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The beryl-group mineral johnkoivulaite, Cs(Be2B)Mg2Si6O18, was compressed hydrostatically in a diamond-anvil cell up to 10.2 GPa. In situ Raman spectroscopy and X-ray crystallography revealed a P6/mcc-to-P 3 c1 (second-order) phase transition on isothermal compression at the critical transition pressure Pc = 4.13 ± 0.07 GPa. The elastic parameters determined for the volume elasticity of the two polymorphs correspond to a Birch-Murnaghan equation of state with K0 = 148 ± 2 GPa and K ′ = 0 for P < Pc and K0 = 75.5 ± 0.9 GPa with K ′ = 4 for P > Pc. The low-P polymorph shows anomalously linear compression behavior, as reported for several other beryl-derived framework structures. The high-P polymorph, which was found to follow a a ′ = a ⋅ 3 , c ′ = c superstructure according to P 3 c1, is almost twice as compressible as its low-P form. This is unique for any beryl-derived structure and can be attributed to the high degree of freedom for atomic displacements in the superstructure. The reduced symmetry can also be understood as the effect of the driving mechanism of the transformation. The extra-framework Cs channel components counteract any lateral deformation of the channels parallel to [0001] within the microporous framework and, similar to pezzottaite, are responsible for maintaining the trigonal/hexagonal lattice metrics. [ABSTRACT FROM AUTHOR]

A new mineral of the beryl group, johnkoivulaite, Cs(Be2B)Mg2Si6O18, was recovered from the gem gravels in the Pein Pyit area of the Mogok region in Myanmar. Thus far, only a single crystal has been identified. It has dimensions of about 5.8 x 5.7 x 5.5 mm. This specimen has an irregular shape but still has discernible crystal form with geometric growth patterns observed on the crystal faces. The crystal of johnkoivulaite is grayish-violet in color and strongly pleochroic, going from nearly colorless with E⊥c to dark bluish-violet with E||c. Johnkoivulaite has a Mohs hardness of about 7 1/2 and a measured density of 3.01(10) g/cm³. It is uniaxial (-) with ω = 1.607(1) and ε = 1.605(1) (white light). Electron microprobe analyses gave the empirical formula of (Cs0.85K0.10Na0.01) (Be1.88B1.12) (Mg1.66Fe0.27 Mn0.01Al0.05) (Si5.98)O18 with Be calculated by stoichiometry and confirmed by LA-ICP-MS measurements. Johnkoivulaite is hexagonal, P6/mmc (no. 192) with a = 9.469(2), c = 9.033(2) Å, V = 701.5(3) ų, and Z = 2. Johnkoivulaite is isostructural with beryl and exhibits partial substitution of B for Be at the distorted tetrahedral site, Mg for Al at the octahedral site, and Cs in the channel sites within the stacked Si6O18 rings. This substitution can be written as (CsMg2B)(□Al2Be)-1. Johnkoivulaite, the seventh member of the beryl group, is named in honor of gemologist John Koivula in recognition of his contributions to mineralogy and gemology. [ABSTRACT FROM AUTHOR]

In studies of the structures of silicate minerals and glasses, 29Si NMR spectroscopy has been applied almost exclusively to materials containing relatively low concentrations of ions with unpaired electrons spins, such as most transition metals and rare earths, because of sometimes severe broadening and shifting of resonances in such strongly paramagnetic systems. However, by adapting experimental methods to allow detection of very broad signals, and by examining a much expanded range of frequencies, we show here that accurate spectra can indeed be measured for a series of pure-phase transition metal silicates, including olivines (Fe2SiO4, Co2SiO4, Ni2SiO4), Co-akermanite (CoCa2Si2O7), and clinopyroxenes (CoCaSi2O6, NiCaSi2O6). For the latter two, we also present data for glasses of the same nominal compositions. For all of these phases, NMR peak broadening is large, in many cases to the point where magic angle spinning (MAS) does not enhance resolution; in all cases the observed paramagnetic shifts fall far outside the known range for diamagnetic silicates. There are clearly large effects of local structure on shift, suggesting great potential sensitivity to variations in chemical, electronic, and magnetic structure in both crystalline and amorphous phases. In particular, the spectra for the glasses are very different from those of crystals in both width and position. In most cases, measured spin-spin relaxation times are long enough to avoid major loss of signal during the NMR acquisition, but this may not always be the case in more magnetically dilute solid solutions, where small, broad paramagnetic resonances due to first cation neighbor interactions may in some systems be difficult to detect. To explore this issue, we present new data on a Ni-doped forsterite (Mg1.9Ni0.1SiO4) and on natural San Carlos olivine (Mg1.8Fe0.2SiO4), which improve upon the accuracy of our previous studies of these materials. It is clear that applications of NMR to paramagnetic silicates holds great promise not only for empirical studies of structure of a much wider range of compositions of minerals and glasses, but for future testing and application of advanced theoretical methods to more completely interpret such results. [ABSTRACT FROM AUTHOR]

