Abstract
Compression measurements on cordierite, (Mg, Fe) 2Al 4Si 5O 18· n (H 2O, CO 2, etc.), reveal two vol discontinuities. Room-temperature measurements to 30 kbar located them at 2 and 9 kbar. While the P- T-path of the lower discontinuity has been determined by compression experiments up to 900°C ( P (kbar)≈0.0075 T°C+2), the one of the upper discontinuity relies on indirect evidence from H 2O- and CO 2-content data of Mg-cordierite, which suggest a similar positive slope. When interpreted as an abrupt change in specific volume ( ΔV>O), both discontinuities yield a change in volume on the order of 0.002; the calculated changes in the thermodynamic parameters (e.g. ΔS≈0.4 J K. mol −1) are similarily small. X-ray data give no indication of deviation from the orthorhombic space group Cccm of cordierite. The two volume discontinuities are interpreted as displacive, higher order phase transitions. Comparison of the presented compression data with those of other framework silicates and the ring silicate beryl yield an intermediate behavior for cordierite.
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