Carbonate Minerals by Solid-State 13 C NMR. Spectroscopy

Transcription

1 Quantitative Identification of Metastable Magnesium Carbonate Minerals by Solid-State 13 C NMR Spectroscopy Jeremy K. Moore, J. Andrew Surface, Allison Brenner, Philip Skemer, Mark S. Conradi, Sophia E. Hayes This supporting information contains 13 pages, 2 tables, and 9 figures. S1

2 The magnesium carbonate minerals presented here are summarized in Table S1. Table S1: Summary of metastable magnesium carbonate minerals. Mineral Formula Formula H 2 O/Mg ratio Magnesite MgCO 3 0 Hydromagnesite 4MgCO 3 Mg(OH) 2 4H 2 O 0.8 Dypingite 4MgCO 3 Mg(OH) 2 (5-8)H 2 O Nesquehonite Mg(OH)(HCO 3 ) 2H 2 O 2 The assignment of the magic angle spinning (MAS) NMR resonances to their corresponding static chemical shift anisotropy (CSA) powder patterns was done by slow spinning MAS NMR. Spinning rates (ν R ) of 1.3 khz were used. This method uses the spinning side band pattern of the isotropic chemical shift to map out the CSA powder pattern. The spinning side bands appear at intervals of ν R extending out from the peak for the isotropic chemical shift. The area under the peak of the spinning side band is determined by the area under the CSA powder pattern at that frequency. The slow spinning MAS NMR spectra are shown for magnesite, hydromagnesite, and nesquehonite in Figure S1. S2

3 Figure S1: 13 C{ 1 H} MAS NMR at slow spinning speed (ν R = 1.3 khz) of the synthesized minerals. A) magnesite. B) hydromagnesite. C) nesquehonite. The slow spinning MAS NMR spectrum for the dypingite sample was unable to be recorded because the sample converted during the time between the original data and the slow spinning data being acquired. The sample was left in a capped vial, under no special atmosphere, so effects such as humidity could have affected the outcome of the transformation. Estimates of the error in the fits of the CSA powder patterns are given in Table S2 for the isotropic chemical shift, asymmetry, and anisotropy. These values are able to be calculated S3

4 through Monte Carlo analysis within DMFIT. The error in the CSA principal values will be less than or equal to the error of the isotropic shift. Table S2: Summary of the error in the CSA powder pattern fits. Minerals Magnesite Hydromagnesite Dypingite Nesquehonite 13 C δ iso (ppm) Standard Deviation 13 C η Standard Deviation 13 C δ aniso (ppm) Standard Deviation Characterization of the synthesized magnesite from pxrd and Raman spectroscopy is shown in Figure S2 A and B. Reference patterns for magnesite and forsterite (Mg 2 SiO 4 ) are shown below the pxrd pattern. The Raman spectroscopy peak at 1094 cm -1 is consistent with the ν 1 symmetric stretching vibration of CO 3 2- of magnesite. 1 S4

5 Figure S2: Synthesized 13 C-enriched magnesite (MgCO 3 ) analyzed by A) Raman spectroscopy and B) pxrd. Reference pxrd patterns are shown for C) magnesite (PDF# ) and D) forsterite (PDF# ). The synthesized hydromagnesite was co-mixed with a carbon-free powder, forsterite (Mg 2 SiO 4 ), for filling the rotor used for 13 C NMR measurements prior to the pxrd studies. Therefore, forsterite is seen as a contaminant in this sample. The pxrd and Raman spectroscopy characterization of the synthesized hydromagnesite is shown in Figure S3 A and B. Reference patterns for hydromagnesite and the co-mixed mineral, forsterite, are shown below the pxrd pattern. The peak at 1119 cm -1 in the Raman spectrum is consistent with the ν 1 symmetric stretching vibration of hydromagnesite. 1 S5

6 Figure S3: Synthesized 13 C-enriched hydromagnesite analyzed by A) Raman spectroscopy and B) pxrd. Reference pxrd patterns are shown for C) hydromagnesite (PDF# ) and D) forsterite (PDF# ). Figure S4 shows a 13 C{ 1 H} CPMAS NMR spectra of hydromagnesite with a short contact time. Here the resonance at low frequency splits into two distinct peaks which indicate that there are multiple configurations of this carbon site due to disorder in the crystal. Since the time constants give information about the nearby protons, we can gather that some of the 13 C atoms are closer to a proton than others. The contact time of 250 µs is relatively short in the CPMAS experiment and when τ cp is lengthened, a single peak at ppm is once again detected. S6

