Type of Document	Master's Thesis
Author	Hochella, Michael Frederick
URN	etd-07282010-020312
Title	High temperature crystal chemistry of hydrous Mg- and Fe-rich cordierites.
Degree	Master of Science
Department	Geological Sciences
Advisory Committee	Advisor Name Title Gibbs, Gerald V. Committee Chair Bloss, F. Donald Committee Member Ribbe, Paul H. Committee Member Ross, F. K. Committee Member
Keywords	anisotropic expansion
Date of Defense	1977-07-05
Availability	restricted
Abstract 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-O bond lengths in both cordierites remain unchanged but the mean octahedral bonds (M-O) 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 a, and an expansion parallel to a and b. In the Fe-cordierite, the Fe-octahedron is more flattened, resulting in a 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 a 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 Δρ 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 Δρ 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 375°C, while the peak ascribed to the oxygen associated with H20 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 Δρ 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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