This paper is published as part of a PCCP Themed Issue on: Solid-State NMR Spectroscopy

Guest Editors: Paul Hodgkinson, Durham, UK, and Stephen Wimperis, Glasgow, UK

Editorial

Solid-State NMR Spectroscopy Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b914008p

Perspectives

Recent advances in solid-state NMR spectroscopy of spin I = 1/2 nuclei Anne Lesage, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b907733m

Recent advances in solid-state NMR spectroscopy of quadrupolar nuclei Sharon E. Ashbrook, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b907183k

Papers

Solid-state 17O NMR as a sensitive probe of keto and gem-diol forms of -keto acid derivatives Jianfeng Zhu, Amanda J. Geris and Gang Wu, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b906438a

Anomalous resonances in 29Si and 27Al NMR spectra of pyrope ([Mg,Fe]3Al2Si3O12) garnets: effects of paramagnetic cations Jonathan F. Stebbins and Kimberly E. Kelsey, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904731j New opportunities in acquisition and analysis of natural abundance complex solid-state 33S MAS NMR spectra: (CH3NH3)2WS4 Hans J. Jakobsen, Henrik Bildsøe, Jørgen Skibsted, Michael Brorson, Bikshandarkoil R. Srinivasan, Christian Näther and Wolfgang Bensch, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904841n An analytic expression for the double quantum 1H nuclear magnetic resonance build-up and decay from a Gaussian polymer chain with dynamics governed by a single relaxation time Michael E. Ries and Michael G. Brereton, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b905350f Static solid-state 14N NMR and computational studies of nitrogen EFG tensors in some crystalline amino acids Luke A. O Dell and Robert W. Schurko, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b906114b Solid state deuteron relaxation time anisotropy measured with multiple echo acquisition Robert L. Vold, Gina L. Hoatson, Liliya Vugmeyster, Dmitry Ostrovsky and Peter J. De Castro, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b907343d Application of multinuclear magnetic resonance and gauge-including projector-augmented-wave calculations to the study of solid group 13 chlorides Rebecca P. Chapman and David L. Bryce, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b906627f High-resolution 17O double-rotation NMR characterization of ring and non-ring oxygen in vitreous B2O3 Alan Wong, Andy P. Howes, Ben Parkinson, Tiit Anupõld, Ago Samoson, Diane Holland and Ray Dupree, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b906501f

Probing chemical disorder in glasses using silicon-29 NMR spectral editing Julien Hiet, Michaël Deschamps, Nadia Pellerin, Franck Fayon and Dominique Massiot, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b906399d GIPAW (gauge including projected augmented wave) and local dynamics in 13C and 29Si solid state NMR: the study case of silsesquioxanes (RSiO1.5)8 Christel Gervais, Laure Bonhomme-Coury, Francesco Mauri, Florence Babonneau and Christian Bonhomme, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b907450c Determining relative proton–proton proximities from the build-up of two-dimensional correlation peaks in 1H double-quantum MAS NMR: insight from multi-spin density-matrix simulations Jonathan P. Bradley, Carmen Tripon, Claudiu Filip and Steven P. Brown, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b906400a Manifestation of Landau level effects in optically-pumped NMR of semi-insulating GaAs Stacy Mui, Kannan Ramaswamy, Christopher J. Stanton, Scott A. Crooker and Sophia E. Hayes, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b907588g Motional heterogeneity in single-site silica-supported species revealed by deuteron NMR Julia Gath, Gina L. Hoaston, Robert L. Vold, Romain Berthoud, Christophe Copéret, Mary Grellier, Sylviane Sabo-Etienne, Anne Lesage and Lyndon Emsley, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b907665d Magnesium silicate dissolution investigated by 29Si MAS, 1H–29Si CPMAS, 25Mg QCPMG, and 1H–25Mg CP QCPMG NMR Michael C. Davis, William J. Brouwer, David J. Wesolowski, Lawrence M. Anovitz, Andrew S. Lipton and Karl T. Mueller, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b907494e Intermediate motions and dipolar couplings as studied by Lee–Goldburg cross-polarization NMR: Hartmann–Hahn matching profiles Marcio Fernando Cobo, Kate ina Mali áková, Detlef Reichert, Kay Saalwächter and Eduardo Ribeiro deAzevedo, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b907674c Measurements of relative chemical shift tensor orientations in solid-state NMR: new slow magic angle spinning dipolar recoupling experiments Andrew P. S. Jurd and Jeremy J. Titman, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b906814g Signal loss in 1D magic-angle spinning exchange NMR (CODEX): radio-frequency limitations and intermediate motions Christiane Hackel, Cornelius Franz, Anja Achilles, Kay Saalwächter and Detlef Reichert, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b906527j

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Calculation of fluorine chemical shift tensors for the interpretation of oriented 19F-NMR spectra of gramicidin A in membranes Ulrich Sternberg, Marco Klipfel, Stephan L. Grage, Raiker Witter and Anne S. Ulrich, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b908236k J-Based 3D sidechain correlation in solid-state proteins Ye Tian, Lingling Chen, Dimitri Niks, J. Michael Kaiser, Jinfeng

Lai, Chad M. Rienstra, Michael F. Dunn and Leonard J. Mueller, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b911570f Natural abundance 13C and 15N solid-state NMR analysis of paramagnetic transition-metal cyanide coordination polymers Pedro M. Aguiar, Michael J. Katz, Daniel B. Leznoff and Scott Kroeker, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b907747b

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Anomalous resonances in29Si and

27Al NMR spectra of pyrope

([Mg,Fe]3Al2Si3O12) garnets: effects of paramagnetic cations

Jonathan F. Stebbins* and Kimberly E. Kelsey

Received 9th March 2009, Accepted 6th May 2009

First published as an Advance Article on the web 2nd June 2009

DOI: 10.1039/b904731j

In oxide and silicate materials, particularly naturally-occurring minerals with contents of iron

oxides greater than a few percent, paramagnetic impurities are well-known to broaden MAS

NMR peaks, decrease relaxation times, and even cause overall loss of signal intensity. However,

detection of resolved, discrete peaks that are shifted in frequency by nearby unpaired electron

spins is rare in such systems. We report here high-resolution 27Al and 29Si spectra for synthetic

and natural samples of pyrope garnet ([Mg,Fe]3Al2Si3O12), the latter containing up to 3.5 wt%

FeO. For both nuclides, spectra contain anomalous NMR peaks at frequencies that are 25 to

200 ppm from normal ranges, possibly through pseudocontact shifts induced by paramagnetic

cations. Quantitation of peak areas suggests that signals from nuclides with such cations in their

first shell may be broadened enough to be unobservable, while those with paramagnetics in their

second cation shells may be substantially shifted. Overall spin–lattice relaxation rates are greatly

enhanced by such impurities, and shifted resonances relax much faster than the unshifted main

peaks. A high symmetry crystal structure (in this case cubic), which limits the number of different

cation–cation distances in each shell, combined with a relatively low (non-cubic) symmetry for the

sites hosting the magnetic cations, may be needed to readily detect such features.

