Raymond Joesten

Evolution of mineral assemblage zoning in diffusion metasomatism

Abstract Metamorphic reaction bands consist of mineral assemblage layers, that separate incompatible mineral assemblages, and grow by reaction at their contacts and by precipitation within the layer. The stoichiometry of these reactions is determined by the relative diffusion fluxes within adjacent layers. Using the local equilibrium-steady diffusion model of Fisher (1973), the stoichiometric coefficients of layer growth reactions can be computed from the mass balance at each layer contact and independent ratios of instantaneous fluxes within each layer. Model calculations in a ternary system show that (1) variation in bulk composition across a reaction band, expressed in terms of layer sequence, mineral formulae of phases that comprise monomineralic layers and modal proportions of phases in multimineralic layers is largely determined by the relative fluxes of components that diffuse in the same direction; (2) the fraction of the width of a reaction band produced by replacement of each of the initial reactant assemblages is largely determined by the relative fluxes of components that diffuse in opposite directions; (3) the fraction of the width of a layer produced by internal precipitation is determined by temperature and the relative fluxes of components that diffuse in opposite directions; (4) in a multilayer reaction band, only one layer can grow by reaction at both contacts, and that layer contains the initial contact between initial reactants; and (5) a reaction band cannot grow by constant volume replacement of both reactant assemblages, if the system is closed to diffusion beyond layer contacts with the initial reactant rock masses.

Kinetics of diffusion-controlled mineral growth in the Christmas Mountains (Texas) contact aureole

Chert nodules in limestone in the aureole of the Christmas Mountains gabbro are rimmed by wollastonite in the interval 102-25 m from the intrusive contact and by tilleyite or spurrite and wollastonite within 25 m of gabbro. Wollastonite rims thicken from 4.5 mm at 102 m to 7 mm at 80 m to 30 mm at 49 m and diminish in thickness from 79 to 53 mm over the interval 13-2 m from the gabbro. Wollastonite has two textural components. Polygonal matrix wollastonite grains coarsen from 0.011 mm at 102 m to 0.053 mm at 46 m to 0.2-0.3 mm within 20 m of the contact, whereas scattered wollastonite porphyroblasts appear at 30 m and coarsen to impingement with a diameter of 1.07 mm at 12 m. Kinetic models for normal grain growth and for layer thickening set a function of the square of the grain diameter or layer thickness equal to the product of a material constant and a temperature-time integral that includes the Arrhenius function for the diffusion coefficient. The time index for non-isothermal diffusion-controlled mineral growth is the temperature-time integral numerically evaluated along T-t curves for the thermal history of the contact aureole. The thermal history of the Christmas Mountains aureole is obtained using a numerical model for a cylindrical intrusion, radius = 823 m, that matches maximum temperatures in the contact aureole of 600, 940, 1000, and 1030 °C at 115, 23, 13, and 0 m by convection for 700 yr followed by crystallization and cooling. The log of the T-t integral varies linearly with distance from the gabbro, with a slope proportional to activation energy and an intercept proportional to the material constant. A plot of the log of the square of the grain diameter of matrix wollastonite against distance from the gabbro is linear, and the normal grain-growth model yields the following Arrhenius function for the diffusion of oxygen in wollastonite grain boundaries. D GB O 2 /δ = 9.333 x 10 -4 [m/s] exp (-185/RT) [kJ/mole], where δ is grain-boundary width. The fixed ratio of tilleyite to wollastonite rim thickness on nodules from the inner aureole yields a molar ratio of tilleyite to wollastonite in C|T|W|Q structures of 1T:24W and requires consumption of calcite and quartz in the ratio 29C:26Q. Truncation of fully coarsened impingement wollastonite by columnar tilleyite implies that the tilleyite layer grew at expense of wollastonite and that wollastonite coarsened at temperatures greater than 940 °C. Solution of a system of mass balance, conservation, and flux ratio equations describing diffusion-controlled growth of a C|T|W|Q nodule matches the net mass transfer and observed textural features for Onsager diffusion-coefficient ratios of L CaO /L SiO 2 = 42 and L CaO /L CO 2 Grain diameter of impingement wollastonite varies linearly with the radius of spherical wollastonite nodules that ceased growth on consumption of the quartz core, implying that coarsening coincided in time with wollastonite rim growth. Grain-boundary cross section of wollastonite decreased by two orders of magnitude during layer growth. Coupling of the kinetics of coarsening of impingement wollastonite with diffusion-controlled growth of the wollastonite layer yields the following Arrhenius function for the Onsager diffusion coefficient for diffusion of CaO in wollastonite grain boundaries. δL GB CaO = 1.731 x 10 -4 /RT [mole 2 /J.s]exp (-220/RT) [kJ/mole]. The pre-exponential term for grain-boundary diffusion of SiO 2 in wollastonite is obtained from the relation L CaO /L SiO 2 = 42. Values of the diffusion coefficients derived from analysis of grain coarsening and layer growth in the Christmas Mountains contact aureole satisfy the diffusion-compensation relations for grain-boundary diffusion of oxygen and cations in oxides based on laboratory experiment. The value of the coefficient for diffusion of CaO in tilleyite grain boundaries at 1000 °C based on a time-integrated aver-age temperature is essentially identical to that obtained for wollastonite using the full non-isothermal analysis, suggesting that the values of Onsager coefficients for grain-boundary diffusion of a given cation are not strongly dependent on identity of solid phase Wollastonite rims grow to 82% to 95% of their ultimate thickness during the period of heating that coincides with magmatic convection. Because growth of the wollastonite and tilleyite layers does not begin until temperature rises to 600 and 940 °C, respectively, rim growth is not synchronous across the aureole.

