Talk:Schreinemaker's analysis

Improving the Terminology

Until 1990 (at least) Schreinemakers' 'rules' were used by chemical petrologists to roughly sketch the regions of stability of pure minerals ('the phases discussed here') about their thermodynamic invariant points on p,T-phase diagrams. These regions are very important in determining the most intense environment metamorphic rocks experienced. F.A.H. Schreinemakers, a Dutch chemist of the latter 19th Century, published relations among the univariant curves in both Dutch and English, but provided no algorithm. It would probably do no harm to restrict the article to p,T-diagrams.

Because Schreinemakers studied the relative order of stable (and metastable) curves at the invariant point, the axes are really dp and dT, what Gibbs termed intensities. The axes are not intensive variables, for the rules fail for the axes density and molar entropy (both intensive variables). (The Gibbs Phase Rule is valid for intensive variables; thus the need for this term.)

Correctly drawing and labelling the axes will not make the diagram thermodynamically accurate. For this, the coefficients for two independent reactions are needed. From the signs of the phases in the phase reactions, once can determine the relative order of the curves and their metastable extensions, though their orientation is not determined. That is, drawn on transparent paper, the paper can be flipped over: which orientation is correct is not calculated. (This is because the Clapeyron equation used is written dp/dT=change in entropy/change in volume, as is common. If written as (change in entropy)dT + (change in volume)d(-p) = 0, one could probably determine which orientation is correct.)

Perhaps a clear statement of the Morey-Schreinemakers Coincidence Theorem could be given, a statement of Schreinemakers rules given, and a reference at least given to why the latter follows from the former.

The term 'assemblage' is used incorrectly. Classical thermodynamics is founded upon the concept of state. Equilibrium reactions describe thermodynamic phenomena, not states: no set of equilibrium reactions can necessarily describe the thermodynamic state of a mineral assemblage. An assemblage of phases is the collection of those phases present in the system; it consists of those phases that react and those that do not. Those that do not change the system's volume, extract or emit heat, and precipitate or dissolve material. A phase assemblage is the partition of a thermodynamic system into phases.

Finally, the one reference given is very expensive. It would be nice to include historical references and some examples of applications in the primary literature. Geologist (talk) 02:43, 22 November 2013 (UTC)