1. Introduction

High-resolution solid-state NMR has been widely applied to

oxide and silicate materials, including those synthesized in the

laboratory or in industrial processes and those that occur as

natural minerals, often providing unique information about

short-range structure and ordering state.1–4 Especially for

naturally occurring minerals, however, an important limita-

tion has often been the loss in resolution and hence of

information content that can be caused by even minor con-

tents of paramagnetic cations, most notably ferrous and ferric

iron, with Fe being one of the four most abundant elements

in the Earth. Early reports showed severe broadening of29Si MAS NMR peaks, and even loss of an observable signal,

in minerals such as olivine ([Mg,Fe]2SiO4) with a few wt%

FeO.5,6 These suggested that the method might not be useful

for many common groups of natural minerals, except for their

(usually) synthetic, Fe-free counterparts. This presumption

remains largely still correct. However, in some cases, MAS

spectra with a useful resolution can be obtained for silicates

containing moderate concentrations of paramagnetic impurities,

which can provide important constraints on the effects on peak

width, relaxation and resonance shifts. As such interactions

become better understood, they may be able to yield new

information about the distribution of paramagnetic cations in

silicate crystal structures and hence about order/disorder and

the extent of solid solution or phase separation.7–10

Most MAS NMR studies of silicates with significant

contents of paramagnetics have focused on their dramatic

effects on peak widths and relaxation. Early studies of crystal-

line aluminosilicates such as clays and feldspars showed strong

dependence on Fe contents of 29Si peak widths and spin–

lattice relaxation rates.11–13 For example, a comparison

between a synthetic pyrope garnet (Mg3Al2Si3O12) and a

natural sample with only about 2 wt% FeO (similar to the

samples described here) showed large effects on both 29Si and27Al peak widths;14 even doping with a fraction of 1% of FeO

can degrade resolution in 29Si spectra of synthetic garnet solid

solutions.15 29Si spectra of aluminosilicate glasses, although

already quite broad because of disorder and resulting chemical

shift distributions, are broadened further as iron oxide

contents become greater than a few wt%, and intensity is

transferred into spinning sidebands as dipolar coupling to

unpaired electron spins increases the static linewidth.16 Similar

effects have recently been systematically studied for high-field27Al spectra of high-pressure aluminosilicate glasses, in which

useful resolution among peaks for different Al coordinations is

maintained to about 5 wt% FeO.17 Related changes caused by

addition of paramagnetic rare earth element cations have been

pursued in detail for 29Si and 89Y in synthetic oxynitrides.18

Detailed studies of the shapes of relaxation curves have

helped to constrain relaxation mechanisms, particularly the

importance or unimportance of nuclear spin diffusion, and of

paramagnetic centers, for nuclides including 1H, 13C, 27Al

and 29Si.19–22

In contrast, only a few high-resolution NMR studies of solid

oxides and silicates have reported discrete resonances that are

shifted by the direct effects of unpaired electron spins, rather

than simply missing or broadened signals. For example, 119Sn

Dept. of Geological and Environmental Sciences, Stanford University,Stanford, CA 94305, USA. E-mail: stebbins@stanford.edu;Tel: +1 (1)650 723 1140

6906 | Phys. Chem. Chem. Phys., 2009, 11, 6906–6917 This journal is �c the Owner Societies 2009

PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics

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and 89Y MAS NMR on rare earth pyrochlore phases clearly

showed not only the expected enhancement of relaxation rates

from the electron-nuclear dipolar interactions of the para-

magnetic REE3+ cations, but also well-resolved peaks that

were shifted by both through-bond hyperfine coupling (Fermi

contact shift) and through-space dipolar coupling (pseudo-

contact shifts).7,8 The first of these effects can reach 1000s of

ppm for cations separated by only a single anion;23 the second

are generally smaller, can be either negative or positive, and

are apparent only when the magnetic susceptibility at the site

of the magnetic ion is anisotropic, i.e. when the ligand field has

lower than cubic symmetry.4,7 In 29Si MAS NMR spectra of

synthetic zircon (ZrSiO4) containing up to about 1 mol% V4+,

two small peaks were observed whose intensities were

correlated with the dopant level, with contact shifts of up to

about 30 ppm.9 For Tb-doped Y3Al5O12, broadening of 27Al

and 89Y resonances, and the appearance of shifted peaks for

the latter nuclide, were attributed to pseudocontact interaction

with the magnetic Tb3+ ion.10 Contact shifts have been

more widely described in solid-state 13C MAS NMR spectra

of transition metal and rare earth compounds such as

carboxylates and acetates,24,25 where this effect has provided

structural information.26 Contact-shifted resonances have

even been reported for 2H and 6Li in ions on the surface of

goethite (a-FeOOH).27,28 Very recently, we have described29Si spectra of isotopically-enriched Mg2SiO4 polymorphs

forsterite and wadsleyite in which numerous small ‘‘extra’’

peaks appeared outside the normal range of chemical shifts forIVSi in silicates, and hypothesized that these may also result

from pseudocontact shifts.29,30

Silicate garnets, with the general formula X2+3 Y3+

2 Z4+3 O12,

are common in metamorphic rocks and in the deeper parts

of the Earth’s upper mantle. Their structure and crystal

chemistry have been extensively studied because of the wide-

spread use of garnets in deducing pressures and temperatures

from coexisting mineral equilibria.31–34 Cation order/disorder

in synthetic silicate garnet solid solutions has been investigated

in detail by several methods, including high-resolution NMR

in systems with low paramagnetic contents.14,15,35–37 Aluminate

garnets (e.g. yttrium aluminium garnet or YAG) are also

important in solid state lasers and other technologies.38 A

number of single crystal (primarily 27Al) NMR studied have

been made on garnets, some of which have described large line

splittings due to unpaired electron-nuclear dipolar inter-

actions, which are strongly orientation-dependent, but which

are expected to be averaged to zero by rapid magic-angle

spinning.39,40 Most silicate garnets have cubic symmetry and a

simple structure that can make them especially interesting as

case studies in spectroscopy. In this report, we describe an

example that appears to be quite unusual among high-resolution,

solid state NMR investigations of silicates: pyrope garnets

([Mg,Fe]3Al2Si3O12), which contain up to 3.5 wt% FeO and

yet retain well-enough resolved 29Si and 27Al spectra to show

discrete, shifted resonances for sites that are probably within

the second cation shell of the paramagnetic centers. We take

advantage of this resolution to quantify effects of para-

magnetic cations on signal loss, peak widths and relaxation,

which may be more generally applicable to other aluminosilicate

and oxide materials.

2. Samples and methods

Two synthetic pyrope garnets studied here were synthesized

from glasses at 2.2 GPa and 1125 1C as described previously,41

and were nominally Mg3Al2Si3O12 in composition with about

0.1 wt% Co3O4 added, originally to speed spin–lattice relaxa-

tion. Data shown here are for a sample (‘‘SP’’) with natural

isotopic abundance; similar results were obtained for an17O-enriched sample. Powder XRD showed the presence of a

few% of a clinopyroxene, presumably a metastable aluminous

enstatite ([Mg,Al][Si,Al]O3). This was low enough in abun-

dance to contribute only a small peak to 27Al NMR spectra

and to not be detected in 29Si spectra. Electron microprobe

data (Table 1) showed the sample to be on-composition but to

be slightly heterogeneous, probably because of minor amounts

of pyroxene.

Natural pyrope garnets from the Dora Maira (Italy) ‘‘ultra-

high pressure metamorphism’’ locality,42,43 were obtained as

hand-picked, crushed grain separates from Excalibur Mineral

Co., originally as a source of the rare mineral ellenbergerite.44

Pyropes from this locale are apparently the garnets closest in

composition to the pure-Mg end member that have been found

in nature. Samples used were of two types, one that is white in

color and contains about 1.4 wt% FeO, the other that is light

pink and contains about 3.5 wt% FeO. Several grains of each

type, 3 to 4 mm in size and free of inclusions visible under

50� magnification, were selected for study by NMR. Data

presented are for two of these, one white (‘‘DW’’) and one

pink (‘‘DP’’); results from other samples are very similar.

