J. Phys. Chem. B 2002, 106, 773-778 17O
773
Multiple-Quantum MAS NMR Study of Pyroxenes Sharon E. Ashbrook,†,‡ Andrew J. Berry,§ and Stephen Wimperis*,† School of Chemistry, UniVersity of Exeter, Stocker Road, Exeter EX4 4QD, United Kingdom, Physical Chemistry Laboratory, UniVersity of Oxford, South Parks Road, Oxford OX1 3QZ, United Kingdom, and Research School of Earth Sciences, Australian National UniVersity, Canberra ACT 0200, Australia ReceiVed: August 7, 2001; In Final Form: NoVember 9, 2001
Two-dimensional triple-quantum 17O (I ) 5/2) MAS NMR powder spectra were obtained for orthoenstatite, clinoenstatite, and protoenstatite (MgSiO3 polymorphs) and diopside (CaMgSi2O6). The spectra were analyzed to yield the 17O isotropic chemical shifts (δCS) and quadrupolar coupling parameters (PQ ) CQ(1 + η2/3)1/2) of the oxygen sites in each sample. High-resolution isotropic projections exhibit resolved 17O peaks equal in number to the number of crystallographically inequivalent oxygen sites. Bridging and nonbridging oxygens were distinguished by their characteristic PQ values. Further assignments were possible by comparing the isotropic 17O NMR spectra for this series of similar compounds with that reported previously for forsterite (Mg2SiO4). Full assignments of the three oxygen sites in protoenstatite and diopside and of the six sites in orthoenstatite and clinoenstatite are suggested.
Introduction Solid-state 17O NMR spectra are often broad as a result of the interaction of the 17O quadrupole moment with the electric field gradient (EFG) at the nucleus. NMR techniques, such as double rotation (DOR),1,2 dynamic angle spinning (DAS),3,4 and multiple-quantum magic-angle spinning (MQMAS),5 have been developed that allow the resolution of discrete oxygen sites in silicate minerals and glasses6-12 through the removal of this quadrupolar broadening. Broad differences in the oxygen environment, such as the ionic radius of the coordinating cations or between bridging (Si-O-Si) and nonbridging sites, have been related to the chemical shift.7,13-15 For bridging oxygens, isotropic shifts have also been related to the Si-O-Si bond angle.16-18 In contrast, it is not clear what structural parameters might be useful in assigning resonances corresponding to nonbridging oxygens. Ab initio calculations of NMR parameters are currently not sufficiently accurate to allow multiple sites with similar chemical environments to be distinguished.19 We have recently demonstrated how it is possible to make 17O NMR assignments based upon the careful comparison of a series of closely related compounds.20 This approach may identify the importance of various structural controls and provides a data set of well-defined oxygen sites and NMR parameters to which future theoretical models may be compared and refined. The pyroxenes are an important group of rock-forming minerals. They occur in nearly all igneous rocks, some metamorphic settings, and comprise up to 25% of the Earth’s upper mantle. Pyroxenes have the general formula M1M2Si2O6 and consist of chains of corner-sharing SiO4 tetrahedra linked by cations.21 Each tetrahedron comprises both bridging (two SiO3-Si) and nonbridging (one apical O1 and one O2) oxygen atoms. The M1 site is approximately octahedrally coordinated by nonbridging oxygens (four O1 and two O2). The M2 site is * To whom correspondence should be addressed. Fax: +44-1392263434. E-mail:
[email protected]. † University of Exeter. ‡ University of Oxford. § Australian National University.
