Charles T. Prewitt

Charles Prewitt's research involves the synthesis and characterization of mineral analogs and new materials that are interesting to both geoscientists and materials scientists. Along with Staff Members Larry Finger and Robert Hazen, and Research Scientist Hexiong Yang, Prewitt maintains and operates the Geophysical Laboratory's x-ray diffraction lab and, with the retirement of Finger in July 1999, has taken on an expanded role. Prewitt also uses synchrotron facilities in the U.S. and abroad for his work. He has several current research projects.

Funds from the W. M. Keck Foundation were used to match a National Science Foundation, Division of Earth Sciences grant to purchase a Bruker CCD single-crystal x-ray diffractometer. Larry Finger and Prewitt were the principal investigators. The instrument was delivered in October 1998 and, after learning how to operate the equipment and reduce the data, a number of different scientists have used it for a variety of projects. The researchers are pleased with its capabilities and believe it will have a major impact on productivity, enabling scientists to perform experiments that would be difficult or impossible with the older diffractometers. The early operation of the instrument was aided substantially by a predoctoral fellow, Przemyslaw Dera, who is a graduate student at Adam Mickiewicz University in Poznan, Poland. Dera returned to Poland in March to complete his Ph.D. thesis and will return to Carnegie in January 2000 as a postdoctoral fellow to continue working with the group. Other scientists who have used the equipment include Finger, Prewitt, Hexiong Yang, Jeff Post (Smithsonian Institution), Altaf Carim (Penn State University), and two summer interns, Kenneth Kehoe (University of Wisconsin) and Jason Nicholas (Franklin and Marshall College). Several papers based on data from the diffractometer have been submitted for publication, and several others are in preparation.

Prewitt is also interested in research on hydrous magnesium silicates, particularly in the addition of components other than those found in the MgO-SiO₂-H₂O system. High-pressure hydrous silicate phases have attracted much attention for the past 30 years because of the implications they have for the presence of water in the Earth's mantle and the effects they have on Earth processes. Previous studies on hydrous silicates were focused primarily on the MSH (MgO-SiO₂-H₂O) system, and a number of phases were identified and labeled as 10Å, 3.65Å, A, B, C, D, E, F, G, and superhydrous B (the "alphabet phases"). Because the mantle contains elements other than those in the MSH system, Jürgen Konzett, Charles Prewitt, and Hexiong Yang investigated more chemically complicated hydrous silicate phases synthesized at high pressures and temperatures using the multianvil apparatus. The synthesis and characterization include the following hydrous phases:

^M4K-substituted K-richterite, K(KCa)Mg₅Si₈O₂₂ (OH)₂
(15 GPa, 1400°C);

Clinopyribole, K_0.96Ca_1.56Na_2.51Mg_6.01Al_1.12 Si₁₂O₃₄(OH)₂
(10 GPa, 1200°C);

Hydrous phase (aenigmatite structure), Na₂(Mg_5.30 Al_0.48)Si_5.91O₁₈(OH)₂
(10 GPa, 1250°C);

Na-phase X, (Na_1.16K_0.01)(Mg_1.89Al_0.14)Si₂O₇H_0.65
(10 GPa, 1250°C);

K-phase X, K_1.54Mg_1.93Si_1.89O₇H_1.04
(16 GPa, 1300°C)

Identification and structure refinements of these phases not only provide researchers with insights into crystal chemistry of high-pressure hydrous silicates, they also expand the knowledge of deep Earth mineralogy. The table shows several new phases with structures related to Phase E, one of the major candidates to be actually present in the mantle, but as yet not identified as a mineral. Addition of Fe³⁺ results in structures with different unit cells and symmetries but similar structural components.

Fig. 15. This image shows the crystal structure of Phase D, MgSi₂H₂O₆. The dark gray octahedra represent MgO₆ and the light gray ones, SiO₆. This material has the highest known pressure-temperature stability of any hydrous silicate and may be present as a mineral in Earth's lower mantle.