High temperature crystal chemistry of hydrous Mg- and Fe-rich cordierites.
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A re-examination of the water orientation in the cavities of the Mg-cordierite using neutron and X-ray Î Ï maps confirms the orientation obtained previously by spectroscopic methods for type I water, i.e., H-O-H near (100) with the H-H vector nearly parallel to a. However, no evidence was found in the Î Ï maps for type II water. A peak ascribed to the alkali atoms that centers the six-embered ring becomes elongated parallel to a upon heating through 375Â°C, while the peak ascribed to the oxygen associated with H20 is absent in the Mg-cordierite at 775Â°C and in the Fe-cordierite at 375Â°C. After heating the Mg-cordierite to 775Â°C the peak reappeared in the Î Ï maps computed from data recorded at 24Â°C, but it no longer showed an elongation parallel to a as it did before heating.
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