NMR spectroscopy of minerals and allied materials

Sharon E. Ashbrook and Daniel M. Dawson DOI: 10.1039/9781782624103-00001

Nuclear Magnetic Resonance (NMR) spectroscopy has played an important role over many years in understanding the structure and reactivity of minerals. The advent of high-resolution NMR techniques, higher magnetic field strengths and recent improvements in theoretical calculations have widened the potential use and application of NMR in mineralogy and solid-state chemistry. Here we review work from the period 2010–2014, focussing primarily on materials formally classified as minerals, but mentioning allied materials that are wholly or partly synthetic, where significant structural or mineralogical insight has been demonstrated.

1 Introduction

There are over 4600 formally recognised types of mineral, i.e., elements or chemical compounds that occur naturally as a result of geological processes. Minerals are usually defined as naturally-occurring, stable solids with a specific chemical composition (within some defined limits) and exhibiting an ordered atomic structure. In the past, minerals were typically considered to be inorganic and abiogenic, with biological substances, e.g., bones and shells, excluded from classification, although this latter point has always been the subject of some debate. However, today, many classification schemes include all biominerals, and a specific class of organic minerals is also recognised. The distinction between minerals and rocks, however, should be noted – the latter typically being aggregates containing one or more minerals and exhibiting structural and chemical heterogeneity."

The study of minerals has long been recognised to be of considerable importance – not only for understanding the fundamental physical and chemical properties of the materials that make up the surface and inner depths of our planet, but to understand the effects of variations in pressure or temperature upon these properties, and the changes that can occur due to weathering or chemical processes. Many minerals also find industrial use in, e.g., ceramics, cements, fertilisers, catalysts and glasses, making an understanding of their structure, composition and reactivity vital. A large number of materials are structurally and/or chemically related to minerals, and can be produced either by chemical modification/substitution of a mineral or by an entirely synthetic approach. While not strictly minerals (as they are not naturally formed), they nonetheless provide additional possibilities for application, and their study may well also provide insight into that of the parent/related mineral.

All of the 90 natural elements have some geochemical interest, but the bulk (~90%) of the Earth's crust is composed of silicate and alumino-silicate minerals, with elements such as Fe, Ca, Na, K and Mg also of importance, as shown in Fig. 1a. The inner regions of the Earth, i.e., where pressures and temperatures increase, are also typically composed of silicate minerals, but with increased Mg and Fe content, as shown in Fig. 1b. Figure 1 also shows the changes in the major mineral component of the Earth with increasing depth results in the designation of 'layers', e.g., the change from a-(Mg,Fe) 2SiO4 to ß-(Mg,Fe)2SiO4 at ~410 km, signifying the boundary between the upper mantle and the upper transition zone, with further transitions to ?-(Mg,Fe) 2 SiO4 at the boundary with the lower transition zone, and to (Mg,Fe)SiO3 perovskite in the lower mantle.

The requirement for an ordered atomic structure in a mineral has resulted in much previous mineralogical study being carried out using crystallographic diffraction. However, many minerals form extensive solid solutions (i.e., they exhibit a variation in chemical composition) where the exact ordering/position of substitution is not known. Diffraction provides information on the average structure, but is rarely able to provide the atomic-level detail required to understand how and why the structure and/or properties of a mineral vary with composition. This is particularly true where the difference in scattering factors is small (e.g., Al3+ and Si4+), concentrations are low, or dynamics play a significant role. The sensitivity of NMR spectroscopy to the local structure, through the variation of interactions such as the chemical shielding, J-coupling or quadrupolar coupling, provides an ideal tool for structural investigation of minerals, and the recent developments in hardware and software, enabling high-resolution NMR spectra of solids to be acquired with good sensitivity, have considerably widened the application of this technique. Despite these advances, complex spectral lineshapes can be observed for disordered materials. However, over the last 10 years, the approach of combining experiment with theoretical calculations of NMR parameters (typically using density functional theory, DFT) has grown to enable the assignment of spectral resonances and the prediction of spectra for many possible models when a structure is less well defined.

In this chapter we review the NMR spectroscopy of minerals published in the period 2010–2014. We assume a basic working knowledge of the methods used to obtain high-resolution NMR spectra of solids (e.g., MAS, decoupling, MQMAS, etc.,) and some knowledge of prior significant work on minerals, e.g., the use of 29Si NMR to study Si/Al ordering in aluminosilicates. More complete reviews on these can be found in ref. 2, 3, 5 and 6. We shall initially concern ourselves with silicate minerals, dividing these according to their structural features, e.g., materials containing isolated units, chains, layers or frameworks of silicate tetrahedra, before turning to non-silicate minerals (which we shall categorise according to their chemical type). Although we shall focus primarily on materials in the more formal classification of minerals described above, we shall also mention wholly or partly synthetic allied materials, where significant mineralogical insight has been shown.

