R. James Kirkpatrick

29Si MAS NMR study of the structure of calcium silicate hydrate

This paper presents the results of a comprehensive investigation of single-phase calcium silicate hydrate (CSH) with known compositions using the combined capabilities of 29Si magic angle spinning (MAS) NMR, powder X-ray diffraction (XRD), and chemical analysis of the solution and solid. CSH gels with C/S ratios ranging from 0.4 to 1.85 have been synthesized by hydration of highly reactive β-C2S and aqueous reaction of fumed silica with CaO and separately with highly reactive β-C2S. The main findings include the following. (1) CSH shows continuity and diversity in both composition and structure and forms a continuous structural series. (2) Phase-pure CSH has C/S ratios between 0.6–1.54. (3) SiOH and CaOH bonds both occur in CSH, with the abundance of the former decreasing and that of the latter increasing with increasing C/S ratio. Model calculation indicates that there are between 0.13–0.43 SiOH bonds per tetrahedron and the CaOH/Ca ratio varies between nearly 0 and 0.64. An ideal formula for CSH with a C/S ratio of 1.5 is: Ca4.5[Si3O8(OH)](OH)4 · nH2O. A defect-tobermorite structural model is proposed for CSH in which individual layers have the basic structure of 1.4-nm tobermorite but contain a significant concentration of defects and are more disordered. Stacking disorder between adjacent layer and disorder within individual layers may both contribute to the local and long-range disorder and thus to the diversity of CSH.

Nuclear magnetic resonance investigation of the structures of phosphate and phosphate-containing glasses: a review

This paper presents a review of the nuclear magnetic resonance (NMR) data for phosphate and phosphate-containing glasses obtained primarily within the past 10 years and of the structural interpretations based on those data. Compositions discussed include P2O5, alkali and alkaline earth phosphates, aluminophosphates, borophosphates, fluorophosphates, and phosphate-containing silicate and aluminosilicate glasses. 31P NMR data, in conjunction with 27Al, 29Si, 11B, 7Li, and 23Na data if appropriate, have proven very powerful in providing direct evidence about the local structural environments present in the these materials and in many cases have allowed interpretation of the physical and chemical behavior of these glasses in terms of polyhedral structures.

Structure and Cation Effects on Phosphorus-31 NMR Chemical Shifts and Chemical-Shift Anisotropies of Orthophosphates*

Abstract We have obtained high-resolution “magic-angle” sample-spinning phosphorus-31 NMR spectra of a variety of orthophosphates, and have derived chemical-shift tensor elements and anisotropies from spinning-sideband patterns. An empirical correlation between isotropic chemical shift and Z / tsr is noted, where Z is the cation charge and r the cation radius. The chemical-shift anisotropy varies in an approximately linear fashion with the PO bond length, and with the deviation of the OPO bond angle from the tetrahedral value. These results may be of use in testing models of phosphate bonding in amorphous systems, such as glasses and ceramics.

High-resolution 23Na, 27Al and 29Si NMR spectroscopy of framework Aluminosilicate glasses

The results of high-resolution 23Na, 27Al and 29Si NMR spectroscopy of aluminosilicate glasses with MO/Al2O3 or L2OAl2O3 = 1 (M = +2, L = +1 cations) shows that these glasses have a fully polymerized tetrahedral framework structure with only a defect level of non-bridging oxygens. The chemical shifts of the peak maxima for all three nuclides become less shielded (less negative or more positive) with decreasing Si/ (Si + Al). For 29Si and 27Al, this variation parallels the variation of framework crystalline silicates. The 23Na chemicAl shifts for the glasses becomes less shielded (less negative) with decreasing Na/(Na + K), opposite to the trend for crystalline alkali feldspars. Few data exist for the 23Na chemical shifts of crystalline samples, and the structural causes of variation in 23Na chemical shifts are not well understood. The 27Al and 29Si chemical shifts of the glasses do not vary significantly with different large (modifier) cations. The 29Si chemical shifts provide estimates of average Si-O-T (T = Si, Al)bond angles and Si-O bond distances and the 27AL and 29Si chemical shifts and peak breadths are consistent with a decrease in the tetrahedralring order (number of tetrahedra per ring) with decreasing Si/(Si + Al). The data presented here for fully polymerized glasses form a base from which the data for aluminosilicate glasses containing both fully polymerized sites and less polymerized sites can be interpreted.

