Lee A. Groat

Linear birefringence and X-Ray diffraction studies of the structural phase transition in titanite, CaTiSiO5

AbstractThe antiferroelectric phase transition in titanite characterised by a collinear displacement of Ti-atoms from their central octahedral position is investigated using linear optical birefringence and X-ray diffraction techniques. Both methods indicate a continuous transition near 496 K and extra contributions to δΔn and X-ray intensity signals at higher temperatures. The critical exponent of the macroscopic order parameter is found to be β = 0.14 ± 0.02 and the transformation is interpreted in terms of a two-dimensional quasi-spin model. Topological features of the structure agree well with the spatial distribution of the diffuse scattering of the superstructure reflection 40

Plate tectonic gemstones

\bar 3

The amblygonite (LiAlPO4F)-montebrasite (LiAlPO4OH) solid solution: A combined powder and single-crystal neutron diffraction and solid-state 6Li MAS, CP MAS, and REDOR NMR study

.

A synchrotron radiation, HRTEM, X-ray powder diffraction, and Raman spectroscopic study of malayaite, CaSnSiO 5

Abstract We report on temperature dependencies of infrared (IR) fundamental, combination, and overtone vibrations of hydroxyl species (OH) in nominally anhydrous minerals (e.g., titanite), ferroelectric crystals (KTa1-xNbxO3, KTN), layer silicates (talc, mica, and pyrophyllite), and hydrous minerals such as apatite and synthetic hydrous/deuterated garnets [Ca3Al2(O4H4)3 and Ca3Al2(O4D4)3] for the temperature range of 20-300 K. Data obtained by high-resolution FTIR spectroscopy show that increasing temperature generally leads to a decrease in the band height and band area of fundamental vibrations of hydroxyl species, whereas the combination and first-overtone bands commonly show different temperature dependencies. The results show that in the investigated temperature range, the variations of the band height and area for different OH bands (especially for combinations and overtones) on cooling or heating do not reflect changes in OH concentrations in the materials, but relate to temperature-dependent absorption coefficients. The observations imply that absorption coefficients calibrated at room temperature cannot necessarily be used for the determination of hydroxyl contents at other temperatures

Oxy-schorl, Na(Fe2+2Al)Al6Si6O18(BO3)3(OH)3O, a new mineral from Zlatá Idka, Slovak Republic and Přibyslavice, Czech Republic

The gemstones jadeite and ruby generally form as a result of the plate tectonic processes subduction and collision. Jade made of jadeite (jadeitite) forms when supercritical fluids released from subducting oceanic crust condense in the overlying mantle wedge, 20–120 km deep in the Earth. Jadeitite deposits thus mark the location of exhumed fossil subduction zones. Ruby, the red gem variety of corundum, forms during amphibolite- and granulite-facies metamorphism or melting of mixed Al-rich and Si-poor protoliths, 10–40 km deep in the crust. Suitable conditions generally exist where passive-margin carbonates and shales are involved in continental collision. Most ruby deposits formed during Ediacaran-Cambrian (ca. 550 Ma) collisions that produced the East African–Antarctic orogen and the supercontinent Gondwana, or during Cenozoic collisions in south Asia. Ruby is thus a robust indicator of continental collision. As a result of these diagnostic properties, we propose the term “plate tectonic gemstones” (PTGs) for jadeitite and ruby. The PTGs are a new type of petrotectonic indicator that are mostly found in Neoproterozoic and younger rocks. The PTGs as petrotectonic indicators that form deep in the Earth have the added advantage that their record is unlikely to be obliterated by erosion, although the possibility of destruction via retrogression needs to be further assessed. Recognition of the PTGs links modern concepts of plate tectonics to economic gemstone deposits and ancient concepts of beauty, and may aid in exploration for new deposits.

Joëlbruggerite, Pb3Zn3(Sb5+,Te6+)As2O13(OH,O), the Sb5+ analog of dugganite, from the Black Pine mine, Montana

Abstract The influence of F substitution on the local structure of Al in the tetrahedral and octahedral sheets of synthetic Al-rich phlogopite samples with nominal gel compositions of K(Mg3-xAlx)[Al1+xSi3-xO10] (OH)y(F)2-y between 0.0 ≤ x ≤ 0.8 and 0.5 ≤ y ≤ 1.8, was studied by 27Al MAS, MQMAS, {1H/19F} → 27Al 2D CPMAS (HETCOR) and {1H/19F} 27Al REDOR solid-state NMR and by IR spectroscopy. Changes in intensity of the absorption bands in the OH-stretching region of the IR spectra clearly indicate the incorporation of octahedral Al. Signals from the different phases can be separated in the 27Al MQMAS NMR spectra by generation of an isotropic dimension in F1. The 27Al quadrupolar parameters of the four phases were estimated from 27Al MAS NMR spectra obtained at 104.26 and 208.42 MHz. The quadrupolar coupling constant and isotropic chemical shift increases with increasing Al content for the IVAl site in phlogopite. The VIAl site shows a clear increase of the asymmetry parameter and CQ with increasing F content. The estimated 27Al signal areas show the lowest amount of impurity phases at high OH contents and a stabilization of VIAl sites by hydroxyl groups. The {1H} → 27Al 2D CPMAS (HETCOR) NMR experiment at short contact times provides information about site neighborhoods of tetrahedral Al sites and Mg3OH as well as Mg2AlOH sites, whereas magnetization is only transferred to the octahedral Al sites from hydroxyl groups in Mg2AlOH sites. The {19F} → 27Al 2D CPMAS (HETCOR) NMR spectrum is dominated by IVAl sites coupled to the Mg3F complex in phlogopite. Resonances from Mg2AlF complexes are not observed. Finally, the {1H/19F} 27Al REDOR experiments support the results of the 2D CPMAS (HETCOR) experiments.

