Peter J. Heaney

Focused ion beam milling: A method of site-specific sample extraction for microanalysis of Earth and planetary materials

Abstract Argon ion milling is the conventional means by which mineral sections are thinned to electron transparency for transmission electron microscope (TEM) analysis, but this technique exhibits significant shortcomings. In particular, selective thinning and imaging of submicrometer inclusions during sample milling are highly problematic. We have achieved successful results using the focused ion beam (FIB) lift-out technique, which utilizes a 30 kV Ga+ ion beam to extract electron transparent specimens with nanometer scale precision. Using this procedure, we have prepared a number of Earth materials representing a range of structures and compositions for TEM analysis. We believe that FIB milling will create major new opportunities in the field of Earth and planetary materials microanalysis, particularly with respect to ultraprecious mineral and rock samples.

THE WIDESPREAD DISTRIBUTION OF A NOVEL SILICA POLYMORPH IN MICROCRYSTALLINE QUARTZ VARIETIES

An x-ray examination of more than 150 specimens of fine-grained quartz varieties from around the world has revealed that more than 10% and as much as 80% of the silica in many samples is actually moganite, a little-known silica polymorph. Rietveld refinements of 50 powder x-ray diffraction patterns produced by fibrous quartz (agate, chalcedony) and nonfibrous quartz (chert, flint) indicate that the concentrations of moganite within each subgroup are widely distributed. The large amount of moganite (>30%) found in cherts from arid, alkaline environments may resurrect length-slow silica as an indicator of evaporitic regimes, and the absence of moganite in weathered and hydrothermally altered silica samples may be a useful measure of fluid-rock interaction.

Synchrotron powder X-ray diffraction study of the structure and dehydration behavior of palygorskite

Abstract Rietveld refinements using synchrotron powder X-ray diffraction data were used to study the crystal structure and dehydration behavior of sepiolite from Durango, Mexico. The room-temperature (RT) sepiolite structure in air compares well with previous models but reveals an additional zeolitic H2O site. The RT structure under vacuum retained only ~1/8 of the zeolitic H2O and the volume decreased by 1.3%. Real-time, temperature-resolved synchrotron powder X-ray diffraction data and Rietveld refinements were used to investigate the behavior of the sepiolite structure from 300 to 925 K. Rietveld refinements revealed that most of the zeolitic H2O is lost by ~390 K, accompanied by a decrease in the a and c unit-cell parameters. Above ~600 K the sepiolite structure folds as one-half of the crystallographically bound H2O is lost. Rietveld refinements of the “anhydrous” sepiolite structure reveal that, in general, unit-cell parameters a and b and volume steadily decrease with increasing temperature; there is an obvious change in slope at ~820 K suggesting a phase transformation coinciding with the loss of the remaining bound H2O molecule.

A proposed mechanism for the growth of chalcedony

The structural disparities that distinguish chalcedony from macrocrystalline quartz suggest that different crystallization mechanisms are operative during the growth of these two forms of silica. Although the paragenesis of chalcedony has provoked marked disagreement among researchers, a review of previous studies supports the idea that chalcedony can precipitate from slightly saturated aqueous solutions at relatively low temperatures (<100° C). These conditions for deposition suggest a model for chalcedony crystallization that involves the assembly of short-chain linear polymers via bridging silica monomers. This assembly occurs through a spiral growth mechanism activated by a screw dislocation withb=n/2 [110], wheren is an integer. The proposed model can account for a number of peculiarities that have been observed in chalcedony at the microstructural scale, such as: (1) the direction of fiber elongation along [110] rather than [001]; (2) the periodic twisting of chalcedony fibers about [110]; (3) the high density of Brazil twin composition planes; (4) the common intergrowth of moganite within chalcedony.