EPMA data shows that these have nearly ideal pyrope

compositions with minor almandine (Fe3Al2Si3O12) and very

small grossular (Ca3Al2Si3O12) components (Table 1). The

measured Si/Al ratios are equal within uncertainty to 1.50,

indicating that any ferric iron content (e.g. andradite com-

ponent, Ca3Fe23+Si3O12) is minimal, as also observed in more

in-depth studies by others.42 In DW, signal from a small

amount of included kyanite (Al2SiO5) was detected by both27Al and 29Si NMR, as shown below.42 Powder XRD detected

only a trace of pyroxene in SP, of kyanite in SW and no

impurity phases in DP.

For estimation of signal loss due to paramagnetic impurities,

several well-characterized intensity standards were used. For29Si, we chose a synthetic forsterite (Mg2SiO4), whose

spectrum and relaxation behavior has recently been described

in detail.29,30 For 27Al we used a synthetic gibbsite (reagent

Al(OH)3) and corundum (a-Al2O3) made from the gibbsite by

heating at 1500 1C for 20 h.27Al NMR spectra were obtained with a Varian Unity/Inova

600 spectrometer at 14.1 T (156.4 MHz), using a Varian/

Chemagnetics ‘‘T3’’-type probe with 3.2 mm ZrO2 rotors

spinning at 20 kHz, with 0.2 ms radiofrequency pulses,

135 kHz pulse power, and spectral widths of 400 kHz to

2 MHz. Spectra were also acquired at lower spinning speeds

to compare line shapes. 29Si spectra were collected with a

Varian Infinity-Plus 400 spectrometer at 9.4 T (79.5 MHz)

with a similar probe, rotors spinning at 20 to 23 kHz, and

spectral widths of 100 to 400 kHz. Either single-pulse acquisi-

tion, with 0.67 ms pulses (solid 301 tip angle), or, in some cases,

a spin-echo sequence (90-t-180) with an echo delay of one

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rotor period, were employed; both the saturation-recovery or

inversion-recovery methods were used to study spin–lattice

relaxation rates. Some 27Al MAS spectra were also acquired at

9.4 T for comparison of peak widths with data from higher

field. Frequencies are referenced to 0.1 M aqueous Al(NO3)3or to tetramethylsilane (TMS). For both nuclides, spectra were

collected with wide ranges of pulse delays to detect differential

relaxation among different components of the spectra, to

optimize signal-to-noise and to obtain fully relaxed spectra

for quantitative analysis of total signal intensities. Spectro-

meter deadtimes were about 10 ms. Deadtime effects were

corrected by back-calculation of the time-domain signal using

a linear prediction algorithm. 27Al spectra, both central peaks

and spinning sidebands, were simulated with the Varian

STARS software package.45 Chemical shift anisotropy and

internuclear dipolar couplings were not simulated, as they

are expected to be negligible in these materials under these

experimental conditions (high symmetry, closest Al–Al distance

of 0.50 nm, fast spinning).

3. Results

3.1 27Al NMR spectra and peak assignments

Spectra obtained for the gibbsite and corundum standards

were fully consistent with previously published results. The

former was accurately simulated with two equally populated

Al sites with isotropic chemical shift (diso) values of 10.4 and

11.5 ppm, quadrupolar coupling constants (CQ) of 1.97 and

4.45 MHz, and quadrupolar asymmetry parameters (Z) of 0.73and 0.44, respectively;46 the latter with diso of 16.0 ppm, CQ of

2.4 MHz, and Z of 0 as required by symmetry.47,48 Fully

relaxed spectra were obtained with pulse delays of 10 s for

gibbsite and 30 s for corundum.

The 27Al spectrum for the synthetic pyrope (SP), when

plotted at full scale (Fig. 1a and 2), appears as expected for

a single octahedral (VIAl) site in an ordered crystal, with a

narrow central peak near to 0 ppm and a spinning side-

band manifold that were fitted with diso = 3.0 � 0.2 ppm,

Table 1 Electron microprobe data for selected pyrope samples

Synthetic pyrope (SP) Dora Maira white pyrope (DW) Dora Maira pink pyrope (DP)

Weight%:SiO2 44.9(9) 44.1(2) 43.6(3)TiO2 — 0.03(2) 0.05(1)Al2O3 25.9(9) 25.4(2) 24.7(2)FeO 0.01(2) 1.4(1) 3.5(3)MnO — 0.00(1) 0.08(6)CoO 0.06(4) — —MgO 30.3(4) 29.1(2) 26.6(4)CaO 0.02(1) 0.2(1) 0.65(4)Totals 101.2(7) 100.3(4) 99.2(7)

Cations per formula unit (X2+3 Y3+

2 Z4+3 O12)

Si 2.98(5) 2.972(12) 3.000(6)Ti — 0.002(1) 0.003(1)Al 2.03(8) 2.019(11) 2.005(11)Fe 0.001(1) 0.081(3) 0.204(17)Mn — 0.000 0.005(3)Co 0.003(2) — —Mg 2.99(3) 2.924(15) 2.731(3)Ca 0.001(1) 0.017(4) 0.048(3)

Fig. 127Al MAS NMR spectra (14.1 T) for pyrope samples.

Throughout, ‘‘SP’’ denotes synthetic pyrope, ‘‘DW’’ and ‘‘DP’’ are

white and pink natural samples from Dora Maira locality. (a) vertical

scales �1 (normalized to most intense peak) (b) Vertical scales

enlarged by 4� (DW and DP) or 20� (SP). ‘‘k’’ marks peaks due to

kyanite impurity.

6908 | Phys. Chem. Chem. Phys., 2009, 11, 6906–6917 This journal is �c the Owner Societies 2009

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CQ = 1.05 � 0.05 MHz, and Z = 0, the latter as required by

symmetry. (Note that in our previous report only the central

transition was analyzed, giving a less accurate value for Z.41)This chemical shift is consistent with previously reported peak

positions at lower fields.14,36,37 The fully relaxed spectrum was

obtained with a pulse delay of 10 s.

Spectra for the Fe-bearing, natural pyropes (DW and DP)

are dominated by central peaks at the same location, but

which are considerably broader (Table 2). The latter effect has

been previously reported for similar samples.14 Peak widths

did not significantly change with spinning speed from 12 to

20 kHz. At 9.4 instead of 14.1 T, resonances shifted down in

frequency by only a fraction of a ppm, consistent with the

small CQ value noted above; peaks were also only slightly

broader at the lower field (Table 2). Fully relaxed spectra were

obtained with pulse delays of 1 and 0.1 s, respectively,

indicating a large effect on relaxation of the Fe2+ (see below).

Each of these spectra also contains two other ‘‘anomalous’’

peaks, labeled A1 and A2, centered at about 68 and 34 ppm,

which have not previously been described. These can be seen

more clearly in the enlarged plots (Fig. 1b), where small A1

and A2 peaks can also be seen at about 67 and 26 ppm in the

data for SP. (Also obvious are the low-frequency features of a

trace of kyanite in DW.) For DW and DP, the A1 and A2

peaks also appear in the spinning sidebands (Fig. 2), whose

manifolds are similar in overall width and shape to that of SP,

but which have extra intensity towards the center. No other

anomalous peaks were observed. As described below, the A2

peak in the synthetic pyrope, and the A1 and A2 peaks in the

Fe-containing natural pyropes, relax considerably faster than

the main peaks, and can thus be considerably accentuated in

spectra with short pulse delays.

