Daniel R. Hummer

Ultralow viscosity of carbonate melts at high pressures

Knowledge of the occurrence and mobility of carbonate-rich melts in the Earths mantle is important for understanding the deep carbon cycle and related geochemical and geophysical processes. However, our understanding of the mobility of carbonate-rich melts remains poor. Here we report viscosities of carbonate melts up to 6.2 GPa using a newly developed technique of ultrafast synchrotron X-ray imaging. These carbonate melts display ultralow viscosities, much lower than previously thought, in the range of 0.006-0.010 Pa s, which are ~2 to 3 orders of magnitude lower than those of basaltic melts in the upper mantle. As a result, the mobility of carbonate melts (defined as the ratio of melt-solid density contrast to melt viscosity) is ~2 to 3 orders of magnitude higher than that of basaltic melts. Such high mobility has significant influence on several magmatic processes, such as fast melt migration and effective melt extraction beneath mid-ocean ridges.

Thermal expansion of anatase and rutile between 300 and 575 K using synchrotron powder X-ray diffraction

High-precision unit-cell parameters for the TiO2 polymorphs anatase and rutile at temperatures between 300 and 575 K have been determined using Rietveld analysis of synchrotron powder XRD data. Polynomial models were used to express the tetragonal unit-cell parameters as a function of absolute temperature, with a (anatase)=1.759 37x10-8xT2+6.418 16x10-6xT+3.779 84, c (anatase)=6.6545x10-8xT2+4.0464x10-5xT+9.4910, V (anatase)=2.237 58x10-6xT2+1.027 77x10-3xT+135.602, a (rutile)=-6.636 42x10-11xT3+1.005 01x10-7xT2-1.009 9310-5xT+4.586 34, c (rutile)=-4.115 50x10-11xT3+6.405 94x10-8xT2+4.675 61x10-7T+2.951 81, and V (rutile)=-2.7790x10-9xT3+4.2386x10-6xT2-3.3551x10-4xT+62.100. The polynomial expressions were used to calculate linear (alpha) and volume (beta) thermal expansion coefficients of anatase and rutile between 300 and 575 K. At 298.15 K, these values were alphaa=4.46943x10-6 K-1, alphac=8.4283x10-6 K-1, and beta=17.3542x10-6 K-1 for anatase, and alphaa=6.99953x10-6 K-1, alphac=9.36625x10-6 K-1, and beta=28.680x10-6 K-1 for rutile.

Synthesis and crystal chemistry of Fe3+-bearing (Mg,Fe3+)(Si,Fe3+)O3 perovskite

Abstract We have synthesized magnesium-iron silicate perovskites with the general formula Mg1-xFe3+x+ySi1-y O3, in which the iron cation is exclusively trivalent. To investigate the crystal chemistry of Fe3+-bearing perovskite, six samples (both with and without Al) were analyzed using scanning electron microscopy, electron microprobe, X-ray diffraction, and Mössbauer spectroscopy. Results indicate that Fe3+ substitutes significantly into both the octahedral and dodecahedral sites in the orthorhombic perovskite structure, but prefers the octahedral site at Fe3+ concentrations between 0.04 and 0.05 Fe per formula unit, and the dodecahedral site at higher Fe3+ concentrations. We propose a model in which Fe3+ in the A/B site (in excess of that produced by charge coupled substitution) is accommodated by Mg/O vacancies. Hyperfine parameters refined from the Mössbauer spectra also indicate that a portion of dodecahedral sites undergo significant structural distortion. The presence of Fe3+ in the perovskite structure increases the unit-cell volume substantially compared to either the Mg end-member, or Fe2+-bearing perovskite, and the addition of Al did not significantly alter the volume. Implications for increased compressibility and a partially suppressed spin transition of Fe3+ in lower mantle perovskite are also discussed.

Speciation of l-DOPA on Nanorutile as a Function of pH and Surface Coverage Using Surface-Enhanced Raman Spectroscopy (SERS)

The adsorption configuration of organic molecules on mineral surfaces is of great interest because it can provide fundamental information for both engineered and natural systems. Here we have conducted surface-enhanced Raman spectroscopy (SERS) measurements to probe the attachment configurations of DOPA on nanorutile particles under different pH and surface coverage conditions. The Raman signal enhancement arises when a charge transfer (CT) complex forms between the nanoparticles and adsorbed DOPA. This Raman signal is exclusively from the surface-bound complexes with great sensitivity to the binding and orientation of the DOPA attached to the TiO(2) surface. Our SERS spectra show peaks that progressively change with pH and surface coverage, indicating changing surface speciation. At low pH and surface coverage, DOPA adsorbs on the surface lying down, with probably three points of attachment, whereas at higher pH and surface coverage DOPA stands up on the surface as a species involving two attachment points via the two phenolic oxygens. Our results demonstrate experimentally the varying proportions of the two surface species as a function of environmental conditions consistent with published surface complexation modeling. This observation opens up the possibility to manipulate organic molecule attachment in engineered systems such as biodetection devices. Furthermore, it provides a perspective on the possible role of mineral surfaces in the chemical evolution of biomolecules on the early Earth. Adsorbed biomolecules on mineral surface in certain configurations may have had an advantage for subsequent condensation reactions, facilitating the formation of peptides.

