Abstract
Structural refinements have been completed for a Mg–rich cordierite using data recorded at 24°, 375°, 775° and 24°C (after heating to 775°) and for an Fe–rich cordierite at 24° and 375°C. The mean T–O bond lengths in both cordierites remain unchanged but the mean octahedral bonds (M–О) 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 octahedron is more flattened, resulting in с being smaller and a and b being larger than the cell dimensions of the Mg-cordierite. Upon heating Fe-cordierite, there is no evidence for a rotation of the rings, and a, b, and с increase as the M–O bonds expand.
X-ray Δρ maps calculated for the Mg-cordierite showed approximate positions and relative amounts of channel constituents. The peak ascribed to the alkali and other atoms that centers the six-membered rings becomes elongated parallel to с upon heating through 375°C. However, the peak ascribed to the oxygen associated with H20 in the 24° and 375° maps is absent in the 775°C maps. It reappears in maps computed from the 24° (after heating) data. In both cordierites, small amounts of hematite were produced during heating (prematurely halting data collection on the Fe-cordierite), and apparently formed by combination of octahedral and channel iron with oxygen from the channel water molecules.
A re-examination of the water orientation in the channels of the Mg-cordierite using neutron and X-ray Δρ maps does not clearly show either type I [H–O–H in the (100) plane with the H–H vector parallel to с] or type II [H–O–H in the (100) plane with the H–H vector parallel to b] water, as previously suggested by spectroscopic studies. Instead, our Δρ maps indicate that the water molecule lies in a plane tilted ∼29° from (100) and that the H–H vector is tilted ∼19° from c.
