Treatment of phase transitions

Some phase transitions, e.g. the α-quartz/β-quartz transition, are spontaneous, and under reference conditions β-quartz is not preserved. The thermophysical parameters and functions of β-quartz are not accessible at temperatures below 855 K. To avoid the difficulties associated with their estimation, modifications of phases with spontaneous phase transitions were not treated as separate phase components. Any changes in the thermodynamic parameters and functions were considered at transition conditions and treated as first order transitions. In the case of additional lambda behavior, as in quartz, the lambda shape of the transition in cp is given by the original cp function (cf. Hemingway 1987, for quartz).

Using tabulated values for the transition temperature Tt, and the entropy and volume change at Tt, ΔSt and ΔVt at 0.1 MPa, the transition boundary in P-T space can be calculated using the relationship:

Phase transitions for cristobalite, hematite, iron, larnite, leucite, magnetite, nepheline, quartz, tridymite and wollastonite were treated in this way. All other polymorphs, e.g. andalusite, kyanite and sillimanite, which are preserved at reference conditions (298 K, 0.1 MPa), were treated as separate phases.

phase transition	Tt at 0.1 MPa	ΔSt at Tt	ΔVt at Tt
cristobalite (α) <==> cristobalite (β)	0523	2.57	1.29
hematite (α) <==> hematite (β)	0950	1.95	0.00
iron (α) <==> iron (α, Currie point)	1042	0.92	0.00
iron (α) <==> iron (γ)	1184	0.76	0.00
larnite <==> larnite (α)	0970	1.57	7.51
leucite (tetragonal )<==> leucite (cubic)	0955	1.93	0.00
magnetite (α) <==> magnetite (β)	0848	2.85	0.00
nepheline (α) <==> nepheline (β)	0467	0.00	0.00
nepheline (β) <==> nepheline (γ)	1180	0.77	0.00
quartz (α) <==> quartz (β)	0844	0.74	0.17
tridymite (α) <==> tridymite (β)	0432	0.42	0.29
lollastonite (low)<==> wollastonite (high)	0995	0.20	0.00
wollastonite (high ) <==> wollastonite (cyclo)	1398	4.14	0.31