coordinated by four nonbridging oxygens (two O1 and two O2) and either two, three, or four bridging oxygens depending upon the size of the cation. In enstatite (MgSiO3), magnesium is 6-fold coordinated on both the M1 and M2 sites. Three polymorphs are stable or metastable under ambient conditions: orthorhombic orthoenstatite22,23 (Pbca) and protoenstatite24 (Pbcn), and monoclinic clinoenstatite25 (P21/c). In diopside26 (C2/c), magnesium occupies the M1 site and calcium is 8-fold coordinated on the M2 site. In each structure O1 is coordinated by two M1, one M2, and one Si; O2 by one M1, one M2, and one Si; and O3 by one M2 (two M2 in diopside) and two Si. There are two distinct silicate chains (A and B or 1 and 2) in orthoenstatite and clinoenstatite producing two similar oxygen sites for each of the three site types, or a total of six oxygen species. The A chain is less kinked and contains smaller tetrahedra than the B chain. In protoenstatite and diopside the silicate chains in adjacent layers are symmetrically related, resulting in a single chain type (more similar to A than B) and only three oxygen sites. A number of alkaline earth silicates (including clinoenstatite and diopside) have previously been studied by 17O MAS14,27 and DAS/DOR6,7 NMR. Simulations of the MAS spectra assumed the presence of only three oxygen sites since the resolution of this technique is insufficient to allow modeling of additional sites arising from crystallographic inequivalencies. The values of the quadrupolar coupling constant, CQ ) eQVzz/ 4π0h, were found to define two types of site corresponding to bridging and nonbridging oxygens. For both types of site, trends in chemical shift and CQ with cation radius and electronegativity were also established.14 DAS and DOR allowed the resolution of all oxygen sites in clinoenstatite and other structures and, from the measured values of the quadrupolar “product” PQ ) CQ(1 + η2/3)1/2, confirmed the 1:2 ratio of bridging and nonbridging oxygen sites. No attempts have been made to discriminate between the different bridging and nonbridging oxygens. We have chosen to treat clinoenstatite and diopside as model compounds in our 17O MQMAS NMR studies of silicate minerals. They contain multiple oxygen sites of different
10.1021/jp0130362 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/03/2002
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structural types that have been resolved by other NMR techniques and against which our spectra may be compared. In addition, we present results for orthoenstatite and protoenstatite and for the first time suggest full assignments of all oxygen sites in the enstatites and diopside. Experimental Section Sample Preparation. The pyroxenes were synthesized using MgO and SiO2. The preparation of these oxides from H217O has been described previously.28 CaO was not enriched and was freshly prepared by the decomposition of CaCO3 at 1000 °C. A stoichiometric oxide mix was used in all syntheses. MgSiO3 is stable as clinoenstatite, orthoenstatite, and protoenstatite under different temperature and pressure conditions.29,30 Protoenstatite is a high-temperature phase that is metastable at room temperature. It was prepared by heating at 1400 °C under Ar for 12 h in a vertical tube furnace followed by quenching in H2O.31,32 Variable amounts of the lowtemperature polymorph, clinoenstatite, are also formed. This inversion continues at room temperature to give a noticeable increase in the clinoenstatite content over a period of months. NMR spectra were recorded within several weeks of synthesis, after which time no appreciable difference in the X-ray diffraction pattern could be detected. Clinoenstatite was prepared by annealing protoenstatite at 550 °C under Ar for 24 h to produce complete inversion. The upper temperature stability limit is too low to allow direct synthesis from the oxides in the clinoenstatite stability field. Similarly, the maximum temperature of the orthoenstatite field is insufficient for reaction at atmospheric pressure. The reaction does occur at higher pressures and orthoenstatite was prepared at 1100 °C and 15 kbar using a piston cylinder apparatus. Diopside was synthesized by heating at 1300 °C under Ar for 12 h. The nominal levels of 17O isotopic enrichment are 35% for orthoenstatite, clinoenstatite, and protoenstatite and 29% for diopside. NMR Experiments. 