Elise Knittle

Stability and equation of state of Fe3C to 73 GPa: Implications for carbon in the Earth's core

We have measured the volume and lattice parameters of Fe3C-cementite as a function of pressure to 73 GPa using synchrotron-based x-ray diffraction. Several samples were laser heated and temperature-quenched from >1,500 K while held at high-pressure. No pressure or temperature induced phase transitions are observed in this study, and the 300 K isothermal equation of state of Fe3C yields a best-fit bulk modulus, K0T, of 175±4 GPa and a of 5.2±0.3. The bulk sound speed of Fe3C under outer core conditions is calculated: barring higher pressure polymorphism or marked changes in compressibility on melting, carbon could produce some or all of the offset between seismic observations (PREM) and measurements on pure iron and iron-nickel alloys. Additionally, we model the thermodynamic stability of this phase at the conditions of Earths mantle, and determine that in the presence of free iron (i.e. during core formation) the formation of Fe3C is strongly favored relative to carbonates.

Static compression of ε‐FeSi and an evaluation of reduced silicon as a deep Earth constituent

The volume of e-FeSi has been measured to pressures of 50 GPa. No high pressure transformations are observed in this system, including after laser-heating at high pressures. The bulk modulus of this phase is 209 (±6) GPa, with a pressure derivative of 3.5 (±0.4). This high bulk modulus will slightly elevate the thermochemically-inferred pressure conditions at which chemical reactions between iron and mantle silicates should commence at mid-mantle depths. Moreover, because of the relatively low pressure derivatives of the bulk moduli of FeSi alloys manifested both in our data and in previous shock results, silicon as the sole light alloying component of the outer core is unlikely to produce a sufficiently large change in the bulk sound speed of the outer core relative to pure iron liquid. Therefore, silicon is not likely to be the primary alloying component in the outer core, unless its effect on the elasticity of the outer core is fortuitously offset by other light alloying constituents: its role is probably that of a subsidiary constituent. Thus, silicon is unique (to date) among proposed major outer core alloying constituents in being potentially precludable on purely geophysical grounds.

Static compression of chondrodite: Implications for water in the upper mantle

We have determined the equation of state of chondrodite (2Mg2SiO4·Mg(OH,F)2), a possible water-bearing phase in the upper mantle, to 42.3 GPa at room temperature using x-ray diffraction through the diamond cell. The isothermal bulk modulus of chondrodite, determined using the Birch-Murnaghan equation of state, is 136.2 GPa with a pressure derivative of 3.7. Using our results, we have constructed density models for hydrous upper mantle compositions. These models indicate that up to 2 weight percent water could be present in a pyrolitic mantle, and be undetectable using seismically-determined density constraints. We have also investigated the stability of chondrodite under lower mantle pressure and temperature conditions using the laser-heated diamond cell. From in-situ x-ray diffraction at high pressure, and x-ray diffraction and infrared reflectance and absorption spectra of quenched run products, we conclude that chondrodite breaks down to MgSiO3 perovskite, MgO and a hydrated melt at lower mantle pressures and temperatures.

The stability and equation of state of majoritic garnet synthesized from natural basalt at mantle conditions

The high-pressure stability and equation of state of majoritic garnet synthesized from natural basalt have been determined using the laser-heated diamond cell and x-ray diffraction. Natural mid-ocean ridge basalt (MORB) and tholeiitic basalt transform from a garnet-dominated mineral assemblage to a silicate perovskite-dominated mineralogy near 42 (±3) GPa. Static compression data (at 300 K) for the majoritic garnet synthesized from the natural tholeiitic basalt yields an isothermal bulk modulus, K0T, of 226.2 (±9.3) GPa with dK0T/dP constrained to be 4. An equally acceptable fit to the data is obtained if K0T is constrained to be 180 GPa and dK0T/dP is 7 (±1). Using our new equation of state and phase stability results for basaltic (majorite-structured) garnet, we calculate density models for the subducting oceanic crust at mantle pressures and temperatures, and find that the basaltic crust is 0.4 g/cm³ (9%) less dense than the lower mantle at 660 km depth. If we assume that the subducted crust can delaminate from the underlying lithosphere, a simple buoyancy model for the crustal portion of subducting slabs indicates that basaltic crust may not sink below 660 km in depth.

