One Component Diagrams - depict the stability fields of polymorphs - H2O System Recall the Phase Rule: f = c - p + 2
Components: chemical endmembers that describe a system; usually chosen such that the smallest number of components are necessary to describe a system, commonly in terms of oxides for chemically simple systems and in terms of mineral components in petrologic systems with reactions of interest.
Phase: a homogeneous substance with a well-defined set of physical properties, may be either solid, liquid, or vapor.
Degrees of Freedom: the minimum number of variables that must be fixed in order to uniquely define the state of a system.
Back to the water phase diagram: one phase regions are divariant, two phase curves are univariant, triple points are invariant points critical point is that at which phases become indistinguishable
- compare displacive and reconstructive polymorphic transformations for: H2O silica minerals aluminum silicates graphite-diamond calcite-aragonite olivine-h.p. spinel
Two-Component diagrams, T-X diagrams - most commonly constructed with horizontal composition bar and vertical temperature axis. - different forms of T-X diagrams result from the degree of solid solution between the endmembers. - complete solid sloution: olivine, plagioclase - incomplete solid solution: albite-orthoclase - no solid solution: albite-silica, forsterite-silica Liquidus diagrams with complete solid solution: - Liquidus is a line (surface) along which compositions of melt are in equilibrium with a solid phase or phases. - Solidus is a line (surface) along which compositions of a crystalline phase are in equilibrium with the melt.
Liquidus diagrams with incomplete solid solution: - Eutectic point is an invariant point which is the minimum melting point of the system and at which two or more phases are in equilibrium with the melt. - Peritectic Point is an inflection point along a liquidus at which two or more phases can be in equilibrium with a unique melt. Metamorphic P-T grid and rections that evolve H2O and CO2: Anthophyllite=enstatite+quartz+water Calcite+quartz=wollastonite+CO2 (c.f., CC=CaO+CO2) Muscovite+Quartz=Sillimanite+Orthoclase+H2O