17O MAS NMR spectra were obtained either at a Larmor frequency of 54.2 MHz on a Bruker MSL 400 spectrometer equipped with a widebore 9.4 T magnet or at a Larmor frequency of 81.4 MHz on a Chemagnetics Infinity 600 spectrometer equipped with a wide-bore 14.1 T magnet. Powdered samples were packed inside 4 mm MAS rotors. The recycle intervals were optimized experimentally and, in addition, a number of “dummy” transients were also acquired. The twodimensional triple-quantum 17O MAS NMR experiments were performed using a phase-modulated split-t1 pulse sequence (shown in Figure 14b of ref 33). The ppm scales were referenced to H2O. Further experimental details are given in the figure captions. 17O-enriched
Results The conventional 17O MAS NMR spectra of orthoenstatite, clinoenstatite, protoenstatite, and diopside, recorded at B0 ) 9.4 T, are shown in Figure 1. All four minerals possess more than one crystallographically distinct oxygen species. The spectra of the enstatite polymorphs (Figure 1a-c) all show a composite resonance at ∼30 ppm, displaying features associated with second-order quadrupolar broadening but comprising overlapping line shapes from a number of crystallographically distinct sites. A similar resonance, centered at ∼50 ppm, is also observed in the spectrum of diopside (Figure 1d). Triple-quantum 17O MAS NMR was used to remove fully the inhomogeneous second-order quadrupolar broadening and resolve the distinct oxygen species.5 We have employed a “split-
Figure 1. 17O (54.2 MHz) MAS NMR spectra of (a) orthoenstatite, (b) clinoenstatite, (c) protoenstatite, and (d) diopside. Spectra are the result of averaging (a)-(c) 240 and (d) 700 transients with recycle intervals of (a)-(c) 2 s and (d) 5 s. The MAS rate was (a) 10.0 kHz, (b) 9.0 kHz, (c) 9.5 kHz, and (d) 9.0 kHz. The ppm scale is referenced to H2O.
t1” MQMAS technique, which results in a two-dimensional spectrum with ridge line shapes lying parallel to the δ2 axis.33 A projection onto the δ2 axis results in the conventional MAS spectrum, but a high-resolution, or isotropic, spectrum may be obtained directly from a projection onto the δ1 axis. Figure 2a shows the triple-quantum 17O (54.2 MHz) MAS NMR spectrum of orthoenstatite. Four intense ridge line shapes are observed, two of which are very closely spaced, with indications of two longer ridges appearing with higher δ1 shifts. The values of the quadrupolar product, PQ ) CQ(1 + η2/ 1/2 3) , and the isotropic chemical shift, δCS, for each inequivalent oxygen may be obtained from the δ1 and δ2 positions (in ppm) of the center of gravity of each two-dimensional ridge line shape.28,34 For split-t1 triple-quantum MAS spectra of spin I ) 5/ nuclei: 2
δCS )
PQ )
x
31δ1 + 10δ2 27
ν02(37δ1 - 17δ2) 162000
(1)
(2)
where ν0 is the Larmor frequency in Hz. Fitting of δ2 crosssections through the ridge line shapes provided independent estimates of the individual quadrupolar parameters CQ and η for most of the resolved oxygen sites in Figure 2 and was used as a check on the PQ values obtained. The values of PQ and δCS extracted for orthoenstatite from Figure 2a are given in Table 1. The four most intense peaks correspond to low PQ values (∼3 MHz), characteristic of those observed for nonbridging oxygen species, while the two peaks at higher δ1 shifts yield significantly higher values of PQ. These species appear