An XAFS study of the crystal chemistry of Fe in orthopyroxene

Abstract X-ray absorption fine-structure (XAFS) spectra at the Fe K edge in Bamble enstatite (Mg0.88Fe0.12)SiO3 were analyzed in conjunction with theoretical XAFS spectra to determine the bonding configuration of Fe in this structure. The structural analysis involved determination of the Fe distribution between the octahedral M1 and M2 sites, and Fe-O bond lengths in the M2 site, into which Fe strongly partitions. Our analysis yielded bond lengths for Fe in the M2 site of 1.97(2), 1.98(2), 2.09(2), 2.16(2), 2.43(2), and 2.65(10) Å, in agreement with bond lengths determined from X-ray and neutron-diffraction analysis of the two orthopyroxene end-members. The average Fe-O bond length in the M2 site is 2.22(2) Å, longer than that of the Mg end-member (2.151 Å) but approximately the same as that of the Fe end-member (2.228 Å) of the orthopyroxene solid-solution series. Octahedral distortion of the M2 site may be greater than that of either the Fe or Mg end- member. The presence of a minor amount of Fe3+ was inferred by our analysis of the M1 site and was also suggested by our bond-valence calculations, which yielded a charge of 2.07 for Fe in the M2 site and a charge of 2.78 for Fe in the M1 site. Simple calculations using our data and those of other studies show that the average Fe-O bond length in the M2 site is constant along the Fe-Mg join in the orthopyroxene solid-solution series.

The high-pressure behavior of micas: Vibrational spectra of muscovite, biotite, and phlogopite to 30 GPa

Abstract The infrared spectra of natural samples of muscovite, biotite, and phlogopite are characterized to pressures of ~30 GPa, as is the Raman spectrum of muscovite to ~8 GPa. Both far-infrared and midinfrared data are collected for muscovite, and mid-infrared data for biotite and phlogopite. The response of the hydroxyl vibrations to compression differs markedly between the dioctahedral and trioctahedral micas: the hydrogen bonding in dioctahedral environments increases with pressure, as manifested by shifts to lower frequency of the hydroxyl-stretching vibrations, whereas cation-hydrogen repulsion likely produces shifts to higher frequency of the hydroxyl vibrations within trioctahedral environments. An abrupt decrease in frequency and increase in band width of the hydroxyl-stretching vibration in muscovite is observed at pressures above ~18-20 GPa, implying that the previously documented pressure-induced disordering is associated with the local environment and shifts in location of the hydroxyl unit in this material. The far-infrared vibrations of muscovite indicate that its compressional mechanism changes above 5-8 GPa, as the K-O stretching vibration with a zero-pressure frequency near 112 cm-1 shifts in its pressure dependence from 6.9 cm-1/GPa below this pressure range to 0.78 cm-1/GPa above it. Thus, it appears that the magnitude of interlayer compression is decreased above this pressure, and hence that the compression of muscovite may become less strongly anisotropic. The mid-infrared bands that are primarily produced by vibrations of the tetrahedral layer broaden under pressure in both muscovite and biotite: within biotite, a spectral region that may be associated with higher coordination of tetrahedral cations increases in amplitude above about 25 GPa. The corresponding bands in phlogopite undergo less broadening, and their behavior is fully reversible on decompression.

Introduction to Mineralogy

A course in mineralogy is a rite of passage for most undergraduate Earth sciences majors. As fluency with minerals is so basic for deciphering a range of geologic processes, many Earth scientists can recall long hours in the lab memorizing mineral samples, their chemical formulae and crystal systems, and perhaps staring through a petrographic microscope wondering, what exactly is 2V? In this venerable field with so many classic textbooks, one might ask why another mineralogy text is warranted. Introduction to Mineralogy is organized in a traditional way, with Part I covering the topics of symmetry, crystallography, crystal chemistry and structure, and crystal growth. Part II covers physical properties of minerals and methods for studying mineral structures and chemistry (including optical mineralogy and x-ray diffraction), and Part III presents the systematic mineralogy of all of the mineral groups.