7 Figure S4: 1 H- 13 C CPMAS NMR with τ cp = 250 µs and ν R = 5 khz of the hydromagnesite mineral. Dypingite characterization from pxrd and Raman spectroscopy is shown in Figure S5 A and B, with the reference pxrd pattern for dypingite shown below the pxrd pattern. The Raman spectrum has a peak at 1121 cm -1 which is consistent with the ν 1 symmetric stretching mode of dypingite. 2 To distinguish dypingite s pattern from hydromagnesite s, the reflection with the most obvious change in dypingite is its lowest reflection detected at 5.8 o (2θ). S7

8 Figure S5: Synthesized 13 C-enriched dypingite analyzed by A) Raman spectroscopy and B) pxrd. A reference pxrd pattern is shown for C) dypingite (PDF# ). Prior to the pxrd study the synthesized nesquehonite was co-mixed with forsterite for filling the rotor for 13 C NMR measurements, due to limited sample; therefore, forsterite is seen as a contaminant. Figure S6 A and B show characterization of the synthesized nesquehonite from pxrd and Raman spectroscopy. Nesquehonite and forsterite reference patterns are shown below the pxrd pattern. The Raman spectroscopy has a single peak at 1100 cm -1, which is consistent with the ν 1 symmetric stretching vibration of nesquehonite. 1 S8

9 Figure S6: Synthesized 13 C-enriched nesquehonite analyzed by A) Raman spectroscopy and B) pxrd. Reference pxrd patterns are shown for C) nesquehonite (PDF# ) and D) forsterite (PDF# ). We synthetically prepared nesquehonite-like samples that exhibited the two resonances (as seen in Figure S7) that were discussed in the manuscript. Incomplete sample purification and the presence of excess Na + from the synthesis appears to lead to the second resonance at the lower shift value. S9

10 Figure S7: 13 C{ 1 H} MAS NMR with ν R = 5 khz of contaminated nesquehonite mineral. The contaminant peak has a chemical shift of ppm. Since nesquehonite has only one crystallographically inequivalent carbon, the second resonance must be a separate mineral phase. From the 13 C NMR it can be concluded that this phase will also have a carbonate-like structure. The extra reflections in the pxrd pattern match the mineral northupite, Na 3 Mg(CO 3 ) 2 Cl, seen in Figure S8. This conclusion is confirmed with Raman spectroscopy (Figure S8) as a shoulder at 1114 cm -1, as well as 23 Na{ 1 H} MAS NMR, seen in Figure S9. Since northupite has only one crystallographically inequivalent carbon, and this is expected in the same general region as other carbonates, we conclude this is a northupite impurity in nesquehonite, arising from incomplete purification. S10

11 Figure S8: Synthesized 13 C-enriched nesquehonite containing an impurity analyzed by A) Raman spectroscopy and B) pxrd. Reference pxrd patterns are shown for C) nesquehonite (PDF# ), D) northupite (PDF# ), and E) halite (PDF# ). The 23 Na{ 1 H} MAS NMR was obtained at a frequency of MHz with a decoupling frequency of MHz. Each spectrum was acquired with 4 scans and a 60 s recycle delay. S11

12 Figure S9: 23 Na{ 1 H} MAS NMR of A) nesquehonite (with an impurity present in the 13 C NMR spectrum) and 23 Na resonances for both northupite and NaCl and B) a pure nesquehonite sample, (which shows no impurity in the 13 C NMR spectrum) with only a small 23 Na resonance for NaCl (Figure 9B is x10). The 23 Na NMR resonance at 6.7 ppm observed in both samples is consistent with the chemical shift expected for NaCl. The additional resonance for 23 Na observed in the nesquehonite sample (with the impurity present) is consistent with a mineral possessing both a single magnetically inequivalent carbon and a sodium site. The pxrd shows the presence of northupite, which has one carbon site; therefore, we attribute the 23 Na resonance at -0.6 ppm to this mineral species. S12

13 (1) Edwards, H. G. M.; Villar, S. E. J.; Jehlicka, J.; Munshi, T. FT-Raman spectroscopic study of calcium-rich and magnesium-rich carbonate minerals. Spectrochim. Acta. A. Mol. Biomol. Spectrosc. 2005, 61, (2) Frost, R. L.; Bahfenne, S.; Graham, J. Raman spectroscopic study of the magnesiumcarbonate minerals-artinite and dypingite. J. Raman Spectrosc. 2009, 40, (3) Frost, R. L.; Dickfos, M. J. Raman spectroscopy of halogen-containing carbonates. J. Raman Spectrosc. 2007, 38, S13