only at lower contour levels in Figure 2a and with reduced intensities in the isotropic projection, reflecting the broad nature of the ridges and the nonuniform excitation of triple-quantum coherence. The two-dimensional triple-quantum 17O (54.2 MHz) spectrum of clinoenstatite in Figure 2b also exhibits a series of ridge line shapes and appears very similar to that observed for orthoenstatite in Figure 2a. The isotropic projection confirms the presence of six inequivalent oxygen species with isotropic chemical shifts and quadrupolar parameters, as shown in Table 1. As in orthoenstatite, the four ridge line shapes at lower δ1 shifts yield PQ values usually associated with nonbridging
17O
Multiple-Quantum MAS NMR Study of Pyroxenes
J. Phys. Chem. B, Vol. 106, No. 4, 2002 775 TABLE 1: 17O Isotropic Triple-Quantum MAS Shifts (δ1), 17O Isotropic Chemical Shifts (δ ), 17O Quadrupolar CS Products (PQ), and Tentative Assignments of the Oxygen Species in Orthoenstatite, Clinoenstatite, Protoenstatite, and Diopsidea mineral
δ1 (ppm)
δCS (ppm)
PQ/MHz
orthoenstatite 54.2 MHz
27(1) 30(1) 34(1) 35(1) 47(1) 56(1)
42(1) 46(1) 52(1) 54(1) 64(3) 73(3)
2.8(1) 2.9(1) 3.0(1) 3.0(1) 4.3(3) 4.9(3)
O21 O22 O11 O12 O31 O32
clinoenstatite 54.2 MHz
27(1) 30(1) 34(1) 35(1) 47(1) 56(1)
41(1) 45(1) 51(1) 54(1) 64(3) 75(3)
2.8(1) 2.8(1) 3.0(2) 3.0(2) 4.3(3) 4.8(3)
O21 O22 O11 O12 O31 O32
protoenstatite 54.2 MHz
26(1) 27(1) 30(1) 34(1) 35(1) 47(1) 48(1) 56(1)
39(1) 40(1) 45(1) 52(1) 53(2) 63(3) 66(3) 75(3)
2.7(1) 2.8(1) 2.8(1) 2.8(1) 3.0(2) 4.3(3) 4.3(3) 4.7(3)
O2 impurity impurity O1 + impurity impurity impurity O3 impurity
diopside 54.2 MHz
40(1) 51(1) 51(1)
63(1) 72(2) 84(2)
2.8(1) 4.3(2) 2.7(2)
O2 O3 O1
diopside 81.4 MHz
37(1) 44(1) 49(1)
63(1) 70(1) 85(2)
2.9(2) 4.3(2) 2.7(2)
O2 O3 O1
forsterite 54.2 MHz ref 28
32(1) 37(1) 39(1)
48(1) 61(1) 64(1)
2.8(1) 2.4(1) 2.6(1)
O1 O3 O2
a
Figure 2. Two-dimensional triple-quantum 17O (54.2 MHz) MAS NMR spectra of (a) orthoenstatite, (b) clinoenstatite, and (c) protoenstatite recorded using phase-modulated split-t1 experiments. In (a) and (b) 480 transients were averaged for each of 300 t1 increments of 50 µs with a recycle interval of (a) 3 s and (b) 2.5 s, while in (c) 384 transients were averaged for each of 300 t1 increments of 50 µs with a recycle interval of 2 s. Contour levels are drawn at (a) 8, 16, 32, and 64%, (b) 10, 30, 50, 70, and 90%, and (c) 4, 8, 16, 32, and 64% of the maximum value. The MAS rate was (a) 10.0 kHz, (b) 9.0 kHz, and (c) 9.5 kHz. Spinning sidebands in the projections are marked with an asterisk.
oxygens, while the two ridges at higher δ1 shifts correspond to significantly higher PQ values, between 4 and 5 MHz. The values of δCS and PQ shown in Table 1 for the six oxygen species in clinoenstatite do not agree with values obtained in earlier 17O static,14 DOR/DAS,6 and DAS NMR studies7-nor do these three studies agree with each other. However, the following points can be noted: (i) the static 17O NMR study assumed only three oxygen sites and suffered from inherently low spectral resolution; (ii) the DOR/DAS study involved fitting of DAS line shapes acquired while spinning at 79.2° rather than the magic angle (54.7°) and relied, therefore, upon potentially poor estimates of CQ and η; (iii) the pure DAS study, although performed at two different B0 field strengths, yielded δCS and PQ values that are in severe conflict with several qualitative features of the spectrum in Figure 2b (for example, the data in ref 7 suggest that the two peaks corresponding to higher PQ values should both have δ1 shifts of ∼56 ppm in Figure 2b, rather than appear as the very well-resolved pair of peaks at δ1 shifts of ∼47 and ∼56 ppm actually observed); (iv) the present study, the first since 1992, finds internally consistent δCS and
assignment
For comparison, literature values for forsterite are also given.