The A1 and A2 peaks fall in the ranges typical of IVAl andVAl in aluminosilicates, respectively, and well above the

known range for VIAl.1,4 However, especially for VAl, it is

extremely unlikely that this assignment could be correct: no

known impurity phase could give rise to such a resonance, and

defects that could produce such large amounts of VAl in a

natural, well-crystallized, anhydrous garnet have not been

reported in any of numerous studies.31–34 For example, large

fractions of oxygen vacancies would seem to be required, but

are not known to occur in these materials: missing oxygens

would produce energetically highly unstable, three-coordinated

Si sites. The A1 and A2 peaks are several times larger in DP,

which contains 2.6 times as much FeO as DW, and the A2

peak is much smaller in SP, which contains little or no FeO

and only 0.6% CoO. The A1 peaks in DW and DP vary in

intensity and width with the A2 peaks and are thus probably

primarily of the same origin, although in principle minor

amounts of signal in this region could be contributed by IVAl

in impurity phases, such as the phengite mica that is common

in the source rocks of these pyropes, although not detected in

these samples. Analyzed compositions reported here and

previously42 do not appear to allow for a major fraction ofIVAl in the pyropes, as they are close to ideal stoichiometry.

The relatively large A1 peak in SP is probably mostly or

entirely due to the known presence of a minor amount of

aluminous pyroxene. As discussed below, the resonant

frequency shifts of the A2 peaks in all of the samples, and of

the A1 peaks in the Fe-bearing pyropes, are thus probably the

result not of normally considered chemical shifts resulting

from the local bonding environment (distances, coordination

Fig. 2 As in Fig. 1, with frequency scale widened to include most

prominent spinning sidebands in one half of each spectrum. ‘‘Sim’’

denotes simulation with diso = 3.0 ppm, CQ = 1.05 MHz, and Z = 0.

‘‘c’’ marks central peaks, all others are spinning sidebands. The main

central peaks are each about 4� as high as the most intense sideband,

and are thus off scale.

Table 2 Positions of NMR peak maxima and full-widths at halfmaxima. Spinning speed (ss) is 20 kHz unless otherwise noted; field is9.4 T for 29Si and 14.1 T for 27Al unless otherwise noted

Nuclide Sample, peak Maximum/ppm FWHM/ppm

29Si SP, main –72.0 0.3S1 –73.6 1.0S2 –76.0 1.0

DW, main –71.9 4.5S1 –79.8 6a

S2 –88.1 6a

S3 –95 14a

S4 +134 25DP, main –71.3 9a

S1 –79 9a

S2 –87 9a

S3 –95 15a

S4 +138 38

27Al SP, main 2.7 2.3SP, main, 9.4 T 2.2 1.9A1 67.2 7.0A2 25.6 2.0

DW, main 2.8 5.8DW, main, 9.4 T 2.3 6.3A1 68 13A2 34 12

DP, main 2.6 10.5DP, main, ss=12 kHz 2.6 10.7DP, main, 9.4 T 2.2 12.0A1 64 16A2 34 15

a Approximate value, from fit.

This journal is �c the Owner Societies 2009 Phys. Chem. Chem. Phys., 2009, 11, 6906–6917 | 6909

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number, etc.), but are due to magnetic interactions between

the observed nuclei and the unpaired electron spins of

the Fe2+ (or Co2+) cations. The fast relaxation of these

components supports this suggestion.

3.2 29Si spectra and peak assignments

Data for the forsterite standard were presented recently;29,30

fully relaxed, albeit noisy, spectra were obtained with a pulse

delay of 1 h. In spectra for the synthetic pyrope collected with

pulse delays of about 10 s or longer, only the expected, single,

very narrow peak was seen at �72.0 ppm (Table 2), consistent

with several previous studies.14,37 At shorter delays when the

main peak was partially saturated (far from fully relaxed), two

other small peaks (S1, S2) appeared at �73.6 and �76.0 ppm

(Fig. 3), as well as a partially-resolved shoulder at �71.2 ppm.

In the Fe-containing natural pyropes, the main peaks are

much broader, as reported previously for similar samples.14

Two or possibly three peaks or shoulders also appear at lower

frequencies (also denoted S1, S2, S3, although not at the same

positions as in SP, Table 2), which have apparently not

previously been observed. As in the synthetic (and as for the

‘‘anomalous’’ 27Al peaks described above), the S1, S2, and S3

peaks in the natural pyropes relax faster than the main peak.

As for the 27Al spectra, these grow in width and intensity with

Fe content. Although these peaks are certainly in the ‘‘normal’’

range for IVSi in silicates, impurity phases are not abundant

enough to account for these features; in fact, the only detected

extra phase was about 2–3% kyanite, seen as two narrow

peaks at �83.2 and �84.2 ppm in DW.22 The presence of the

grossular component in pyrope does lead to a series of 29Si

NMR peaks that shift down in frequency as Ca cations replace

Mg neighbors,49,50 but measured Ca contents are too low, and

the frequency shifts too large, for this to explain the observed

spectra. The correlation of these ‘‘anomalous’’ peak intensities

with the content of paramagnetic impurities, and their rapid

relaxation, suggest that like the A1 and A2 peaks, their shifts

are related to interactions with unpaired electron spins.

More dramatically, a new 29Si peak appears in spectra of the

natural pyropes at about +138 ppm (Fig. 4), far outside

the known range for silicates, which extends up to only about

�60 ppm.3,4 The identity of this feature (‘‘S4’’) as a central

resonance, and not a spinning sideband, was confirmed by

varying the sample spinning rate. The only likely cause of

such a large shift can again be linked to interactions with

paramagnetic Fe2+ cations.

3.3. Peak areas: 29Si

As will be discussed below, clues to the origin of the observed

anomalous NMR peaks include their relative areas, and the

total absolute signal intensities as measured by the sums of all

of the peak areas relative to a known standard. For the 29Si

spectra, total peak areas were measured simply by direct

integration of the various peaks, including their detectable

spinning sidebands, after baseline correction and correction

for minor detected impurities (e.g. kyanite in DW). These were

normalized to sample weight and measured Si content for

comparison to the signal from an undoped, natural isotopic

forsterite standard.29,30 The greatest source of uncertainty in

this comparison for the natural pyropes is probably the

relatively low signal-to-noise ratio obtainable for fully relaxed

spectra of the standard, introducing an error of about �5%into this absolute calibration. The synthetic pyrope relaxed

even more slowly, increasing the uncertainty even more for

this sample (�5 to 10%). However, the total areas of the

spectra for the two Fe-bearing garnets are more accurately

comparable relative to each other, as these both relaxed

rapidly. Results are shown in Table 2, as the % of the total

signal reduction below the value expected from the standard.

Fig. 3 29Si MAS NMR spectra (9.4 T) for pyrope samples. Spectra

for SP and DW with 0.1 and 1 s pulse delays are shown to illustrate

more rapid relaxation of S1 and S2 peaks. The 1 s data for DW is

processed with a minimum of smoothing to allow detection of kyanite

peaks, which are labeled ‘‘k’’.