Real-Time Analysis of Student Comprehension: An Assessment of Electronic Student Response Technology in an Introductory Earth Science Course

Electronic student response technologies (SRT) are capable of assessing teaching and learning methods in real time, and they offer an exceptional means of introducing active learning protocols in classes with large enrollments. These wireless systems allow students to key in responses with remote control units to questions posed by an instructor in the classroom. Student responses then are displayed in real time, allowing both students and instructors to gauge student comprehension instantaneously. From Spring 2002 to Spring 2003, we utilized SRT in 4 sections of a high-enrollment introductory Earth Science course (Geosc 020: Planet Earth) at Penn State University. We conducted a multi-faceted assessment of the use of SRT in our course that included quantitative and qualitative perception data from students enrolled in the course and faculty/administrator visitors to our classroom. Our preliminary assessment of the pedagogical merits of SRT in our course suggests that this technology is an effective tool for introductory geoscience education.

Neutron and temperature-resolved synchrotron X-ray powder diffraction study of akaganéite

Abstract Rietveld refinements using neutron powder diffraction data were used to locate H atom positions and obtain a more precise crystal structure refinement for akaganéite [Fe3+7.6Ni2+0.4O6.35 (OH)9.65Cl1.25·nH2O]. Difference Fourier maps clearly showed H atoms positions near those O atoms at the midpoints of the tunnel edges. The O-H vectors point toward the Cl sites at the center of the tunnel, and weak hydrogen bonds likely form between the framework O atoms and Cl. The Cl position is near the center of a prism defined by the eight hydroxyl H atoms. The Cl atoms fill ~2/3 of the tunnel sites, suggesting an ordering scheme in a given tunnel with every third tunnel site vacant. Such an arrangement allows the Cl anions to increase their separation distance along a tunnel by displacing away from one another toward their respective adjacent vacancies. The Fe-O octahedra in akaganéite are distorted with Fe-(O, OH) distances ranging from 1.94 to 2.13 Å and show three longer and three shorter Fe-O distances; as expected the longer distances are associated with the OH- anions. Temperature-resolved synchrotron X-ray powder diffraction data and Rietveld refinements were used to investigate changes in the akaganéite structure and its transformation into hematite as it was heated from 26 to 800 °C. Rietveld refinements revealed surprising consistency in all unit-cell parameters between room temperature and ~225 °C, resulting in nearly zero thermal expansion of the akaganéite structure over a 200 °C interval. Above ~225 °C, the unit-cell volume gradually decreased, primarily in response to decreases in c and b, and an increase in the b angle. The a parameter remained nearly constant until ~225 °C and increased thereafter. Akaganéite started to transform to hematite in the temperature range 290 to 310 °C with no evidence for maghemite as an intermediate phase.

Kinetic and thermodynamic properties of moganite, a novel silica polymorph

Abstract A growing body of evidence reveals that much of the silica that crystallizes at the Earths surface is a finely intergrown mixture of quartz and moganite. To better understand the behaviour of both solid and aqueous silica in these systems, the kinetics and thermodynamic properties for endmember moganite have been determined as a function of temperature from 25° to 200°C. Because endmember moganite has yet to be found in nature or synthesized in the laboratory, these properties were determined indirectly by (1) measuring quartz dissolution rates at pH 3.5, (2) measuring the dissolution rates of quartz/moganite mixtures of various proportions at pH 3.5 to deduce the endmember moganite dissolution rate, (3) using the principle of detailed balancing and the assumption that silica polymorphs have equal precipitation rates (Rimstidt and Barnes, 1980) to compute the equilibrium constant for the quartz to moganite transformation reaction, and (4) regressing these data together with corresponding values for quartz to generate endmember moganite thermodynamic properties. Equations describing the temperature dependence of the specific dissolution rate of quartz, k+,Si,qtz (mole/m2/s), and moganite, k+,Si,mog (mole/m2/s) at pH 3.5 and far from equilibrium are ln k +,Si,qtz = −0.0463 − 80480 RT for quartz ln k +,Si,mog = −0.975 − 70502 RT for moganite which is consistent with activation energy of 80.5 and 70.5 kJ/mol for quartz and moganite, respectively. The specific dissolution rate of moganite is 7.4 times faster than that of quartz at pH 3.5 and 25°C. The surface area of quartz/moganite mixtures increase exponentially with increasing moganite content. It follows that the apparent dissolution and precipitation rate of quartz/moganite mixtures also increases exponentially with moganite content. The standard state enthalpy and Gibbs free energy of formation for moganite from the elements at 25°C and one bar was calculated to be −900.723 and −851.314 kJ/mole which is 10 and 5 kJ/mol, respectively, more positive than those for quartz. The standard state entropy at these conditions is 58.245 J/mole/K, which is 17 J/mol/K greater than that for quartz. The logarithm of the equilibrium constant for moganite hydrolysis is −3.14 at 25°C and one bar, which corresponds to a solubility of 44 mg/kg silica. In contrast, the logarithm of the equilibrium constant for quartz hydrolysis is −4.00 which corresponds to a solubility of 6 mg/kg silica. The difference in the hydrolysis constants decreases with increasing temperature. The relative rapid dissolution rate of moganite and its thermodynamic instability with respect to quartz is consistent with the observation (Heaney and Post, 1992) that moganite is depleted in weathered chert and chalcedony, and it supports the diagenetic silica sequence proposed by Heaney (1995), who documented a scarcity of moganite in rocks older than 100 m.y. It also follows that the high abundance of moganite in recent arid environments is likely due to the lack of water available to mediate the dissolution of moganite and simultaneous precipitation of quartz.