Pressure, temperature and fluid conditions during emerald precipitation, southeastern Yukon, Canada: fluid inclusion and stable isotope evidence

Abstract The amblygonite-montebrasite series of minerals, common constituents of granitic pegmatites and topaz-bearing granites, show complete solid solution with ideal composition LiAlPO4(F, OH). These compounds are ideal for studying F ↔ OH solid solution in minerals because natural members of the series generally show little deviation from the ideal composition. In this study, we used powder and single-crystal neutron diffraction and solid-state 6Li MAS, CP MAS, and REDOR NMR techniques to study the effect of F ↔ OH substitution on the series. Lattice parameters refined from single-crystal neutron diffraction data show increasing b and decreasing a, c, and V with increasing F/(F + OH). The volume is highest for the OH end-member because of the presence of an additional atom (H). The a and c parameters decrease with increasing F/(F + OH) because the O-H vector is close to the a-c plane and the Al-OH/F vectors are approximately parallel to c. Lattice parameters refined from neutron powder diffraction patterns collected at lower T show that thermal contraction increases with F/(F + OH), presumably because the F anion takes up less space than the OH molecule. The results show that the OH/F position is always fully occupied. The H displacement ellipsoid shows little change with occupancy, which obviously corresponds negatively with increasing F/(F + OH). However, the Li displacement ellipsoid becomes extremely large and anisotropic with increasing F fraction. Most of the distortion is associated with the U3 eigenvalue, which lies between the c and c* directions. Ueq values corresponding to the Li atom show a greater reduction with decreasing temperature than the other atoms. The temperature dependence of Li is the same regardless of F content. Even when extrapolated to absolute zero the Li displacement ellipsoid is very large, which implies a large static disorder. At the montebrasite end of the series, there are five short Li-j (j = O, OH, F) distances and one very long Li-O4 bond. With increasing F content, the Li-O4 distance decreases and the Li-O3f distance increases, such that at the amblygonite end, the coordination is 4 + 2. The disorder in the Li site is obviously caused by the substitution of F for OH. The driving force is the loss of the hydrogen bond to O4, which causes the Li-O4 bond to strengthen and improves the bond valence to O4. The results show that the H atom position is imbedded within the distorted octahedral oxygen coordination of the Li atom. To represent the disorder better, we used a split Li site model. The results show that Li1 occupancy increases and the Li2 occupancy decreases with increasing F content, and that the Li1-Li2 distance is longer for intermediate compositions than for the end-members. The 6Li MAS NMR experiments provide important structural information complementary to the neutron diffraction results. The spectra of samples in the amblygonite-montebrasite series show two well-resolved peaks, confirming the presence of Li disorder over two distinct sites, and highly resolved 6Li MAS NMR spectra are obtained at the very high magnetic field strength of 18.8 T. The peaks at -0.3 ppm and -0.9 ppm were unambiguously assigned to the Li2(OH) and Li1(F) sites found in the neutron diffraction structures. The isotropic chemical shifts are consistent with the coordinations of these Li sites found in the neutron diffraction structures. The relative intensities of the two peaks across the series of samples reflect the varying F/(F + OH). In addition to confirming the assignments of the peaks, it is possible to measure H-Li2 and F-Li1 internuclear distances by 6Li{1H} and 6Li{19F} CP and REDOR NMR that are consistent with the corresponding distances from the neutron diffraction structures. The 6Li{1H} and 6Li{19F} CP and REDOR results indicate that the Li disorder is random throughout the crystals rather than over large domains, a conclusion that cannot be made from diffraction experiments. Variable temperature 6Li MAS NMR spectra confirm that the disorder is static and there is no dynamic exchange involving F, OH, or Li. Each Li ion has access to only one of the two observed sites as determined by the presence of either OH or F in its immediate environment and there is no possibility of a dynamic exchange.

The crystal structure of namuwite, a mineral with Zn in tetrahedral and octahedral coordination, and its relationship to the synthetic basic zinc sulfates

Abstract Synchrotron radiation, high-resolution transmission electron microscopy (HRTEM), X-ray powder diffraction, and Raman spectroscopy were used to study the structure and thermal behavior of malayaite, CaSnSiO5. No indications of deviation from A2/a symmetry and no structural transitions were observed between 100 and 870 K. HRTEM revealed that the material is free of domains and antiphase boundaries. However, the lattice constants, cell volume, and Raman-active phonons show a thermal discontinuity near 500 K, which is possibly related to variation of the coordination sphere around the highly anisotropic Ca position.