Fig. 429Si MAS spin-echo NMR spectra (9.4 T) for natural pyrope

samples, with vertical scales enlarged by about 10� from those in

Fig. 3, and much wider frequency range. ‘‘*’’ marks spinning

sidebands for main peak at �72 ppm, ‘‘**’’ marks sidebands for

‘‘anomalous’’ peak S4 at about +135 ppm. Spectrum for DP was

collected with 0.1 s pulse delay, for DW 0.2 s.

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For all three samples, these are small values, close to zero

within the relatively large uncertainties.

The individual areas for the S4 peaks were again obtained

by direct integration of the +138 ppm peak and its observable

spinning sidebands. For S1, S2, and S3, component areas were

obtained by fitting spectra with 80% Gaussian peak shapes.

Because of peak overlap, we report only the sums of these

components in Table 2, which are at least good estimates of

the area shifted from the main peaks. As will be discussed

further below, the fraction of the signal in the anomalous

peaks increases substantially with the content of paramagnetic

cations (Co, Fe, and very minor Mn). In addition, the fraction

of the signal in sidebands for the S4 component is much higher

than that for the overall total.

3.4 Peak areas: 27Al

For the 27Al spectra, quantification is made more precise by

the much higher signal to noise ratios and more rapid relaxa-

tion for even the Fe-free samples and standards, but more

complex by the presence of five spin transitions for this spin =

5/2 nuclide. The total observable signal was determined from

spectra with widths of 2 MHz, wide enough to include all

satellite transition (�1/2–3/2 and �3/2–5/2) signals except fora minor fraction of that for the large CQ site in the gibbsite.

(The latter was corrected using the simulated sideband

manifold intensities for the full theoretical spectrum.4,46) All

peaks and sidebands were integrated after a baseline correc-

tion and included in the totals (Table 3). Center peaks areas,

which are for convenience more commonly used to assess

signal intensities, were also determined by integration.

Overlapping contributions from the center spinning sidebands

of the satellite transitions were estimated by simulations

with the parameters given above (e.g. about 10% for the

pyropes) and subtracted, to give the areas due only to the

�1/2 transition.

As confirmed by the line shape simulation, the central peak

for the synthetic pyrope contains almost all of the signal for

the central (�1/2) spin transition. This is the expected result

when the chemical shift anisotropy in negligible and any

dipolar broadening present (e.g. Al–Al homonuclear coupling)

is much smaller than the spinning frequency.

For the DM pyropes, estimation of the relative intensities of

the A1 and A2 ‘‘anomalous’’ peaks was facilitated by the

observation that at 14.1 T field, with the small CQ and

relatively large observed paramagnetic broadening, each of

the peaks is nearly symmetrical, so that Gaussian/Lorentzian

line shapes could be used to fit the spectra and approximate

the relative areas of the components. However, in these

samples, magnetic dipolar coupling to the abundant para-

magnetic impurities may broaden the overall resonance for the

central transition sufficiently so that at least the innermost

sidebands contain some of this signal, and this effect may not

be equivalent for A1, A2, and main peaks. This can be readily

seen in Fig. 2 by the growth in sideband intensity towards the

central peak in the natural pyropes, in contrast to the syn-

thetic, and the obviously greater relative intensities of the A1

and A2 components in the inner sidebands when compared

to the central peak. To approximate this contribution, we

measured the area of each satellite sideband for the synthetic

Table 3 Relative areas (in %) of minor components of central peaks and spinning side bands (ssb) in 29Si and 27Al spectra. When differentialrelaxation was observed, results are based on relaxed spectra

Sample SP DW DP Gibbsite

Paramagnetics, per f.u. 0.003 (Co) 0.081 (Fe+Mn) 0.209 (Fe+Mn) —29Si NMR peaks

% loss in total signala 10(10) 2(4) 5(4) —% loss, predictedb 0.03 0.8 2.0 —S1+S2+S3, % area 1.0(5) 29(3) 46(4) —S4, % area — 10(2) 17(2) —S1+S2+S3+S4, obs. 1.0(5) 31(3) 63(5) —S1+S2+S3+S4, pred.c 1.2 32 84 —% of total signal in ssb o2 10 17 —S4, % in ssb — 63 62 —

27Al NMR peaks% loss in total signalad 9(2) 14(2) 38(5) 5(1)% loss, predictedb 1 16 42 —% Loss, central peak(s)e 5(2) 20(2) 55(2) �1(1)A1, % of total area 3.5(5)f 2(1) 6(2) —A2, % of total area 0.4(1) 11(2) 22(2) —A1+A2, obs. 0.4(1) 13(2) 28(2) —A1+A2, pred.c 0.60 16 42 —% total signal in ssb E0 11 35A1, % in ssb — 13 27A2, % in ssb — 40 42

a Relative to intensity standard, normalized to amount of SiO2 or Al2O3 in sample; large uncertainties for 29Si data are primarily due to low signal

to noise ratios in fully relaxed spectra for standard (synthetic forsterite) and in synthetic pyrope. b Predicted from assumption that signals from

first cation neighbors to paramagnetic impurities are lost from observed spectra, eqn (2) (27Al) or eqn (3) (29Si) in text. c Predicted from assumption

that peaks are shifted by paramagnetic ion in second cation shell, eqn (4) (27Al) or eqn (5) (29Si) in text. d Based on areas of all central peaks and

sidebands for all transitions. e Based on area of central peak(s), corrected for contribution from overlapping satellite transition sidebands. f This

peak only probably represents IVAl in a pyroxene impurity in synthetic pyrope, not a shifted VIAl peak. It is thus not included in the sum below.

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pyrope (relative to the central peak), and subtracted this from

each sideband in the Fe-bearing garnets. The remaining

intensity was divided among the A1, A2, and main peak

components of the sidebands by fitting. This approach

assumes that the quadrupolar parameters, which govern the

intensities of the satellite sidebands in the absence of para-

magnetics, are the same in all three samples. This is justified,

because as discussed below, the signal that is actually observed

is probably only that for Al sites that do not have a first

neighbor Fe cation, hence the local electric field gradient for

the sites that contribute to the spectra should be all be similar:

any perturbations in the local electric field gradient from an

Fe2+ for Mg2+ substitution at a second cation neighbor

distance should be minor.

Results for these estimations of peak areas, referenced to the

signal observed for corundum, are summarized in Table 2. For

the gibbsite standard, the central peak area is within error of

that expected; its total signal (all transitions) is reduced

by a few percent, possibly because of some residual dipolar

broadening from the abundant H in this material. The total

signal for the synthetic pyrope is about 9% low. This origin of

this deficit is uncertain, but may be related to the presence of

the pyroxene impurity, and/or to imperfections in crystallinity

resulting from a relatively low synthesis temperature. The

overall signal loss for the Fe-containing pyropes is consider-

ably larger, and is greatest in the higher iron sample. Because

some of the signal for the central (�1/2) transition is trans-

ferred into the innermost sidebands as noted above, the deficit

in the area of the central peak is even more pronounced than

the loss of total signal. The areas of the A1 and A2 peaks in the

higher-iron pyrope are considerably greater; in both natural

pyropes these components have a greater proportion of central

transition intensity in the sidebands.

