High pressure single crystal X-ray diffraction study on α -quartz

Distinguishing cellular from abiotic spheroidal microstructures in the ca. 3.4 Ga Strelley Pool Formation

The morphogenesis of most carbonaceous microstructures that resemble microfossils in Archean (4-2.5 Ga old) rocks remains debated. The associated carbonaceous matter may even-in some cases-derive from abiotic organic molecules. Mineral growths associated with organic matter migration may mimic microbial cells, some anatomical features, and known microfossils-in particular those with simple spheroid shapes. Here, spheroid microstructures from a chert of the ca. 3.4 Ga Strelley Pool Formation (SPF) of the Pilbara Craton (Western Australia) were imaged and analyzed with a combination of high-resolution in situ techniques. This provides new insights into carbonaceous matter distributions and their relationships with the crystallographic textures of associated quartz. Thus, we describe five new types of spheroids and discuss their morphogenesis. In at least three types of microstructures, wall coalescence argues for migration of carbonaceous matter onto abiotic siliceous spherulites or diffusion in poorly crystalline silica. The nanoparticulate walls of these coalescent structures often cut across multiple quartz crystals, consistent with migration in/on silica prior to quartz recrystallization. Sub-continuous walls lying at quartz boundaries occur in some coalescent vesicles. This weakens the "continuous carbonaceous wall" criterion proposed to support cellular inferences. In contrast, some clustered spheroids display wrinkled sub-continuous double walls, and a large sphere shows a thick sub-continuous wall with pustules and depressions. These features appear consistent with post-mortem cell alteration, although abiotic morphogenesis remains difficult to rule out. We compared these siliceous and carbonaceous microstructures to coalescent pyritic spheroids from the same sample, which likely formed as "colloidal" structures in hydrothermal context. The pyrites display a smaller size and only limited carbonaceous coatings, arguing that they could not have acted as precursors to siliceous spheroids. This study revealed new textural features arguing for abiotic morphogenesis of some Archean spheroids. The absence of these features in distinct types of spheroids leaves open the microfossil hypothesis in the same rock. Distinction of such characteristics could help addressing further the origin of other candidate microfossils. This study calls for similar investigations of metamorphosed microfossiliferous rocks and of the products of in vitro growth of cell-mimicking structures in presence of organics and silica.

Modelling the structural variation of quartz and germanium dioxide with temperature by means of transformed crystallographic data

The pseudocubic (PC) parameterization of O₄ tetrahedra [Reifenberg & Thomas (2018). Acta Cryst. B74, 165-181] is applied to quartz (SiO₂) and its structural analogue germanium dioxide (GeO₂). In α-quartz and GeO₂, the pseudocubes are defined by three length parameters, a_PC, b_PC and c_PC, together with an angle parameter α_PC. In β-quartz, α_PC has a fixed value of 90°. For quartz, the temperature evolution of parameters for the pseudocubes and the silicon ion network is established by reference to the structural refinements of Antao [Acta Cryst. (2016), B72, 249-262]. In α-quartz, the curve-fitting employed to express the non-linear temperature dependence of pseudocubic length and Si parameters exploits the model of a first-order Landau phase transition utilized by Grimm & Dorner [J. Phys. Chem. Solids (1975), 36, 407-413]. Since values of tetrahedral tilt angles about ⟨100⟩ axes also result from the pseudocubic transformation, a curve for the observed non-monotonic variation of α_PC with temperature can also be fitted. Reverse transformation of curve-derived values of [Si+PC] parameters to crystallographic parameters a, c, x_Si, x_O, y_O and z_O at interpolated or extrapolated temperatures is demonstrated for α-quartz. A reverse transformation to crystallographic parameters a, c, x_O is likewise carried out for β-quartz. This capability corresponds to a method of structure prediction. Support for the applicability of the approach to GeO₂ is provided by analysing the structural refinements of Haines et al. [J. Solid State Chem. (2002), 166, 434-441]. An analysis of trends in tetrahedral distortion and tilt angle in α-quartz and GeO₂ supports the view that GeO₂ is a good model for quartz at high pressure.

