Order disorder in sillimanite

✍ Scribed by S. K. Saxena

• Book ID: 104745338
• Publisher: Springer
• Year: 1974
• Tongue: English
• Weight: 335 KB
• Volume: 45
• Category: Article
• ISSN: 0010-7999
• DOI: 10.1007/bf00371168

✦ Synopsis

Ala+--Si 4+ disordering in sfllimanite cannot be considered as ideal mixing on tetrahedral sites. Ideal free energy of mixing in sillimanite corresponding to slight disordering results in changing the kyanite-sillimanite phase equilibrium boundary by impossible magnitude. A regular solution model for the sites yields the following expression for the free energy of mixing: ~ = w {(xAI.~ 1 )2 + (xA:.~ ~)2 } + RT in XA:.T 1 XAI.~2.

Calculations using the phase equilibrium data show that even small disequilibrium in intracrystalline distribution results in significant change in pressure of the kyanite-sillimanite "equilibrium". If information on interaction energies such as W is not available for disordering silicates, it is not advisable to use the ideal configurational entropy in calculations of phase equilibria.