3.5 Spin–lattice relaxation

Spin–lattice relaxation data presented here are based on

saturation-recovery measurements. After a train of pulses to

saturate the magnetization, an incremented time delay (t) isallowed for partial recovery, then an observe pulse is used to

measure the signal. Directly integrated or fitted peak areas (M)

were normalized to that observed in the longest-delay spectrum

as an approximation of the fully relaxed value (MN) and

analyzed with a ‘‘stretched exponential’’ equation:19,20

M/MN = A{1 � exp[–(t/T0)b]} (1)

Here, A is a constant that would be precisely 1 if the longest-

delay magnetization was exactly that of the fully relaxed

sample; commonly relaxation may not be 100% complete so

that A is slightly less than 1. A value of either less than or

greater than 1 can result simply from fitting imprecision as

well. In cases where the magnetization equilibrates rapidly

throughout the sample (for example by physical or spin

diffusion), all spins may relax at the same rate and a single

exponential behavior (b = 1) is commonly observed. In this

case, T0 is equivalent to a single ‘‘spin–lattice relaxation time’’

T1. However, it is common for 29Si in silicates that relaxa-

tion is dominated by through-space dipolar coupling

between nuclear spins and the unpaired electronic spins of

paramagnetic impurities, e.g. transition metal cations. In this

case, relaxation of each spin will depend strongly on the

distance to an impurity center. This heterogeneity often results

in relaxation that is well-approximated by a ‘‘stretched’’

exponential with b = 0.5.19,20,22,30,51 In cases where mixed

relaxation mechanisms are present, relaxation curves may be

best fitted with values of b between 0.5 and 1.20

Experimental data were fitted in linear space (M/MN vs. t),as this gave correct weighting of uncertainties for each point

(Table 3). However, fitted curves are compared with data on

log–log plots (Fig. 5 and 6) to better test the shape of the

predicted curve and to examine behavior near to saturation.

To facilitate comparison among curves with different b values,

we define a parameter T50, where the magnetization reaches

50% of the fully recovered value. For the synthetic pyrope,29Si data for the main peak were well-fitted with b fixed at 0.5

(w2 was 15� lower than for b = 1), as expected from recent

studies of Mg2SiO4 polymorphs,30 diverging only slightly at

the very shortest t values where uncertainties on a log scale are

greatest. The relaxation rates of the tiny S1 and S2 peaks could

not be readily measured because of their low signal-to-noise

ratios. However, they clearly relaxed more rapidly than the

main peak, being detectable only at short pulse delays (Fig. 3).

For the white Fe-containing pyrope (DW), peak overlap

precluded independent measurements of relaxation for the

main and the S1 and S2 peaks, although again the intensities

of the latter were enhanced in short-delay spectra (Fig. 3). The

integrated sums of the main peak plus the S1 and S2 intensities

are again well-fitted with b fixed at 0.5 (Fig. 5) (w2 was

4� lower than for b = 1). Relaxation could not be readily

measured for the DP sample because it was so rapid as to

make saturation incomplete; analysis of inversion–recovery

experiments was difficult because of differential relaxation

among severely overlapping components. The 1.4 wt% FeO

in the DW pyrope lowered T50 by a factor of about 3600

compared to the synthetic pyrope.

The relatively good resolution between the main peak in the27Al spectrum of DW, and the summed ‘‘anomalous’’ peaks

A1 and A2, allowed independent analysis of their relaxation

behavior. In contrast to the 29Si data, both were well-fitted

Fig. 5 Log10–log10 plot of29Si saturation–recovery data for SP and

DW. Lines are the results of fits to data in linear space.

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with b = 1 (w2 was 4 to 20� lower than for b = 0.5). The

anomalous peaks relaxed almost an order of magnitude faster.

Uniquely among these samples, the relaxation curve for the

main peak of SP was not well-fitted with either b= 0.5 or 1. A

fitted value of b = 0.73 gave a good match to data with some

systematic deviation at very short t values (Fig. 6) (w2 was 6 to

12� lower than for b = 1 or 0.5).

4. Discussion

4.1 Signal loss and peak shifts

Relatively few NMR studies have been made of naturally

occurring silicate minerals that contain significant quantities

of paramagnetic transition metal cations such as Fe2+, as

early work reported major broadening of peaks, loss of

resolution, and loss of total signal intensity for materials such

as typical natural olivine ([Mg,Fe]2SiO4) with about 5 to

10 wt% FeO.5,6,13 In contrast, a number of NMR studies

have been reported on other crystalline oxides with high

contents of paramagnetic rare earth elements (REE). These

typically have much shorter electronic spin-relaxation times

than transition metal cations and therefore couple less

efficiently with typically observed nuclear spins, probably

reducing the effects on relaxation times and line widths.7 For

example, MAS NMR spectra for 119Sn in REE stannates with

the cubic pyrochlore structure showed peak shifts of 100s to

1000s of ppm, due to through-bond electronic perturbations

from first-neighbor magnetic cations (Fermi contact shifts).7

In solid solutions containing a REE and diamagnetic Y, effects

of magnetic ions in the second or third shell cation shell to the

Sn could also be seen. These much smaller shifts (a few to 10s

of ppm) could clearly be attributed to through-space magnetic

dipolar interactions between a paramagnetic cation in an

asymmetric (i.e. non-cubic point symmetry) site and the

nuclear spin (pseudocontact shift).7 The latter were also shown

to be important in the shifts in 89Y peak positions in the same

solid solutions.8 For both nuclides, multiple peaks could be

attributed to varying numbers of paramagnetic neighbors,

allowing deductions about the distributions of these cations

and the extent of solid solution. Spin–lattice relaxation was

also much more rapid for the shifted peaks. For the pseudo-

contact interaction, the size and sign of the shift depend on the

orientation of the principal magnetic axes at the paramagnetic

center relative to its vector with the nuclear spin being

observed, giving the possibility of several different shifts, either

positive or negative (or zero), for a single type of impurity in a

crystal.7,8

In a more recent study of zircon (ZrSiO4) containing up to

about 1 mol% V, thought to be present as paramagnetic V4+

(as also suggested by the loss of the 51V NMR signal for V5+

in the starting materials), two extra 29Si resonances were

detected, whose frequencies, although within normal ranges

for this nuclide in non-magnetic oxides, were apparently

shifted by contact interactions.9 The signal in these peaks

scaled with V concentration, they relaxed faster than the main,

unshifted peak, and correlations between temperature effects

on bulk susceptibility and shift were noted. The two shifted

peaks were interpreted as being related to V4+ in tetrahedral

vs. dodecahedral sites, but multiple Si–V distances caused by

the relatively low symmetry of the structure could complicate

this analysis.

In a recent study of the forsterite and wadsleyite poly-

morphs of Mg2SiO4, numerous small (each roughly

0.1–0.2% of total area), fast-relaxing 29Si MAS NMR peaks

were detected, many of which were well outside the known

range for SiO4 groups in silicates, e.g. between�28 and�60 ppmand at �128 ppm.29 These were hypothesized to result from

pseudocontact shifts caused by 10s to about 100 ppm of

transition metal impurities, and were detected primarily

through very high signal-to-noise ratios obtained by isotopic

enrichment. Such peaks were absent in spectra of the ring-

woodite polymorph, the cubic (spinel) structure of which may

have eliminated the necessary magnetic anisotropy at the

paramagnetic cations.

