Thursday, December 29, 2011

Wish I had seen this one years ago

Earlier this month I discovered that it is possible for me to download entire textbooks from the internet. Now, it may be that the reason I can do this is because I am on a university computer and they have an account with the publisher which makes it possible, but since a high percentage of readers who might be interested in the book I am about to talk to you about are likely to associated with a university in some capacity, there is a reasonable chance that you will have access to it as well, if not as a download, perhaps in paper at your uni library.

The book which I am enjoying reading just now is Phase Equilibria in Metamorphic Rocks, Thermodynamic Background and Petrological Applications, written by Thomas M. Will, published in 1998 by Springer, DOI: 10.1007/BFb0117723. This is volume #71 of a Lecture Notes in Earth Sciences series.

When I first started my PhD project in the field of metamorphic petrology I was coming into the field cold—it had been years since my last geology course of any sort, and I had had no background courses on topics like thermodynamics or the many calculations associated with phase equilibria. Consequently, when I started reading papers which included formulas that explained how the authors had arrived at their estimates for the pressure and temperatures at which their minerals formed I found myself skipping over the equations and looking for the sentences that explained what was done. Over time and after reading many papers which did this sort of thing I started to gain a partial understanding of the topic. Enough to apply the tools to my own samples and arrive at numbers that, hopefully, actually reflect the history of the samples.

However, while I could follow a recipe for calculating pressure and temperature of formation for metamorphic minerals, I choose which recipe to apply based on the fact that my samples were similar to those in the published papers, not because I had any real understanding of the models which underpinned the calculations. Terms like “ideal” vs “non-ideal” mixing painted pictures in my mind because I know what those words mean grammatically, not because I actually knew the difference between them as applied to the crystal structure of a mineral.

If you are in a similar boat, and would like to actually see good definitions of those terms, along with others like “entropy of mixing”, “activity-composition (or a - x)”, this may be the book for you. So far I have only managed to read the first 40 pages, but already I have a *much* better understanding of why it is that one gets such hugely different results in terms of pressure/temperature estimates depending on which model one uses for a given mineral, and a much better understanding why programs like Perple_X have so many models available to choose from. The author is kind enough to work through example calculations in a step-by-step basis, so that the reader can learn how it is done. Sure, when actually doing the geology you aren't going to do these calculations by hand—at the very least you will have a template full of formulas set up in a spread sheet, if not using a more complicated program to do the work for you, but it is always nice to understand *how* the program does the calculations—this makes it much easier to spot if a typo in the data-entry stage resulted in a geologically implausible answer being spit out by the program.

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