Dear Dr. Polyakova,

I greatly enjoied reading your excellent article. Having had a lifetime interest in Tridymite, I was very pleased with your evaluation of Tridymite and would like to communicate a few old observations I made. Quartz converts to Tridymite when heated with variou Na-

oxysalts above 870 C, such as carbonate, silicate, nitrate , molybdate

and some phosphates and borates. But reaction with Na2SO4 is

different. When heated in moist air at 1200 C it will quickly convert

to Tridymite, but if heated in vacuum , there will only be a small amount of cCritobalite, but no Tridymite. This is due to the very low

activity of Na2O in vacuum. This can be illustrated by heating Quartz with Na2SO4 in a gas mixture of SO2 +O2 with controlled SO3 potential, based on thermodynamic calculations.

I only made a short discussion of this at the Am.Ceram. Soc.