Ionic network analysis of tectosilicates: the example of coesite at variable pressure

The method of ionic network analysis [Thomas (2017). Acta Cryst. B73, 74-86] is extended to tectosilicates through the example of coesite, the high-pressure polymorph of SiO₂. The structural refinements of Černok et al. [Z. Kristallogr. (2014), 229, 761-773] are taken as the starting point for applying the method. Its purpose is to predict the unit-cell parameters and atomic coordinates at (p-T-X) values in-between those of diffraction experiments. The essential development step for tectosilicates is to define a pseudocubic parameterization of the O₄ cages of the SiO₄ tetrahedra. The six parameters a_PC, b_PC, c_PC, α_PC, β_PC and γ_PC allow a full quantification of the tetrahedral structure, i.e. distortion and enclosed volume. Structural predictions for coesite require that two separate quasi-planar networks are defined, one for the silicon ions and the other for the O₄ cage midpoints. A set of parametric curves is used to describe the evolution with pressure of these networks and the pseudocubic parameters. These are derived by fitting to the crystallographic data. Application of the method to monoclinic feldspars and to quartz and cristobalite is discussed. Further, a novel two-parameter quantification of the degree of tetrahedral distortion is described. At pressures in excess of ca 20.45 GPa it is not possible to find a self-consistent solution to the parametric curves for coesite, pointing to the likelihood of a phase transition.

Extending the single-crystal quartz pressure gauge up to hydrostatic pressure of 19 GPa

In situhigh-pressure diffraction experiments on single-crystal α-quartz under quasi-hydrostatic conditions up to 19 GPa were performed with diamond-anvil cells. Isotropic pressures were calibrated through the ruby-luminescence technique. A 4:1 methanol–ethanol mixture and the densified noble gases helium and neon were used as pressure media. The compression data revealed no significant influence of the pressure medium at room temperature on the high-pressure behavior of α-quartz. In order to describe its compressibility for use as a pressure standard, a fourth-order Birch–Murnaghan equation of state (EoS) with parametersKT0 = 37.0 (3) GPa,KT0′ = 6.7 (2) andKT0′′ = −0.73 (8) GPa−1was applied to fit the data set of 99 individual data points. The fit of the axial compressibilities yieldsMT0 = 104.5 (8) GPa,MT0′ = 13.7 (4),MT0′′ = −1.04 (11) GPa−1(aaxis) andMT0 = 141 (3) GPa,MT0′ = 21 (2),MT0′′ = 8.4 (6) GPa−1(caxis), confirming the previously reported anisotropy. Assuming an estimated standard deviation of 0.0001% in the quartz volume, an uncertainty of 0.013 GPa can be expected using the new set of EoS parameters to determine the pressure.

Phase transitions in solid solution series bismutocolumbite – stibiocolumbite (Bi-Sb)(Nb0.79Ta0.21)O4

The crystal structure of bismutocolumbite, Bi(Nb0.79Ta0.21)O4 with orthorhombic (sp. gr. Pnna) stibio-tantalite structure type at ambient conditions has been de-termined at 2.59 and 9.56 GPa, and 23oC. Crystal data and results of structure refinement using powder X-ray diffrac-tion and Rietveld analysis for Bi-rich stibiocolumbite, (Sb0.52Bi0.48)(Nb0.71Ta0.29)O4, are reported. At the pressure ~2.9 GPa bismutocolumbite exhibits phase transition which is accompanied by the change of its symmetry: sp. gr. Pnna is replaced by the sp. gr. Pn21 a. This kind of transformation is anticipated at ambient pressure in bismutocolumbite- stibiocolumbite solid solution series in the compounds with Sb/(Bi+Sb) ratio higher than 0.52 which characterises the studied crystals of stibiotantalite. Similarly with scheelite-like BiVO4, in bismutocolumbite the reduction of the unit cell volume under HP is mainly a result of Bi-O polyhedral compression, which is accompanied by a rearrangement of (Nb,Ta)O6 octahedra within octahedral sheets parallel to (010). (Nb,Ta)O6 octahedra remain rigid with smaller change in size at high pressure.