In marked contrast to the single, narrow 29Si MAS NMR resonance for pure forsterite (Mg2SiO4), the spectra for synthetic forsterite containing 0.05 to 5% of the Mg2+ replaced with Ni2+, Co2+, or Fe2+ display between 4 and 26 additional, small, paramagnetically shifted peaks that are caused by interactions of the unpaired electron spins on the transition metal cations and the nuclear spins. Analyses of these relative peak areas, their numbers, and comparison of their positions to those in spectra of synthetic monticellites (CaMgSiO4) containing similar levels of transition metals, allows at least partial assignment to the effects of cations in either the M1 octahedral site only or to both M1 and M2 sites. More detailed analyses indicate that in forsterite, Ni2+ occupies only M1, Fe2+ occupies M1 and M2 roughly equally, and Co2+ occupies both M1 and M2 in an approximately 3:1 ratio. These findings for low concentrations agree with expectations from previous studies by other methods (e.g., XRD) of olivines with much higher transition metal cation contents. However, even low concentrations of Mn2+ (e.g., 0.1%), as well as higher Fe2+ contents (e.g., in natural San Carlos olivine) can broaden NMR peaks sufficiently to greatly reduce this kind of information content in spectra. [ABSTRACT FROM AUTHOR]

A suite of Fe-bearing natural and synthetic grossular-rich [(Ca,Fe)3(Al,Fe)2Si3O12] and pyrope-rich [(Mg,Fe)3Al2Si3O12] garnets were investigated using 27Al and 29Si MAS NMR and 57Fe Mössbauer spectroscopy. This was done to study the state of cation order-disorder in garnet solid solutions by analyzing paramagnetically shifted resonances in high-resolution NMR spectra. The Mössbauer spectra, along with electron microprobe results, give the concentrations of Fe2+ and Fe3+ and their site occupancies, even in grossular samples with very low concentrations of Fe. MAS NMR spectra were collected on Fe2+-bearing grossular- and pyrope-rich garnets with up to 25 mol% almandine component and on other Fe3+-bearing grossular samples with up to 9 mol% andradite component. Despite peak broadening and signal loss, structural information was even obtained from garnet with relatively high Fe contents (25 mol% almandine component). Paramagnetically shifted NMR peaks, related to the presence of Fe2+, were observed in grossular samples and are similar in nature to those reported previously for natural, relatively low-Fe2+ pyrope garnets by Stebbins and Kelsey (2009). Additional NMR peaks appear as the concentration of Fe2+ increases, reflecting an increase in the average number of neighboring Fe2+ cations around AlO6 and SiO4 groups. These newly observed peaks hold potential to provide information concerning the presence or absence of short-range ordering in certain Fe-bearing silicate garnets. The effect of Fe3+ on the MAS NMR spectra of garnet appears to be less pronounced, because it does not produce any observable paramagnetically shifted resonances. [ABSTRACT FROM AUTHOR]

We present 27Al and 29Si magic angle spinning nuclear magnetic resonance (MAS-NMR) spectra of Al- and Fe-bearing, high-pressure pyroxene and perovskite samples, synthesized in a multi-anvil apparatus at 26 GPa and 1900 °C at targeted compositions of (Mg1--xFex)(Si1--xAlx)O3 (x = 0.01, 0.025, and 0.05). 27Al MAS-NMR spectra of the perovskite samples indicate that Al3+ replaces both Si4+ in the octahedral site and Mg2+ in the larger 12-coordinated site. NMR signal loss caused by paramagnetic interactions is often a severe complication when performing NMR on materials containing Fe2+,3+; however, careful measurement of signal loss and comparison to total Fe content in these samples sheds light on the nature of Al and Fe incorporation. NMR signal loss for the pyroxenes is linearly related to total Fe content as would be expected in the case of uncorrelated substitution of randomly distributed Al and Fe. However, 27Al signal loss for the perovskite samples increases only slightly between samples with x = 0.01 and 0.025 indicating similar coordination of Al by Fe and non-random distribution. Complete signal loss at Fe/(Fe + Mg) = 0.05 suggests the upper limit of Fe2+ and Fe3+ concentration at which useful NMR data can be obtained for this system. [ABSTRACT FROM AUTHOR]

