Synthesis, composition, thermal stability, and thermodynamic properties of bicchulite, Ca<2) [Al<2) SiO<6) ](OH)<2)

Single-phase bicchulite has been synthesized and its composition established as 2CaO·Al₂O₃·SiO₂·H₂O. Bicchulite crystallizes in space group I43m, with a = 8.825 ± 0.001 A. Seven reversal brackets have been obtained experimentally for the dehydration reaction bicchulite = gehlenite + H₂O in the P_H2O range 500 to 7000 bars. The equilibrium constant, K, for this reaction may be expressed as log К = log f_H2O = 9.02 − 5750/T + 0.0699(P − 1 )/T, with P indicated in bars and T in °K. Based on these experimental data, the standard state thermodynamic parameters for bicchulite are found to be: Data available on the CaO-rich portion of the system CaO·Al₂O₃·SiO₂·H₂O·CO₂, where the compositions of simplified bicchulite-bearing metamorphic rocks would plot, suggest that bicchulite will be restricted to assemblages with a vapor of extremely low Х_СО2.