2 Silicate minerals

As Si and O dominate the Earth's crust and much of the mantle, silicates are the most important class of rock-forming minerals, and exhibit great structural variation owing to the stability of Si–O bonds. Most crustal silicates are based on SiO44- tetrahedra, which may occur in isolation or combine to form more complicated structures. Although rare in nature, six-coordinate Si may also be observed in high-pressure minerals. Silicate minerals are commonly classified according to the way the silicate polyhedra are linked and the degree of polymerisation, as shown in Fig. 2a. Minerals containing isolated tetrahedra are termed ortho- or nesosilicates, while those with two corner-sharing tetrahedra, i.e., Si2O76-, are referred to as pyro- or sorosilicates. Tetrahedra may also form rings (cyclosilicates), chains (inosilicates), sheets (phyllosilicates) or 3D networks (tectosilicates). The ease of 29Si NMR spectroscopy (I = 1/2), and the sensitivity of the 29Si chemical shift to coordination number, type of coordinating atoms, degree of polymerisation (denoted Qn, where n is the number of coordinated oxygens that "bridge" to other silicons), and even the substitution of next nearest neighbour (NNN) atoms, has led to the widespread application for the study of silicate minerals. The 29Si MAS NMR spectrum of the aluminosilicate mineral analcime in Fig. 2b exhibits a change in the chemical shift of ~6 ppm for each NNN Al substituted. More recently, the study of 27Al (I = 5/2), 17O (I = 5/2) and other substituted cations has become more widespread.

2.1 Ortho-, pyro- and ring silicates

The simplest silicate minerals contain isolated SiO44- tetrahedra, corner-sharing tetrahedra in Si2O76-, or small cyclic clusters of tetrahedra. The most important of these is olivine (Mg,Fe)2SiO4, an orthosilicate existing as a solid solution from Mg-rich forsterite to Fe-rich fayalite, which dominates the upper mantle. Olivine contains SiO44- linked through six-coordinate Mg2+/Fe2+. This gives a single Si site, two distinct cation sites and three distinct O. As it provides an Fe-free model for olivine and can readily be synthesised at ambient pressure, forsterite has been extensively studied by NMR. Early work determined a 29Si isotropic chemical shift, diso, of – 62 ppm, typical of Q0 orthosilicates, and the principal components of the Si shielding tensor were determined. More recently, Palke and Stebbins carried out NMR measurements on Si-enriched forsterite and showed that a series of peaks between -28 and -60 ppm, and one at -128.5 ppm (each accounting for 0.1–0.2% of the spectral intensity) result from trace paramagnetic impurities, with a strong linear correlation of shift with T-1. The low natural abundance (0.037%) of the only NMR-active isotope of oxygen, 17O, has restricted most oxygen NMR studies of forsterite to 17O-enriched material." Isotropic 17O NMR spectra have been obtained with composite spinning (DAS/DOR) techniques, and 2D multiple-quantum (MQ) MAS experiments, and have been assigned using periodic DFT calculations. 25Mg NMR parameters were determined by early single-crystal studies, and later high-field (21.1 T) MAS experiments. More recent work refined the parameters using high-field MAS, MQMAS and CPMG (Carr-Purcell Meiboom-Gill) experiments on static samples. The work confirmed CQ for Mg2 (4.31 MHz) and showed that the value for Mg1 (5.33 MHz) was larger than determined in previous work.

Since 2010, most NMR investigations of forsterite have focussed on its carbonation, a reaction relevant to geologic carbon sequestration. Kwak et al., studied the process ex situ, by 29Si and 13C NMR. Hydrolysis of forsterite (at 80 °C and 96 atm) produced Q1 (-84.8 ppm) and Q2(-91.8 ppm) surface species, while reaction with supercritical CO2 and H2O produced Q4 species (-111.6 ppm) and small amounts of Q3 (-102 ppm) and Q2 (-91.8 ppm), suggesting that the formation of magnesite (MgCO3) was more rapid than forsterite hydrolysis. An intermediate dypingite (Mg5(CO3) (OH2 · 5H2O) phase was also identified by 13C MAS NMR. In further work, the authors considered the effect of water content on carbonation, and showed that no reaction occurred for trace amounts of water, while below the saturation level a layer of partially-hydrated/ hydroxylated magnesium carbonates and hydrous amorphous silica species formed on the forsterite surface. Above the saturation level the reaction products were magnesite and amorphous polymerised silica. 13C MAS NMR showed reaction products at shifts between 160 and 175 ppm, with weaker peaks at 164.1 and 166.4 ppm attributed to dypingite, hydromagnesite (3MgCO3 · Mg(OH)2 · 3H2O), and nesquehonite (MgCO3 · 3H2O). Hu and co-workers studied the carbonation of forsterite in situ, using a high-pressure MAS rotor, capable of an internal pressure of 150 bar. Reaction with enriched CO2 was followed by 13C NMR at ~ 2 kHz MAS. In addition to CO2 at 126.0 ppm, a sharp resonance at 161.5 ppm was attributed to mobile HCO3-, an intermediate that disappeared before MgCO3 formed. Felmy et al. investigated the carbonation reaction at lower temperature (35/50 °C) and determined that initial products were nesquehonite and magnesite after 3–4 days, with magnesite and amorphous silica formed at longer times (14 days). Work in 2014 revealed that the particles formed in this reaction exhibited a uniform submicron grain size, with rhombohedral morphologies, not consistent with growth on the forsterite surface. Additional work considered the impact of diffusive transport in the carbonation, with in situC NMR measurements of static samples at 105–120 bar and 80 °C. The carbonate solid produced a broad, axially-symmetric powder pattern with peaks from HCO3- and CO2 also observed.