PQ values over a range of very similar compounds using high quality NMR data provided by the MQMAS technique. Figure 2c shows the triple-quantum 17O (54.2 MHz) MAS NMR spectrum of protoenstatite. Although the isotropic projection shows three intense peaks at δ1 shifts of ∼26, ∼34, and ∼48 ppm, there are additional peaks present in the spectrum at lower intensity. These peaks are attributed to a clinoenstatite impurity (∼30% from a Rietveld refinement of the X-ray diffraction data). The PQ and δCS values given in Table 1 reveal that, of the three most intense peaks, two correspond to small PQ values and one to a significantly higher PQ value. Table 1 also shows that the PQ and δCS extracted from the peaks assigned to the impurity are very similar to those observed for clinoenstatite. Figure 3a shows the two-dimensional triple-quantum 17O (54.2 MHz) MAS NMR spectrum of diopside. Two ridge line shapes are immediately apparent, corresponding to oxygen species whose PQ values (given in Table 1) are 2.7 and 2.8 MHz. In addition, a third oxygen resonance appears in Figure 3a with low intensity, only just visible on the lowest contour level shown. Owing to a combination of a larger PQ value and also a larger isotropic chemical shift, this peak is coincident in the isotropic dimension with one of the more intense ridges, both appearing at δ1 ∼ 51 ppm. All three oxygen species in diopside can be observed separately in the triple-quantum 17O MAS NMR spectrum, recorded at a higher Larmor frequency (81.4 MHz), shown in Figure 3b. In this spectrum all three ridge line shapes are well resolved and are no longer coincident in δ1. The values of PQ
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Figure 3. Two-dimensional triple-quantum 17O MAS NMR spectra of diopside recorded using a phase-modulated split-t1 experiment at Larmor frequencies of (a) 54.2 MHz and (b) 81.4 MHz. In (a) 288 transients were averaged for each of 128 t1 increments of 50 µs with a recycle interval of 6 s, while in (b) 192 transients were averaged for each of 128 t1 increments of 66.7 µs with a recycle interval of 6.5 s. Contour levels are drawn at 12, 32, 52, 72, and 92% of the maximum value. The MAS rate was (a) 8 kHz and (b) 12.8 kHz.
and δCS extracted for the three sites are again given in Table 1 and are in agreement with the values obtained at the lower field strength and with those in the literature determined from conventional MAS14 and DOR/DAS6,7 experiments. Discussion The assignment of all spectra may be made by a careful comparison of the four compounds studied and with reference to the previously assigned spectrum for forsterite (Mg2SiO4).28 Figure 4 displays the isotropic projections of the triple-quantum 17O MAS NMR spectra of orthoenstatite, clinoenstatite, protoenstatite, and diopside taken from the spectra shown in Figures 2 and 3a. For comparison, the corresponding triple-quantum 17O MAS isotropic projection of forsterite28 is also shown. In the pyroxene structure there are two nonbridging and one bridging oxygen for each silicate chain. The simplest enstatite structure is that of the single chain protoenstatite, in which there are only three distinct oxygen sites. Three peaks are accordingly observed in the spectrum (Figure 4c), ignoring those arising from the clinoenstatite impurity. Previous work has established a difference in the magnitude of CQ for bridging versus nonbridging oxygens. This result was derived from studies of a number of compounds in which the PQ or CQ values fell into two groups in a ratio corresponding to the number of bridging to nonbridging oxygens.6,7,14,27 The relationship between quadrupolar couplings and oxygen type has also been reported for silicate glasses.15 In our results the PQ of one site is significantly larger than the other two, which are similar, allowing the peak to be assigned to the bridging O3. The two nonbridging oxygens may be distinguished by comparing the protoenstatite structure with that of forsterite35 (Figure 4e). The O1 site in protoenstatite is coordinated to one Si and three Mg in an approximately tetrahedral arrangement.
