Garnets

Garnets are the best minerals on or in the planet. They contain a huge amount of information about the conditions, mechanisms, and rates of metamorphic and tectonic processes through time. The most excellent garnets are zoned and have lots of inclusions of different minerals, but even seemingly simple garnets may have complex microstructures (visible with electron backscattered diffraction [EBSD] analysis).

At Minnesota, we are doing many different things with garnets:

• using composition/zoning and textures of garnets in high-grade rocks, including high-pressure rocks, to determine reaction history, pressure-temperature conditions, and P-T paths.
• determining microstructures of garnets, including identification of high-angle boundaries in morphologically isolated garnets (apparently single crystals, but actually 2-3 clustered grains).
• measuring the material properties of garnets representing a range of compositions; i.e., hardness, fracture toughness, and Young's modulus measurements of Fe-Mg-Mg-Ca garnets using microindentation and depth-sensing (nanoindentation).
• investigating the conditions at which garnets and inclusions interact chemically and texturally in high-grade metamorphic rocks; at high temperatures, inclusions in garnets may acquire facets that represent crystallographic directions in the host garnet.
• analyzing reaction textures involving garnet -- for example, coronas and symplectites around garnet -- to understand conditions, rates, and paths related to the tectonic history of the rocks.

Garnets may be involved in partial melting, one of the most important processes in the Earth's crust or mantle:

Photo by S. Kruckenerg

Garnet research is currently supported by NSF grant EAR-0510300

Selected garnetocentric publications

Whitney, D.L., Goergen, E.T., Ketcham, R.A., and Kunze, K. (2008) Formation of garnet polycrystals during metamorphic crystallization. Journal of Metamorphic Geology, 26, 365-383.

Whitney, D.L., Broz, M.E., and Cook, R.F. (2007) Hardness, fracture toughness, and elastic modulus of some common metamorphic minerals. American Mineralogist, 92, 281-288.

Whitney, D.L., Cooke, M.L., and Du Frane, S.A. (2000) Modeling of radial fracturing at corners of inclusions in garnet using fracture mechanics. Journal of Geophysical Research, 105, 2843-2853.

Whitney, D.L. (1996) Garnets as open systems during regional metamorphism. Geology, 24, 147-150.

Whitney, D.L., Mechum, T.A., Dilek, Y., and Kuehner, S.M. (1996) Modification of garnet by fluid infiltration during regional metamorphism in garnet- through sillimanite-zone rocks. American Mineralogist, 81, 696-705.

Whitney, D.L. (1991) Calcium depletion halos and Fe-Mn-Mg zoning around faceted plagioclase inclusions in garnet from a high-grade pelitic gneiss. American Mineralogist, 76, 493-501.

See also the Mavi Gezegen article, "Granat" by D.L. Whitney (in Turkish)