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SUMMARY:I wasn't born cubic\, said Ca-rich almandine garnet - Prof. Bernar
 do Cesare\, University of Padova\, Italy
DTSTART:20190430T110000Z
DTEND:20190430T120000Z
UID:TALK108625@talks.cam.ac.uk
CONTACT:Owen Weller
DESCRIPTION:Garnet is a key rock-forming phase of the Earth's crust and up
 per\nmantle\, where it represents the paradigmatic cubic mineral\, optical
 ly\nisotropic. Although strain-induced birefringence is observed in the\nr
 are Ca-Fe3+ hydrogarnets\, the optical anisotropy rarely documented in\nco
 mmon Fe2+-Mg-Mn garnets has not found a general and robust\nexplanation so
  far.\n\nHere we show that anisotropic garnets are much more widespread th
 an\npreviously thought\, in particular in blueschist-facies rocks.\nWe une
 quivocally demonstrate that the origin of birefringence is the\ntetragonal
  nature of garnet\, that it is unrelated to strain\, and that\ncrystals ar
 e twinned according to a merohedral law. We also show the\nabsence of a hy
 drogarnet component\, and that the tetrahedral form\nappears to be typical
  of Fe-Ca-rich compositions\, with very low Mg\ncontents.\n\nThe widesprea
 d occurrence of optically anisotropic OH-free garnets in\nblueschists impl
 ies that this phase does not commonly grow with cubic\nsymmetry at low-tem
 perature conditions (< 400 °C). The\ncrystal-chemistry of garnet\, its th
 ermodynamics and\, in turn\, its use\nin unravelling petrogenetic processe
 s in cold metamorphic\nenvironments\, such as in subduction zones\, need t
 o be re-assessed.
LOCATION:Tilley Lecture Theatre\, Department of Earth Sciences
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