Geology Reference
In-Depth Information
In reading Chapter 2, it is easy to fall into the trap of
supposing that reactions between minerals always
proceed rapidly enough to reach chemical equilib-
rium during the time available, however short that
may be. A little thought, however, indicates that this
cannot be so. We have seen in Figure 1.3, for example,
that aragonite is a metastable mineral at atmospheric
pressure, and its survival in some outcrops of meta-
morphic rock crystallized at high pressure is a sign of
disequilibrium : the rate of inversion to calcite has been
too slow for the reaction to be completed before the
processes of uplift and erosion exposed the rock at the
surface. The same reasoning applies to occurrences
in  surface outcrops of the mineral sillimanite, which
is  stable only at elevated temperatures (Figure  2.1).
Examination of igneous or metamorphic rocks in thin
section often brings to light petrographic evidence of
disequilibrium, in such forms as mineral zoning and
corona structures (Box 3.1). From these examples it is
clear that the rate of progress of geochemical reactions,
and the way they respond to different conditions, are
factors we cannot ignore.
The measurement and analysis of chemical reaction
rates is called chemical kinetics , a science whose simpler
geological applications are the subject of this chapter.
Chemical kinetics provides the theoretical basis for
understanding how (and why) reaction rates depend
upon temperature, a matter of fundamental geological
importance in view of the high temperatures at which
magmas and metamorphic rocks crystallize. It also
provides the algebra upon which radiometric (iso-
topic) dating methods are based (Box 3.2).
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