Mineralogical and chemical evolution of contaminated igneous rocks at a gabbro-limestone contact, Christmas Mountains, Big Bend region, Texas

Reaction between alkali gabbro magma and limestone produced a 0.2- to 3-m-thick zone of nepheline pyroxenite and a 0.1- to 1-m-thick band of calc-silicate skarn at their contact. The sinuous embayed pyroxenite-skarn contact, the presence of rounded skarn xenoliths in pyroxenite, and textural evidence for growth of calc-silicate skarn by replacement of both marble and solid pyroxenite indicate that reaction involved assimilation of carbonate wall rock by magma and loss of Al and Si to the skarn. The uneven modal distribution of euhedral titanaugite and enveloping nepheline in pyroxenite, the restricted occurrence of nepheline syenite as dikes in pyroxenite and skarn and the mineralogical similarity of nepheline syenite and the leucocratic matrix of pyroxenite suggest that pyroxenite represents an accumulation of titanaugite cemented by an alkali-rich residual magma and that nepheline syenite represents a part of the residual contaminated magma that was squeezed out of the clinopyroxene crystal mush. Limestone assimilation is modeled by reaction of calcite and magmatic plagioclase, which results in resorption of plagioclase, growth of clinopyroxene enriched in Fe, Ti, and Al, and solution of nepheline and wollastonite in residual contaminated magma. The bulk composition of pyroxenite evolved by addition of Ca from dissolved limestone, loss of Al and Si to skarn, and local segregation of solid clinopyroxene and nepheline syenite magma. The predominance of pyroxenite among contaminated rocks and their restriction to a narrow zone along the intrusive contact provide little evidence for the genesis of a significant volume of nepheline syenite magma by limestone assimilation.

Enhancement of Catalytic Activities of Octahedral Molecular Sieve Manganese Oxide for Total and Preferential CO Oxidation through Vanadium Ion Framework Substitution

High‐valent vanadium ions were substituted into the synthetic cryptomelane manganese oxide (K‐OMS‐2) framework through a simple and low‐cost reflux method and investigated for total and preferential catalytic oxidation of carbon monoxide. Substitutional doping of V5+ resulted in materials with modified composition, morphology, thermal stability; and textural, redox, and catalytic properties. The catalytic activity increased with V concentration until an optimum amount (≈10 % V incorporated) was reached, beyond that a structural “crash point” was observed, resulting in a material with low crystallinity, nanosphere morphology, and reduced catalytic activity. An increase in O2 concentration in the feed gas resulted in an increase in conversion over 10% V K‐OMS‐2. This most active catalyst was deactivated by moisture only at low temperatures and showed better tolerance than undoped K‐OMS‐2. This catalyst also preferentially oxidized CO to CO2 from 25 °C to 120 °C in large amounts of H2 under dry conditions, without significantly affecting CO conversion. The doped catalyst also showed stable activity and selectivity in long‐run experiments. The mobility and lability of surface oxygen, formation of hydroxyl groups, and enhanced surface redox properties promoted by V doping were strongly correlated with the enhancement of catalytic activities of K‐OMS‐2 nanomaterials.

Nanostructured arrays of semiconducting octahedral molecular sieves by pulsed-laser deposition

Cryptomelane-type manganese oxide (OMS-2) has been widely used to explore the semiconducting and catalytic properties of molecular sieves with mixed-valent frameworks. Selective synthesis of patterned thin films of OMS-2 with hierarchical nanostructures and oriented crystals is challenging owing to difficulties in preserving the mixed valence, porosity and crystalline phase. Here, we report that pulsed-laser ablation of OMS-2 in an oxygen-rich medium produces a three-dimensional nanostructured array of parallel and inclined OMS-2 fibres on bare substrates of (001) single-crystal strontium titanate. Both parallel and inclined OMS-2 fibres elongate along the [001](OMS-2) direction. The parallel fibres interact strongly with the substrate and grow epitaxially along <110>(STO) with lattice misfits of less than 4%, whereas the inclined fibres are oriented with (301) parallel to the substrate surface. The spontaneous orientation of the crystalline OMS-2 domains over the STO surface opens up a new avenue in lattice-engineered synthesis of multilayer materials.

Structural Distortion of Molybdenum-Doped Manganese Oxide Octahedral Molecular Sieves for Enhanced Catalytic Performance.

Due to the excellent catalytic performance of manganese oxide (K-OMS-2) in a wide range of applications, incorporation of various dopants has been commonly applied for K-OMS-2 to acquire additional functionality or activities. However, the understanding of its substitution mechanism with respect to the catalytic performance of doped K-OMS-2 materials remains unclear. Here we present the structural distortion (from tetragonal to monoclinic cell) and morphological evolution in K-OMS-2 materials by doping hexavalent molybdenum. With a Mo-to-Mn ratio of 1:20 (R-1:20) in the preparation, the resultant monoclinic K-OMS-2 shows a small equidimensional particle size (∼15 nm), a high surface area of 213 m(2) g(-1), and greatly improved catalytic activity toward CO oxidation with lower onset temperatures (40 °C) than that of pristine K-OMS-2 (above 130 °C). HR-TEM analyses reveal direct evidence of structural distortion on the cross-section of 2 × 2 tunnels with the absence of 4-fold rotation symmetry expected for a tetragonal cell, which are indexed using a monoclinic cell. Our results suggest that substitution of Mo(6+) for Mn(3+) (rather than Mn(4+)) coupled with the vacancy generation results in a distorted structure and unique morphology. The weakened Mn-O bonds and Mn vacancies associated with the structural distortion may be mainly responsible for the enhanced catalytic activity of monoclinic K-OMS-2 instead of dopant species.