Title page for ETD etd-07282010-020312
|Type of Document
||Hochella, Michael Frederick
||High temperature crystal chemistry of hydrous Mg- and Fe-rich cordierites.
||Master of Science
|Gibbs, Gerald V.
|Bloss, F. Donald
|Ribbe, Paul H.
|Ross, F. K.
|Date of Defense
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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