The entropies of a series of pure-silica molecular sieves (structural codes
*BEA, FAU, MFI, and MTT) are obtained by calorimetric measurements of low-
temperature heat capacity. The third law entropies at 298.15 K are (on the
basis of 1 mol of SiO2): *BFA, 44.91 +/- 0.11 J .K-1 mol(-1); FAU, 44.73 +/
- 0.11 J .K-1. mol(-1); MFI, 45.05 +/- 0.11 J .K-1 mol(-1); MTT, 45.69 +/-
0.11 J .K-1. mol(-1) while the corresponding entropies of transition from q
uartz at 298.15 K are *BEA, 3.4 J .K-1. mol(-1): FAU, 3.2 J .K-1 mol(-1); M
FI, 3.6 J .K-1. mol(-1); MTT, 4.2 J .K-1 mol(-1). The entropies span a very
narrow range at 3.2-4.2 J .K-1 mol(-1) above quartz, despite a factor of 2
difference in molar volume. This confirms that there are no significant en
tropy barriers to transformations between SiO2 polymorphs. Finally, the Gib
bs free energy of transformation with respect to quartz is calculated for e
ight SiO2 phases and all are found to be within twice the available thermal
energy of each other at 298.15 K.