Carbon mineral ecology: Predicting the undiscovered minerals of carbon

Abstract Studies in mineral ecology exploit mineralogical databases to document diversity-distribution relationships of minerals—relationships that are integral to characterizing “Earth-like” planets. As carbon is the most crucial element to life on Earth, as well as one of the defining constituents of a planet’s near-surface mineralogy, we focus here on the diversity and distribution of carbon-bearing minerals. We applied a Large Number of Rare Events (LNRE) model to the 403 known minerals of carbon, using 82 922 mineral species/locality data tabulated in http://mindat.org (as of 1 January 2015). We find that all carbon-bearing minerals, as well as subsets containing C with O, H, Ca, or Na, conform to LNRE distributions. Our model predicts that at least 548 C minerals exist on Earth today, indicating that at least 145 carbon-bearing mineral species have yet to be discovered. Furthermore, by analyzing subsets of the most common additional elements in carbon-bearing minerals (i.e., 378 C + O species; 282 C + H species; 133 C + Ca species; and 100 C + Na species), we predict that approximately 129 of these missing carbon minerals contain oxygen, 118 contain hydrogen, 52 contain calcium, and more than 60 contain sodium. The majority of these as yet undescribed minerals are predicted to be hydrous carbonates, many of which may have been overlooked because they are colorless, poorly crystalized, and/or water-soluble. We tabulate 432 chemical formulas of plausible as yet undiscovered carbon minerals, some of which will be natural examples of known synthetic compounds, including carbides such as calcium carbide (CaC2), crystalline hydrocarbons such as pyrene (C16H10), and numerous oxalates, formates, anhydrous carbonates, and hydrous carbonates. Many other missing carbon minerals will be isomorphs of known carbon minerals, notably of the more than 100 different hydrous carbonate structures. Surveys of mineral localities with the greatest diversity of carbon minerals, coupled with information on varied C mineral occurrences, point to promising locations for the discovery of as yet undescribed minerals.

Cobalt mineral ecology

Abstract Minerals containing cobalt as an essential element display systematic trends in their diversity and distribution. We employ data for 66 approved Co mineral species (as tabulated by the official mineral list of the International Mineralogical Association, http://rruff.info/ima, as of 1 March 2016), representing 3554 mineral species-locality pairs (www.mindat.org and other sources, as of 1 March 2016). We find that cobalt-containing mineral species, for which 20% are known at only one locality and more than half are known from five or fewer localities, conform to a Large Number of Rare Events (LNRE) distribution. Our model predicts that at least 81 Co minerals exist in Earth’s crust today, indicating that at least 15 species have yet to be discovered—a minimum estimate because it assumes that new minerals will be found only using the same methods as in the past. Numerous additional cobalt minerals likely await discovery using micro-analytical methods. Primary Co minerals include 26 species, most of which are hydrothermally deposited chalcogenides. We identify 33 additional plausible as yet undiscovered primary cobalt chalcogenide minerals, including 28 phases with spinel, nickeline, pyrite, and marcasite structural topologies. All 40 secondary cobalt minerals are oxides, and 37 of these phases also incorporate hydrogen. We tabulate an additional 117 plausible secondary Co minerals that are related compositionally and/or structurally to known species. New cobalt minerals are likely to be discovered in specimens collected at the 10 most prolific Co localities, all of which are mining districts with hydrothermal Co mineralization and hosting at least 10 different primary and secondary Co species.

Applications of Time-Resolved Synchrotron X-ray Diffraction to Cation Exchange, Crystal Growth and Biomineralization Reactions

Abstract Advances in the design of environmental reaction cells and in the collection of X-ray diffraction data are transforming our ability to study mineral-fluid interactions. The resulting increase in time resolution now allows for the determination of rate laws for mineral reactions that are coupled to atomic-scale changes in crystal structure. Here we address the extension of time-resolved synchrotron diffraction techniques to four areas of critical importance to the cycling of metals in soils: (1) cation exchange; (2) biomineralization; (3) stable isotope fractionation during redox reactions; and (4) nucleation and growth of nanoscale oxyhydroxides.

Crystal structure of abelsonite, the only known crystalline geoporphyrin

Abstract The crystal structure of the unique nickel porphyrin mineral abelsonite, NiC31H32N4, has been solved using direct methods with 2195 independent reflections to a final R1 = 0.0406. Abelsonite crystallizes in the triclinic space group P 1¯

Corrections to "Thermal expansion of anatase and rutile between 300 and 575 K using synchrotron powder X-ray diffraction" †Powder Diffr. 22, 352-357 "2007…‡

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Origin of Nanoscale Phase Stability Reversals in Titanium Oxide Polymorphs

, with Z = 1 and unit-cell parameters a = 8.4416(5) Å, b = 10.8919(7) Å, c = 7.2749(4) Å, α = 90.465(2)°, β = 113.158(2)°, and γ = 78.080(2)° at the measurement condition of 100 K, in very good agreement with previous unit-cell parameters reported from powder diffraction. The structure consists of nearly planar, covalently bonded porphyrin molecules stacked approximately parallel to (1 1¯