Boron-11 nuclear magnetic resonance spectroscopic study of borate and borosilicate minerals and a borosilicate glass

Borates and borosilicates form a wide class of technologically and geochemically important systems (Z-3), many of which have been characterized quite thoroughly by means of wide-line NMR spectroscopy in pioneering studies by Bray and co-workers (3). More recently, “magic-angle” and “variable-angle” sample-spinning Fourier transform NMR methods have been applied to the investigation of the structures of such materials (4-6) in an attempt to supplement existing nuclear quadrupole coupling constant (e2qQ/h) data with chemical-shift information for both tetrahedral (BOs) and trigonal (BOs) sites. In this Note, we report the results of a study aimed at determining the optimum conditions for obtaining well-resolved boron-l 1 MASS NMR spectra of a wide range of borates and borosilicates. We show that rapid (36 kHz) sample spinning, together with high-power proton decoupling, is generally desirable for rapid acquisition of boron11 NMR spectra of borates and borosilicates, from which accurate trigonal/tetrahedral ratios may be determined. In addition, we demonstrate for the systems we have studied that the B03/B04 solid-state chemical-shift ranges occur in well-defined, nonoverlapping regions (although overlap may of course occur as more systems are investigated). Taken together, our results indicate that high-field boron11 MASS NMR should be a useful, rapid method for quantitatively estimating trigonal/ tetrahedral ratios in many natural and synthetic boron-containing materials, such as glasses, minerals, and some zeolites. NMR spectra were obtained on a home-built 11.7 T spectrometer (corresponding to a boron11 NMR frequency of 160.4 MHz) basically as described previously (4, 6). All spectra were recorded using 2 PS pulse excitation. The solution 90” pulse width for boron trifluoride etherate (BF3. Et20) was 9 I.LS. Chemical shifts are reported in parts per million (ppm) from an external sample of BF3 Et20 and positive values correspond to low-field, high-frequency, paramagnetic, or deshielded shifts.

Raman spectroscopy of C-S-H, tobermorite, and jennite

Raman spectra of single-phase calcium-silicate hydrate (C-S-H) samples with C/S ratios between 0.88 and 1.45 are consistent with a defect tobermorite model for the structure of these materials, in agreement with previously published nuclear magnetic resonance (NMR) spectroscopic data for the same samples. The Raman spectra of C-S-H samples with C/S ratios 1 and Q 2 Si sites and indicate the possible presence of jennite-like environments. Raman spectroscopy, like infrared and NMR, is a probe of local structure on the atomic nearest neighbor and next-nearest neighbor scale, and thus provides significantly different information than diffraction methods. The Raman spectra of the C-S-H have a rich structure and contain peaks for Si-O stretching, Si-O-Si bending, internal deformation of the Si-O tetrahedra and Ca-O polyhedra, and characteristic peaks at lower frequencies. The spectra of jennite and 14A tobermorite are quite similar, confirming the result from NMR spectroscopy that jennite has dominantly Q 2 polymerization. The Raman spectra of 11A and 14A tobermorite are also similar, although our sample of 11A tobermorite has a significant concentration of Q 3 Si sites, which indicates cross-linking of the chains.

29Si and 17O NMR investigation of the structure of some crystalline calcium silicate hydrates

Abstract This paper presents the results of a systematic investigation of the structure of 17O-enriched, hydrothermally synthesized 1.1-nm tobermorite, 1.4-nm tobermorite, jennite, calciochondrodite, xonotlite, and hillebrandite, using 29Si magic angle spinning (MAS) NMR, 1H-29Si cross-polarization magic angle spinning (CPMAS) NMR, and 17O MAS NMR. The 17O and most of the 1H-29Si CPMAS results are the first reported for these phases. Six types of oxygen sites were observed in tobermorite and jennite, including both SiOH CaOH linkages. The structure of 1.4-nm tobermorite is similar to that of 1.1-nm tobermorite with about 26% of the Ca2+′s in the interlayers. The results support the proposed jennite structure in which silicate chains and rows of OH− groups alternately occur along the CaO layers [1]. Jennite contains long, single silicate chains similar to those in 1.4-nm tobermorite, with SiOH sites primarily occurring on bridging tetrahedra, and there seems to be no interlayer Ca2+′s. Although the Si sites in xonotlite and calciochondrodite cross-polarize well, neither contains SiOH linkages.