Figure 4. Isotropic projections of triple-quantum 17O (54.2 MHz) MAS NMR spectra of (a) orthoenstatite, (b) clinoenstatite, (c) protenstatite, (d) diopside, and (e) forsterite. The projections in (a)-(d) are those shown in Figures 2 and 3a. In (e) 480 transients were averaged for each of 96 t1 increments of 250 µs with a recycle interval of 1 s; the MAS rate was 7.6 kHz.
The O2 site is coordinated to one Si and two Mg in a distorted trigonal planar geometry. The three sites in forsterite are all broadly similar to O1 and quite different from O2. In fact, a comparison of the oxygen bond lengths and angles to both Si and Mg indicates that the O1 site in protoenstatite is almost identical to the O1 site in forsterite. (Note that, although the oxygen coordination in protoenstatite was determined at 1080 °C,24 there is almost no change on cooling.31) Thus, we might expect the O1 protoenstatite peak to occur at a similar δ1 shift (with similar δCS and PQ values) to the O1 of forsterite and, indeed, there is a corresponding resonance. This allows O1 to be assigned and, accordingly, the remaining peak must derive from O2. These assignments are consistent with the degree of shielding that might be expected for each environment. The bridging O3 should be the most deshielded due to bonding to two Si for which there is a considerable covalent component.36 Net atomic charges from X-ray-determined electron-density distributions for forsterite and orthoenstatite indicate that, as expected, silicon is less ionic than magnesium.37,38 The importance of covalent bonding is also reflected by a trend of increasing chemical shift/ deshielding with increasing bond length, in general agreement with ab initio calculations of changes in chemical shielding with covalent bond extension.20,39,40 O1 might be expected to be more deshielded than O2 due to the difference between approximately sp3 and sp2 hybridization. In terms of simple Pauling bond
17O
Multiple-Quantum MAS NMR Study of Pyroxenes
strengths O3 is highly overbonded, O1 is charge balanced, and O2 is underbonded, which is manifested by variations in cationoxygen bond lengths (M/Si-O2 > M/Si-O1 > M/SiO3).21,23,41 The differences in PQ or CQ for bridging and nonbridging oxygens are also associated with the difference in covalency of cation-oxygen bonds where the more ionic bonds associated with the nonbridging oxygens result in less p-orbital contribution to the EFG and thus smaller values of CQ.14 There are six peaks in the isotropic spectrum of orthoenstatite (Figure 4a) corresponding to the six oxygen sites arising from two types of silicate chains. There are two bridging oxygens, and these are readily attributed to the peaks with large chemical shifts and large PQ. One of these occurs at the same δ1 shift as the bridging oxygen in protoenstatite, suggesting that it should be possible to assign this peak by comparing the structures. In general, the orthoenstatite A or 1 chain is more similar to the protoenstatite structure. The average Si-O bond length for O31 (1.655 Å) is much closer to O3 in protoenstatite (1.649 Å) than that of O32 (1.676 Å) and may discriminate between the two peaks. Such an assignment would be consistent with the correlation of chemical shift with bond length observed for nonbridging oxygens.20 The chemical shifts of both bridging and nonbridging oxygens are certainly correlated with large changes in bond lengths associated with the cation radius.14 There is also theoretical work18 that indicates that the isotropic chemical shift increases as the Si-O-Si bond angle decreases. The Si-O-Si bond angle for O31 (134.2°) is larger than that of O32 (128.0°) and closer to the O3 value (141.6°). The bond length and angle evidence would suggest assignment of the bridging oxygen peaks as O31 and O32 in order of increasing chemical shift. The O11 and O12 sites are structurally similar. Indeed, there are no significant differences between these sites, O1 in protoenstatite, or O1 in forsterite. Therefore, we assign the peaks at δ1 values of 34 and 35 ppm to O11 and O12 due to the small splitting and similarity in shift to the O1 sites in protoenstatite and forsterite. O11 is tentatively assigned to the peak at δ1 ∼ 34 ppm on the basis of the previously