175°C after which temperature, the intensity in the band approaches zero. The water bending mode is normally observed for liquid water at 1625 cm-1 and for water in the vapour phase at 1595 cm-1. For water which is hydrogen bonded the band is observed in the 1640 to 1650 cm-1 range. The fact that the water bending mode is found at 1700 cm-1 implies that the water is very strongly bonded, in this case to the Mg2+ cation. The loss of intensity in the band by 175°C supports the concept that no water molecules are present at this temperature and that the thermally decomposed nesquehonite is Mg(HCO3)(OH).

Confirmation of the strong hydrogen bonding by water in the nesquehonite structure may be confirmed by the study of the spectra in the OH stretching region. The Raman spectra in the 2500 to 3700 cm-1 region are shown in Figure 8. In the Raman spectrum at 25°C four bands are resolved at 3124, 3295, 3423 and 3550 cm-1. The first three bands are attributed to water stretching vibrations. The last band is assigned to the stretching mode of the OH units. It is noted that the intensity of the water band is lost by 175°C. This confirms the observations made from the HOH bending modes. No water molecules are observed in the thermally decomposed nesquehonite structure after 175°C. The band at 3550 cm-1 shows a strong red shift with thermal treatment. The band is observed at 3578 cm-1 at 175 °C and at 3590 cm-1 at 400 °C. Nesquehonite appears to exhibit similar spectra in the water/ hydroxyl - (H2O/OH ) region to that of the other hydrated magnesium hydroxyl-carbonates such as hydromagnesite and artinite.

The presence of the distinct OH vibrations at elevated temperatures is significant and it suggests that the formula of the synthetic sample used in this study has the structural formula of Mg(HCO3)(OH).2H2O, the magnesium hydrogen- carbonate hydroxy, dihydrate. If the formula of nesquehonite which is commonly quoted in the literature as MgCO3.3H2O, then one would not expect the distinct OH band to be found. This is not the case. The study by White does not show evidence of bicarbonate ions in nesquehonite 58. This evidence suggests the possibility of there

7 being a structural isomer of the mineral either MgCO3.3H2O or in this case Mg(HCO3)(OH).2H2O.

Sequestration of green house gases

In order to understand the potential sequestration of green house gases through reaction with magnesium minerals it is important to have fundamental knowledge of the reactions of Mg(OH)2 and MgO with CO2. The following reaction is envisaged: MgO +CO2 → MgCO3 and MgO +CO2 + H2O → Mg(OH)(HCO3).2H2O (nesquehonite) It is important to understand the stability of such minerals both in terms of temperature and partial pressure of the CO2. The hydration-carbonation or hydration- and-carbonation reaction path in the CO2 —MgO—H2O system at ambient temperature and atmospheric CO2 is of practical significance from the standpoint of carbon balance and the removal of green house gases from the atmosphere. A better understanding of the global masses of Mg and CO2 and the thermal stability of the hydrated carbonates of magnesium provide a practical understanding for carbon dioxide removal. From a practical point of view, the exact knowledge of the reaction path in MgO—CO2—H2O system is of great significance to the performance of Mg(OH)2 and related minerals for green house gas removal. The reaction path involving carbonation of brucite (Mg(OH)2) is particularly complex, as Mg has a strong tendency to form a series of metastable hydrous carbonates. These metastable hydrous carbonates include hydromagnesite (Mg5(CO3)4(OH)2.4H2O, or Mg4(CO3)3(OH)2.3H2O), artinite (Mg2CO3(OH)2.3H2O), nesquehonite (MgCO3.3H2O), and lansfordite (MgCO3.5H2O). The free energy of formation for these hydroxy and hydrous carbonates differs and their formation will depend on the partial pressure of CO2.

Conclusions

This research has defined the thermal stability of one of the hydrous carbonates namely nesquehonite. The stability is limited to a maximum temperature of around 400°C. The mineral decomposes in steps at ~50, 175 and 400°C. ThermoRaman spectroscopy enables the thermal stability of the mineral nesquehonite to be defined and further the changes in the formula of nesquehonite with temperature change can be defined. Indeed Raman spectroscopy enables the formula of nesquehonite to be better defined as Mg(OH)(HCO3).2H2O.

The hydration-carbonation or hydration-and-carbonation reaction path in the MgOCO2H2O system at ambient temperature and atmospheric CO2 is of environmental significance from the standpoint of carbon balance and the removal of green house gases from the atmosphere. The understanding of the thermal stability of the carbonates of magnesium and the relative metastability of hydrous carbonates including hydromagnesite (Mg5(CO3)4(OH)2.4H2O, or Mg4(CO3)3(OH)2.3H2O), artinite (Mg2CO3(OH)2.3H2O), nesquehonite (MgCO3.3H2O), and lansfordite (MgCO3.5H2O) is extremely important to the sequestration process for the removal of atmospheric CO2.

Acknowledgments

8 The financial and infra-structure support of the Queensland University of Technology, Inorganic Materials Research Program is gratefully acknowledged. The Australian Research Council (ARC) is thanked for funding the instrumentation.

9 References

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12 List of Figures

Figure 1 Nesquehonite Model off axis (left) and viewed down a axis (right)

Figure 2 Optical microscopic images of synthetic nesquehonite crystals produced

Figure 3 XRD patterns of nesquehonite and its reference pattern

Figure 4 TGA/DTG Nesquehonite

Figure 5 Ion current curves of evolved gases from the thermal decomposition of nesquehonite

Figure 6 Raman spectra of nesquehonite in the 600 to 1200 cm-1 region

Figure 7 Raman spectra of nesquehonite in the 1200 to 1800 cm-1 region

Figure 8 Raman spectra of nesquehonite in the 2500 to 3800 cm-1 region

Figure 2 Nesquehonite Model off axis (left) and viewed down a axis (right)