The structural environments of cations adsorbed onto clays: 133Cs variable-temperature MAS NMR spectroscopic study of hectorite

Abstract 133 Cs Variable-Temperature Magic-Angle-Spinning Nuclear Magnetic Resonance (VT-MAS NMR) spectroscopy shows that Cs on the clay mineral hectorite occurs in several distinctly different chemical environments, and that motional averaging of Cs between some of these sites occurs above ~ −40°C if water is present in the interlayer. At temperatures above ~ −10°C, spectra for slurries of hectorite in CsCl solutions yield two peaks, one due to Cs in solution, and the other due to Cs motionally averaged on the clay. At temperatures below ~ −60°C, motional averaging of the adsorbed Cs slows sufficiently to allow resolution of two peaks representing different Cs-environments on the clay. We use the Stern-Gouy model to explain these peaks, and assign one to Cs in the Stern layer (relatively tightly bound to the basal oxygens) and the other to Cs in the Gouy diffuse layer. A sample hydrated at 100% relative humidity yields similar results, except that there is no peak for Cs in the solution. Cs-exchanged hectorite dehydrated at 500°C yields peaks for two different sites on the clay, which we interpret to be due to a highly coordinated site (probably 12) and a less coordinated site (possibly 9), both in the interlayer. A sample partially dehydrated at 100°C yields similar peaks, but there also appears to be motional averaging of Cs over these two different Cs-environments in the interlayer.

Pre-eruption history of phyric basalts from DSDP legs 45 and 46: Evidence from morphology and zoning patterns in plagioclase

Phyric basalts recovered from DSDP Legs 45 and 46 contain abundant plagioclase phenocrysts which occur as either discrete single grains (megacrysts) or aggregates (glomerocrysts) and which are too abundant and too anorthitic to have crystallized from a liquid with the observed bulk rock composition. Almost all the plagioclase crystals are complexly zoned. In most cases two abrupt and relatively large compositional changes associated with continuous internal morphologic boundaries divide the plagioclase crystals into three parts: core, mantle and rim. The cores exhibit two major types of morphology: tabular, with a euhedral to slightly rounded outline; or a skeletal inner core wrapped by a slightly rounded homogeneous outer core. The mantle region is characterized by a zoning pattern composed of one to several spikes/plateaus superimposed on a gently zoned base line, with one large plateau always at the outside of the mantle, and by, in most cases, a rounded internal morphology. The inner rim is typically oscillatory zoned. The width of the outer rim can be correlated with the position of the individual crystal in the basalt pillow. The presence of a skeletal inner core and the concentration of glass inclusions in low-An zones in the mantle region suggest that the liquid in which these parts of the crystals were growing was undercooled some amount. The resorption features at the outer margins of low-An zones indicate superheating of the liquid with respect to the crystal.It is proposed that the plagioclase cores formed during injection of primitive magma into a previously existing magma chamber, that the mantle formed during mixing of a partially mixed magma and the remaining magma already in the chamber, and that the inner rim formed when the mixed magma was in a sheeted dike system. The large plateau at the outside of the mantle may have formed during the injection of the next batch of primitive magma into the main chamber, which may trigger an eruption. This model is consistent with fluid dynamic calculations and geochemically based magma mixing models, and is suggested to be the major mechanism for generating the disequilibrium conditions in the magma.

133Cs NMR study of cesium on the surfaces of kaolinite and illite

Abstract 133Cs MAS NMR of Cs-exchanged illite, kaolinite, boehmite, and silica gel is shown to be a powerful tool to investigate the adsorption sites and atomic dynamics of Cs on mineral surfaces. Cesium is adsorbed on these mineral surfaces in primarily two ways: at sites relatively tightly bonded to the surface (Stern layer, Cs1) and at more loosely bonded sites in the diffuse (Gouy) layer (Cs2). For illite, both edge sites and crystallite basal surfaces are important adsorption sites. For kaolinite, edge sites, expandable layers, and probably crystallite basal surfaces are important. The 133Cs NMR chemical shifts for the Cs1 site become more shielded (more negative) as the Si Al ratio of the substrate phase increases, paralleling the chemical shift variations of other cations and consistent with this site being relatively tightly bonded to the surface. The 133Cs NMR chemical shifts of Cs2 do not vary systematically with solid composition due to the larger distance of these sites from the surface and weaker electrostatic attraction to the surface compared to Cs1. Rather, the Cs2 chemical shifts are significantly influenced by relative humidity (R. H.) and Cs population ( Cs H 2 O ratio) on the surface. The Cs1 chemical shifts vary less with these parameters. Cs2 is removed by washing with 1–5 mL of deionized water due to its weak attraction to the surface. The Cs1 chemical shifts become less shielded after washing and with decreasing solution concentration due to a decrease in the Cs surface density. At 100% R. H., Cs in the two sites undergoes motional averaging at frequencies > 100 kHz. With decreasing R. H., peaks for Cs on the two sites are resolved due to decreasing exchange frequencies related to a decreasing number of adsorbed water layers. Motional averaging at 100% R. H. is verified by low temperature experiments with illite.