High temperature crystal chemistry of hydrous Mg- and Fe-rich cordierites
Hochella, Michael F. Jr.
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Structural refinements have been completed using data recorded for a Mg-rich cordierite at 24°C, 375°C, 775°C and 24°C (after heating to 775°C) and for an Fe-rich cordierite at 24°C and 375°C. The mean T-0 bond lengths in both cordierites remain unchanged but the mean octahedral bonds (M-0) lengthen upon heating. The unusually low thermal expansion of the Mg-cordierite is the result of its relatively "rigid" tetrahedral framework and the anisotropic expansion of octahedra isolated from each other. This anisotropic expansion leads to a slight rotation of the six-membered rings, a concomitant collapse of the structure parallel to c, and an expansion parallel to a and b. In the Fe-cordierite, the Fe-octahedron is more flattened, resulting in c being smaller and a and b being larger than the cell dimensions in the Mg-cordierite. Upon heating Fe-cordierite, there is no evidence for a rotation of the rings and a, b and c increase as the M-O bonds expand. A re-examination of the water orientation in the cavities of the Mg-cordierite using neutron and X-ray Δp 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 e. However, no evidence was found in the Δp maps for type II water. A peak ascribed to the alkali atoms that centers the six-membered ring becomes elongated parallel to a upon heating through 375°C, while the peak ascribed to the oxygen associated with H2O is absent in the Mg-cordierite at 775°C and in the Fe-cordierite at 375°C. After heating the Mg-cordierite to 775°C the peak reappeared in the Δp maps computed from data recorded at 24°C, but it no longer showed an elongation parallel to a as it did before heating.
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