Garnet mono- and polycrystals from the Snake

Range metamorphic core complex, Nevada.

(a) Photomicrograph of garnet mica schist showing location and texture of a 2-domain polycrystal, highlighted in the inset (b)

Elements maps of Mg, Fe, Mn, and Ca showing that the smaller garnet can be detected in the Ca map but not in the others. Figure from Whitney & Seaton (2010)

Garnet

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.