A number of other NMR studies of garnets have reported

effects of magnetic cations. For single crystals of mixed

Y3�xYbxAl5O12 (‘‘YAG’’) garnet, large electron-nuclear

dipolar shifts in 27Al resonances were observed as a function

of crystal orientation in the external field.40 Most or all of such

shifts would be averaged by MAS or by tumbling in a liquid,

leaving only a residual contribution or pseudocontact effect if

the magnetic susceptibility at the paramagnetic site is aniso-

tropic. Only the mean magnetic moment of the paramagnetic

Yb3+ cations was considered in the analysis, but again useful

information about cation distributions were obtained.40 An

early single-crystal 27Al NMR study of a very iron-rich natural

garnet reported a similar phenomenon.39 In a high-resolution

NMR study of powders of Tb3+-containing Y3Al5O12,

additional 89Y MAS NMR peaks appeared at higher dopant

concentrations and were attributed to pseudo-contact inter-

actions, while resonances for both this nuclide and 27Al were

severely broadened at high Tb contents.10

A complete analysis of either the Fermi contact shift or the

pseudocontact shifts in pyrope garnets would require detailed

information about the electronic states and the anisotropy of

the magnetic susceptibility at the paramagnetic centers. The

latter is complicated by the lack of a four-fold or three-fold

axis through the dodecahedral (X) site for Mg and Fe in the

garnet, thus precluding simplifying uniaxial symmetry.

Fig. 6 Log10–log10 plot of27Al saturation–recovery data for SP and

DW. Lines are the results of fits to data in linear space.

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Lacking such information, conclusions must remain qualitative.

However, useful constraints may be obtained from the relative

areas of shifted peaks.

Because relaxation by coupling to unpaired electrons

depends very strongly on distance (scaling by the inverse sixth

power,52) 29Si or 27Al nuclear spins that have first-neighbor

Fe2+ (or Co2+ for SP) may relax so rapidly that their

contribution to the spectrum is broadened to the point of

non-observability. (This is less likely to occur in REE systems,

as these cations generally have much shorter electron spin

relaxation times and thus couple more weakly with nuclear

spins.) If paramagnetic cations have concentration M per

formula unit ([Mg,Fe]3Al2Si3O12), then their number per

100 Al is N = 100(M/2), or N = 100(M/3) per 100 Si. If

these are randomly distributed among the X sites, the percen-

tage L of Al or Si in the first shells of the paramagnetic cations

would then simply be the product of N, the number of cations

in the first shell to the X site, and the probability that each of

these cations is the NMR-active nuclide. The latter is 1 for27Al, and 0.047 for 29Si. The number of first neighbor Al

cations to the X site in pyrope is 4, all at 0.32 nm; for Si there

are 2 at 0.29 nm and 4 at 0.35 nm.31,32,53 Thus,

LAl E 100(M/2) � 4 � 1 (2)

LSi E 100(M/3) � 6 � 0.047 (3)

For 29Si, the predicted percent losses in signal are small

(at most a few %) because of the low isotopic abundance:

most paramagnetic cations have only 28Si first-shell neighbors,

not 29Si. These are thus consistent with the observed zero to

few percent signal losses, given the relatively large uncertain-

ties in these measurements. For 27Al however, the predicted

losses in total NMR signal for the natural pyropes are much

larger, since every Fe (or Co) cation affects four neighboring27Al nuclei. The increase in the observed signal loss from DW

(1.4 wt% FeO) to DP (3.5 wt% FeO) scales in proportion to

the Fe content. The signal loss for the synthetic pyrope SP

seems anomalously high, perhaps in part because of its

pyroxene impurity or to imperfect crystallinity, both of which

would result in Al sites with higher than expected CQ values.

As noted above, as the content of paramagnetics increases, a

greater proportion of the signal for the central, �1/2 transitionis shifted into the innermost spinning sidebands by dipolar

broadening, resulting in a proportionately higher loss in the

intensity of the central peaks.

By analogy with previous studies of REE-containing

oxides,7 it seems likely that 27Al or 29Si NMR peaks for nuclei

with paramagnetic cations in their second shells will be the

observable resonances most strongly affected by the pseudo-

contact interaction. To assess this, we can approximate the

probability that a second neighbor cation to Si or to Al will be

a paramagnetic ion from the fraction of X cations that are Fe

(or Co), which is M/3, and the number of second neighbors.

For Al in the octahedral Y site, the latter is 6, all at 0.52 nm;

for Si in the tetrahedral Z site there are 6 at 0.54 nm and 6 at

0.58 nm. The percentages of shifted peak areas would thus be:

SAl E 100(M/3) � 6 (4)

SSi E 100(M/3) � 12 (5)

For 27Al, the observed percentage of the signal shifted into the

‘‘anomalous’’ peaks A1 and A2 scales with the content of

paramagnetics and is roughly approximated by eqn (4)

(Table 3). The shifted peak areas are somewhat over-predicted,

perhaps because there are some orientations of the magnetic

anisotropy at the paramagnetic center which produce a

negligible shift.7 The same is generally the case for 29Si: the

estimates above are approximately correct but tend to be

somewhat higher than observed shifted fractions. In any case,

for DP, peak overlap makes the observed estimates of the

shifted peak areas relatively inaccurate. The areas S4 peaks

(+135 ppm) in the 29Si spectra of the natural pyropes also

increase with paramagnetic content. However, whether these

large shifts are the result of multiple Fe neighbors or to a

particular relative orientation for the pseudocontact shift, or

perhaps to a different mechanism (a Fermi contact shift?) is

difficult to assess at this time. The case for 29Si is also more

complex than for 27Al because of two different second-

neighbor distances. We note that positions of the shifted peaks

for both 29Si and 27Al are different in SP from those in DW

and DP, probably because of differences in the electronic

structures and number of unpaired electrons between Co2+

and Fe2+.

4.2 Peak widths and relaxation

A number of factors could be responsible for the large effect of

increasing paramagnetic content on both the 29Si and the 27Al

peak widths. Residual dipolar broadening from insufficient

spinning rate is probably not a major contributor, as there is

very little increased broadening on lowering the spinning rate

from 20 to 12 kHz (Table 2). Although spin–lattice relaxation

is much faster in the Fe-rich pyropes, it is apparently not rapid

enough to be the major peak broadening mechanism: for

example, for the main peak in the 27Al spectrum of DW, the

inverse of the 27Al relaxation time constant (1/0.019 s = 53 Hz)

is relatively small compared to the peak width (5.8 ppm=900Hz).

This contribution will be more important as relaxation

becomes even faster, for example for the A1, A2 peaks of this

sample and possibly for DP. For 27Al, an increase in the

second-order quadrupolar broadening on addition of Fe can

play only a minor role as well, given the minor effect of this

cation on the overall width and shape of the satellite spinning

sideband manifold (Fig. 2), the similarity of effects of field

strength (9.4 vs. 14.1 T) on the peak positions for the three

samples (Table 2), and the small effects of field on the peak

widths. What may instead be the major controls on MAS peak

widths for both nuclides are distributions in paramagnetic

shifts caused by Fe2+ cations at distances farther than the

second shell, due to varying distances, orientations, and

numbers of such neighbors.

The 1.4 wt% FeO in DM causes a large increase in the

spin–lattice relaxation rates relative to the slightly-doped SP.

For 29Si this results in a 3600-fold reduction in the time to

relax to 50% of the equilibrium value (T50). Despite this effect,

for both samples the relaxation is well-fitted by a stretched

exponential function with b= 0.5, as is expected if the process

occurs primarily by through-space dipolar coupling between

nuclear spins and unpaired electron spins on paramagnetic

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impurity cations.19,20 This situation is characterized by nuclear

spins that have a wide range of relaxation rates because of

wide variations in distance to the impurity cations, which in

these samples is also more directly illustrated by the faster

relaxation of the ‘‘anomalous’’ peaks, which we hypothesize

results from sites especially close to such cations. The common

mechanism of spin diffusion by which energy is rapidly

transferred among nuclear spins, resulting in a uniform

relaxation rate, is relatively ineffective for 29Si at natural abun-

dance because of the low probability of 29Si–29Si neighbors.