Liu et al. studied two polymorphs of ZnSiO4 (willemite), synthesised at 6.5 GPa (phase III) and 8 GPa (phase IV). The structure of phase III resembled olivine, but with four-, rather than six-coordinate, Zn. Phase IV has four-coordinate Zn and Si, but contained Zn2O6 dimers. 29Si MAS NMR confirmed that the two polymorphs were different from those already known, and that all Si was four coordinate.

Over the years there has been much interest in the hydration of the minerals in the Earth's mantle, which is thought to contain at least as much water as the Earth's surface. Mantle silicates are formally anhydrous, with no hydrous minerals stable below the upper mantle, and melts and fluids assumed to be absent owing to the increased pressure. Therefore, low concentrations of water are thought to be incorporated as structurally-bound hydroxyl defects in anhydrous minerals. However, the difficulty with identifying low concentrations (ppm) of defects has led to the study of model systems containing stoichiometric proportions of hydrogen. The humite minerals, consisting of n forsterite-like layers separated by layers of Mg(OH,F)2, have been widely studied using solid-state NMR. There are four minerals in the family – norbergite (n = 1), chondrodite (n = 2), humite (n = 3) and clinohumite (n = 4). These minerals were studied in early work by 1H and 29Si NMR, and in 2010 Davis et al. reported the Mg CPMG NMR spectrum of clinohumite. This was compared to a 1H-25Mg CP CPMG NMR spectrum of acid-leached for-sterite, indicating the formation of humite-like layers when forsterite was exposed to acidic conditions for 310 h. More detailed insight into the structure of the hydroxyl end members has been obtained from 17O MQMAS NMR of enriched materials. Work by Ashbrook and co-workers used 2H NMR to investigate motion of the hydroxyl groups in clinohumite (motivated by an earlier observation using 17O satellite-transition (ST) MAS experiments of microsecond timescale dynamics in chondrodite and clinohumite). Diffraction reveals two distinct H sites for each hydroxyl group, each with an occupancy of 50%. DFT calculations confirmed that 17O MQMAS and STMAS spectra could only be simulated assuming dynamic exchange of the hydroxyl protons between H1 and H2. 2H NMR was then able to confirm the presence of microsecond timescale dynamics. The rate constant obtained (1.0-1.3 x 105 s-1 at 298 K) was in good agreement with that from 17O NMR, and an activation energy of 40 kJ mol-1 was estimated for H1/ H2 exchange.

The substitution of OH- for F- in the humites occurs commonly in nature, and is thought to favour occupancy of H1, owing to the formation of O–H ... F- hydrogen bonds. Diffraction studies of 50% fluorinated humites have been interpreted in terms of full occupancy of a single H site. This seemed to be confirmed by 2H MAS NMR of 50% fluorinated clinohumite, where a single sharp resonance was observed with a temperature-independent linewidth. However, subsequent F NMR revealed four distinct resonances (-166.4, -169.3, -175.1 and -177.7 ppm), indicating multiple F environments. DFT calculations showed that the different shifts resulted from different anions (i.e., OD-/ F-) on neighbouring sites. The assignments were supported by DQ experiments, which revealed unexpected J-couplings between some species. J-couplings are usually thought to imply covalent bonding, whereas the fluorines in clinohumite are coordinated only by Mg. However, DFT calculations of these 19F-19F "through-space" J-couplings were in good agreement with experiment. A natural clinohumite sample (with composition [MATHEMATICAL EXPRESSION OMITTED]) exhibited a very similar 19F spectrum, with an additional low-intensity resonance attributed to F close to Ti, suggesting that anion disorder is not related to the sample preparation, but intrinsic to the mineral.