Using Conceptests to Assess and Improve Student Conceptual Understanding in Introductory Geoscience Courses

Conceptests are higher-order multiple-choice questions that focus on one key concept of an instructors major learning goals for a lesson. When coupled with student interaction through peer instruction, conceptests represent a rapid method of formative assessment of student understanding, require minimal changes to the instructional environment and introduce many of the recognized principles of effective teaching that enhance student learning. In this study, instructors from several different institutions developed over 300 conceptests for the geosciences. These instructors then used this suite of concept questions in a wide range of classroom settings, including large introductory general education Earth Science courses for non-majors at open enrollment institutions, smaller physical geology classes suitable for majors at private colleges, and in introductory geology laboratory settings. Results of pre- and post-class Geoscience Concept Inventory (GCI) testing and qualitative feedback from students and instructors showed that conceptests increased attendance, improved student satisfaction, and enhanced student achievement. Participating instructors found implementation of conceptests into their classes straightforward and required less than 30 minutes of preparation per class. The conceptest question database is available on-line for geoscience instructors.

Observation and Origin of Self-Organized Textures in Agates

One of the most impressive manifestations of spontaneous pattern generation in natural materials is iris agate, which contains submicrometer concentric striations that may cycle several thousand times within an individual specimen. Analysis by secondary ion mass spectroscopy and transmission electron microscopy identified the iris texture as alternating layers of fine-grained, highly defective chalcedony and coarse-grained low-defect quartz. This oscillatory zonation in defect concentration may be ascribed to Ostwald-Liesegang crystallization cycles from silica-rich fluids that are variably polymeric and monomeric. Periodic changes in defect concentration and grain size also are observed with wavelengths of hundreds of micrometers and of centimeters, so that agates reveal textural self-similarity over three length scales.

Time-resolved structural analysis of K- and Ba-exchange reactions with synthetic Na-birnessite using synchrotron X-ray diffraction

Abstract Time-resolved Rietveld refinements using synchrotron X-ray diffraction (XRD) have documented real-time changes in unit-cell parameters in response to cation substitution in synthetic Na-birnessite. Potassium- and Ba-birnessite, like Na-birnessite, were found to have triclinic symmetry. Rietveld analyses of the XRD patterns for K- and Ba-exchanged birnessite revealed decreases in the a, c, and β unit-cell parameters, with a decrease of 1.7 and 0.5%, respectively, in unit-cell volume relative to Na-birnessite. Fourier electron difference syntheses revealed that the changes in the configuration of the interlayer species, and the charge, size, and hydration of the substituting cations, serve as the primary controls on changes in unit-cell parameters. Split electron density maxima with centers at (0 0 0.5) were present for Na, K, and Ba end-members; however, with increased substitution of K+ for Na+, the axis connecting the split-site maxima rotated from an orientation parallel to the b-axis to along the a-axis. Substitution of Ba2+ for Na+ did not result in rotation, but splitting of the interlayer site was more pronounced.