In materials with low paramagnetic contents, relaxation for27Al is typically much faster than for 29Si, as is the case here for

the main peaks for the two nuclides in SP (Table 4), where T50

differs by a factor of 30. This may be because additional

relaxation mechanisms are available for the quadrupolar27Al, which can be relaxed by fluctuations in the local electric

field gradient, not just the fluctuations in the local magnetic

field which are required for the spin-1/2 29Si. In addition, 27Al

is 100% abundant, allowing a greater probability for spin

diffusion. In contrast to the behavior for 29Si, for the

faster-relaxing 27Al resonances (all peaks in DW and the A2

peak in SP), the relaxation curves clearly are best fit by single

exponential equations (b = 1 in eqn (1)). However, the more

slowly relaxing main peak in SP clearly has an intermediate

behavior, requiring a b value of 0.73. This suggests that for27Al, at least in an aluminosilicate with a low concentration of

paramagnetics, a low CQ and at high field and spinning rate,

through-space coupling to the electron spins may still play

some role in the relaxation. At the same time, we note that the

single-exponential behavior does not equate to a uniform

relaxation rate for all spins, as those in the contact-shifted

peaks continue to relax much faster.

4.3 Garnets vs. other silicates and oxides

Resolved resonances that are shifted by the effects of para-

magnetic ions have only occasionally been reported in solid-state

MAS NMR studies of oxides. The cubic overall symmetry of

the garnet structure, combined with the non-cubic point

symmetry of the site containing the magnetic impurity cations,

is unusual among silicates, but both features are shared by the

REE pyrochlore-structured materials where pseudocontact

(and Fermi contact) shifts have been so clearly documented.7,8

As described in the recent study of Mg2SiO4 forsterite and

wadsleyite, the lower overall symmetry of these phases leads to

a much larger number of different Si-cation distances (and

orientations) for each cation shell.29 It was apparent that

discrete pseudocontact shifted peaks were observable only at

low paramagnetic concentrations (10s to about 100 ppm),

which in that case were accidental impurities in the reagents.

In fact, when a deliberate addition of about 0.1 wt% CoO was

made, it served only to speed the overall relaxation rate and to

broaden the resonances. The latter effect may be an example of

the more general case, when significant quantities of para-

magnetic impurities in a relatively low-symmetry structure

produce a more-or-less continuous distribution of shifted

intensities (both up and down in frequency), which is most

commonly detected simply as an especially broad com-

ponent in a partially saturated spectrum. For quadrupolar

nuclides, detection of discrete pseudocontact-shifted peaks

may also be facilitated by small CQ values and resulting

narrow resonances, as in the 27Al spectra of garnets.

In the types of crystalline silicates and oxides that are

commonly studied by solid-state NMR, samples are generally

selected or synthesized to have low paramagnetic contents

(o1%). For at least natural isotopic abundance 29Si, however,

spin–lattice relaxation may still be dominated by through-

space dipolar coupling to such impurities, if other mechanisms

such as coupling to abundant, high-g nuclear spins, such as1H, are weak. At such concentrations, the sites giving rise to

the major resonances probably mostly have at least third

neighbor cation distances to paramagnetic centers. In any

but the most symmetrical structures, this means that nuclides

in crystallographically inequivalent sites see similar statistical

distributions of magnetic centers, and relaxation is affected

similarly. Thus, in such a homogeneous structure, observed

site populations are similar in spectra collected under partially

saturated conditions to those (generally much noisier) spectra

collected at full relaxation. Examples include IVSi groups with

varying numbers of bridging oxygens (‘‘Qn’’ species) in homo-

geneous alkali silicate glasses16,54 and coexisting IVSi and VISi

sites in high-pressure crystalline silicates.37,55 In single-phase

materials in which this kind of relaxation mechanism dom-

inates, marked differential relaxation may thus usually be

limited to the type of very fast relaxation of sites with close

paramagnetic neighbors as described here. Thus, strong

differential relaxation of major components of a spectrum will

most commonly imply the presence of more than one phase,

probably with different paramagnetic contents, of dimensions

larger than the effective range of major influence on relaxation

(probably a few nm). This effect can in fact be exploited to

observe and characterize otherwise undetected phase separa-

tion, for example in silicate glasses formed within the spinodal

for liquid–liquid immiscibility.56,57

Conclusions

Pyrope garnets ([Mg,Fe]3Al2Si3O12) with up to about 7% of

the dodecahedral sites occupied by Fe2+ (3.5 wt% FeO), as

well as a synthetic garnet doped with about 0.1% Co, have

resolved, extra, ‘‘anomalous’’ peaks in both their 29Si and 27Al

MAS NMR spectra that probably result from shifts induced

by the effects of unpaired electron spins, most like through the

pseudocontact interaction. The cubic overall symmetry of

the structure, combined with the relatively low symmetry of

the paramagnetic host site itself, may provide the unusual

Table 4 Summary of fits to spin–lattice relaxation curves for pyropesamples, based on the equation M/MN = A{1 � exp[–(t/T0)b]}.Standard errors in the last place are shown in parentheses

Nuclide Sample, peak A T0/s b T50/s

29Si SP: main peak 0.97(2) 130(15) 0.5a 68DW: main+S1+S2 1.00(2) 0.039(4) 0.5a 0.019

27Al SP: main peak 0.99(1) 3.5(2) 0.73(2) 2.2SP: A2 1.16(2) 0.019(1) 1a 0.011DW: main peak 0.96(1) 0.019(1) 1a 0.014DW: A1+A2 0.94(2) 0.0022(1) 1a 0.0017

a Fixed during fitting.

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environment where such effects can be observed, instead of the

more usual overall broadening of the resonance. At least for27Al, signal from sites with Fe2+ first neighbors appears to be

lost from the spectrum, even when all transitions can be

quantitatively observed; for both 27Al and 29Si, significant

fractions of the �1/2 transition signal are also shifted into the

spinning sidebands by dipolar line broadening. The areas of

resolved, shifted peaks scale with paramagnetic content and

are at least roughly correlated with the calculated fractions of

Si and Al that have Fe2+ or Co2+ in their second cation shells.

Spin–lattice relaxation rates of all components of the spectra

are enhanced greatly by the paramagnetics, with the shifted,

‘‘anomalous’’ peaks relaxing an order of magnitude or more

faster. However, rapid spin–lattice relaxation is probably not

the main contributor to the observed peak broadening, which

may instead be dominated by distributions of dipolar shifts

induced by more distant Fe2+ neighbors. Quantitative under-

standing of all of these effects will require more detailed

studies with varying cations and dopant levels, as well as

detailed knowledge of the electronic and magnetic structure

of the paramagnetic ions in the garnet structure. Variable

temperature studies would also be useful to confirm the

hypothesized contact shifts, as these are expected to have

strong temperature dependence.9 However, some of our con-

clusions about the distance range of effects on signal loss and

on resonance shifts can probably be applied to related silicate

and oxide structures.

Acknowledgements

We thank Charles Geiger for the loan of several other pyrope

samples in which the first hints of the ‘‘anomalous’’ peaks were

first seen, ongoing discussions of garnet structure, and helpful

comments on an early version of this paper, and Bob Jones for

the electron microprobe analyses. Comments by two anonymous

reviewers helped to improve the original manuscript consider-

ably. We acknowledge the support of the U.S. National

Science Foundation, grant EAR-0408410.

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