The natural occurrence of clinohumite in metabasalts and hydrothermally altered peridotites provides a source of water-rich minerals in subducted slabs, making knowledge of their phase relations and crystal chemistry under high pressure-temperature ( P- T) conditions important for understating volatile recycling and geodynamic process in the Earth's mantle. Photoluminescence studies show strong red and green light emissions.These results, together with other recent studies, suggest that mixed 4-, 5-, and 6-fold coordination states may occur more commonly in silicate structures compressed at 300 K than previously recognized. The sequence of high-pressure phases shows increasing polymerization of coordination polyhedra.
Cordierite II has four- and five-coordinated Si and Al, while more » cordierite III has four-, five-, and six-coordinated Si, four- and five-coordinated Al, and five- and six-coordinated Mg. Across the phase transitions there is a significant reduction in the length of the a-axis (~2 Å per phase transition), whereas both the b- and c-axis remain largely unchanged. The structures of cordierite II and III were refined at 7.52(3) GPa at 15.22(15) GPa, respectively. We solved and refined both new triclinic hases in space group P1, and designate them cordierite II and III.
We observed a recently described orthorhombic to triclinic transition, as well as a further transition to a second triclinic phase. The samples were compressed at 300 K in a diamond anvil cell to a maximum pressure of 15.22(15) GPa with a neon pressure-transmitting medium and a gold pressure calibrant.
High-pressure single-crystal X-ray diffraction experiments were conducted on natural cordierite crystals with composition Mg1.907(18)Fe0.127(6)Al4.01(2)Si4.96(3)Na0.026(3)O18.12(9) using a synchrotron X-ray source.
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