determined correlation between chemical shift and bond length.20 The O21 and O22 sites have similar bond lengths but quite different Si-O-Mg angles with O21 corresponding to O2 in protoenstatite. On this basis we assign the peak at δ1 ∼ 27 ppm to O21. The spectra of clinoenstatite (Figure 4b) and orthoenstatite (Figure 4a) are virtually identical. A comparison of the bond lengths and bond angles for each oxygen site indicates that similar oxygen coordination environments are almost indistinguishable despite the different space groups. This is reflected in the spectra, and the clinoenstatite spectrum is assigned accordingly. The assignment of diopside (Figure 4d) follows that of protoenstatite. The site with large PQ is assigned to the bridging oxygen and the other two sites can be assigned on the basis of their relative chemical shifts according to the same arguments used in the assignment of protoenstatite. The increased chemical shifts relative to those of MgSiO3 are consistent with deshielding due to the increase in effective ionic radius (covalent bonding) arising from the substitution of Ca for Mg. This effect has been established empirically and is greater for the nonbridging oxygens due to their shorter M-O bond lengths.7,14 The larger ratio of Ca-O to Mg-O bonds for the O2 (1:1) compared with the O1 (1:2) site could also suggest enhanced deshielding of O2 and a reversal of the nonbridging oxygen assignments. Such an effect has been observed for mixed cation glasses where, for a constant coordination number, the chemical shifts can be
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Figure 5. 17O isotropic chemical shifts (δCS) versus Si-O bond length (average Si-O bond length for bridging oxygens) for the enstatite assignments in Table 1. Data for forsterite, chondrodite, and clinohumite are from ref 20.
modeled as combinations of the pure cation end members.42 However, if this were the case in diopside, then we would expect to find a causative change in bond strength and a corresponding difference in the Ca-O or Mg-O bond lengths between O1 and O2. This is not observed, suggesting that in this example the lattice-controlled coordination geometry has a more dominant effect on the relative chemical shifts of the nonbridging oxygens than the competing contribution from the cation ratio. Figure 5 shows the correlation between isotropic chemical shift, δCS, and Si-O bond length for the enstatites based on the assignments presented here, together with data for forsterite, chondrodite, and clinohumite. The enstatite results are consistent with the linear relationship reported previously.20 Data for both bridging and nonbridging oxygens obey this trend, suggesting a general relationship. The empirical determination of correlations between NMR and structural parameters is necessary to make progress in the assignment of complex spectra since ab initio calculations are currently unable to distinguish between sites now being resolved by techniques such as MQMAS.19 Conclusions A series of minerals with the pyroxene structure was studied by triple-quantum 17O MAS NMR. The structures of these model compounds are very similar, allowing differences in the spectra to be related to subtle changes in the chemical environment. When compared with a reference material containing similar oxygen coordination, all the oxygen sites in orthoenstatite, clinoenstatite, protoenstatite, and diopside could be assigned. The values of δCS obtained from the spectra also correlate linearly with the Si-O bond distance based on these assignments. This work demonstrates the high site resolution possible by 17O MQMAS NMR and the potential of the technique to identify discrete oxygen sites in complex structures based on empirically determined relationships between NMR parameters and coordination environment. Acknowledgment. We are grateful to EPSRC for the award of a studentship (S.E.A.), to the Australian Research Council for the award of a fellowship (A.J.B.), and to the Royal Society for consumables funding. We would particularly like to thank Mr. W. O. Hibberson for assistance with the high-pressure syntheses and Professor R. Dupree and Dr M. E. Smith for
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