We present the results of a variable-temperature (VT) 31P magic angle spinning NMR (MAS-NMR) study of a series of solid solutions between different synthetic rare earth (RE = Y, La, Ce, Pr, Nd, Eu, Dy) orthophosphates (REPO4) taking either the monoclinic monazite or tetragonal xenotime (zircon) crystal structure. Solid solutions were formed by mixing a small amount of a paramagnetic REPO4 material (RE = Ce, Pr, Nd, Eu, Dy) with either diamagnetic LaPO4 or YPO4, which take the monoclinic and tetragonal crystal structures, respectively. Mixtures were made with up to 10 mol% (nominal content) of the paramagnetic component. 31P spectra of these materials contained several paramagnetically shifted resonances indicating some dissolution of the paramagnetic rare earth into the host LaPO4 or YPO4 phase; however, it is clear that none of the samples studied here reached a state of complete solid solution. The use of multiple paramagnetic species in dilute solid solution with two diamagnetic materials taking different crystal structures enabled an investigation of the probable mechanisms of paramagnetic interactions in the 31P NMR experiments. A peak assignment model is introduced for the 31P spectra. Our analysis indicates that the paramagnetic interactions are dominated by the Fermi contact shift with a secondary contribution from the so-called "pseudocontact" shift. [ABSTRACT FROM AUTHOR]

We present variable-temperature (VT, 31 to 204 °C) 29Si magic angle spinning NMR (MAS-NMR) spectra of a synthetic forsterite containing dilute paramagnetic impurities and 27Al and 29Si spectra of two iron-bearing natural pyrope garnet specimens from the Dora Maira ultrahigh-pressure metamorphism locality. Previous NMR studies of these materials reported multiple "anomalous" resonances outside the range of expected frequencies. It has been hypothesized that these peaks were caused by interactions between paramagnetic centers and the resonating nuclei. Our VT MAS-NMR study shows a linear relationship between peak position and inverse temperature for the "anomalous" resonances, confirming their assignment as paramagnetic shifts. A detailed analysis of the peaks in the pyropes indicates that the "anomalous" resonances are caused by a combination of the Fermi contact and pseudocontact interactions. The paramagnetic shifts in the 29Si spectra are likely caused by Fe2+ in the first, second, and third nearest dodecahedral sites while those in the 27Al spectra are likely caused by one or two Fe2+ in the first shell dodecahedral site. [ABSTRACT FROM AUTHOR]

The beryl-group mineral johnkoivulaite, Cs(Be2B)Mg2Si6O18, was compressed hydrostatically in a diamond-anvil cell up to 10.2 GPa. In situ Raman spectroscopy and X-ray crystallography revealed a P6/mcc-to-P 3 c1 (second-order) phase transition on isothermal compression at the critical transition pressure Pc = 4.13 ± 0.07 GPa. The elastic parameters determined for the volume elasticity of the two polymorphs correspond to a Birch-Murnaghan equation of state with K0 = 148 ± 2 GPa and K ′ = 0 for P < Pc and K0 = 75.5 ± 0.9 GPa with K ′ = 4 for P > Pc. The low-P polymorph shows anomalously linear compression behavior, as reported for several other beryl-derived framework structures. The high-P polymorph, which was found to follow a a ′ = a ⋅ 3 , c ′ = c superstructure according to P 3 c1, is almost twice as compressible as its low-P form. This is unique for any beryl-derived structure and can be attributed to the high degree of freedom for atomic displacements in the superstructure. The reduced symmetry can also be understood as the effect of the driving mechanism of the transformation. The extra-framework Cs channel components counteract any lateral deformation of the channels parallel to [0001] within the microporous framework and, similar to pezzottaite, are responsible for maintaining the trigonal/hexagonal lattice metrics. [ABSTRACT FROM AUTHOR]