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Open AccessArticle

A New High-Pressure Phase Transition in Natural Gedrite

1
Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, HI 96822, USA
2
Department of Earth and Planetary Sciences, Northwestern University, Evanston, IL 60201, USA
*
Author to whom correspondence should be addressed.
Crystals 2019, 9(10), 521; https://doi.org/10.3390/cryst9100521 (registering DOI)
Received: 23 September 2019 / Revised: 8 October 2019 / Accepted: 9 October 2019 / Published: 11 October 2019
(This article belongs to the Special Issue High Pressure Synthesis in Crystalline Materials)
High-pressure diamond-anvil cell synchrotron X-ray diffraction experiments were conducted on single-crystal samples of natural orthoamphibole; gedrite; with composition; (K0.002Na0.394)(Mg2)(Mg1.637Fe2.245Mn0.004Ca0.022Cr0.003Na0.037Al1.052)(Si6.517Al1.483)O22(OH)2. The samples were compressed at 298 K up to a maximum pressure of 27(1) GPa. In this pressure regime, we observed a displacive phase transition between 15.1(7) and 21(1) GPa from the orthorhombic Pnma phase to a new structure with space group P21/m; which is different from the familiar P21/m structure of cummingtonite and retains the (+, +, −, −) I-beam stacking sequence of the orthorhombic structure. The unit cell parameters for the new phase at 21(1) GPa are a = 17.514(3), b = 17.077(1), c = 4.9907(2) Å and β = 92.882(6)°. The high-pressure P21/m phase is the first amphibole structure to show the existence of four crystallographically distinct silicate double chains. The orthorhombic to monoclinic phase transition is characterized by an increase in the degree of kinking of the double silicate chains and is analogous to displacive phase changes recently reported in orthopyroxenes, highlighting the parallel structural relations and phase transformation behavior of orthorhombic single- and double-chain silicates. View Full-Text
Keywords: amphibole; phase transition; high-pressure; single-crystal X-ray diffraction; diamond anvil cell; synchrotron source amphibole; phase transition; high-pressure; single-crystal X-ray diffraction; diamond anvil cell; synchrotron source
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Yong, T.; Bina, C.R.; Finkelstein, G.J.; Zhang, D.; Dera, P. A New High-Pressure Phase Transition in Natural Gedrite. Crystals 2019, 9, 521.

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