7 Minerals 7
70
Petrological research requires a strong understanding
of the principles of mineralogy and mineral identification
and a thorough familiarity with the theoretical and experi-
mental studies of rock origins. The present section focuses
on phase equilibrium, upon which the link between the
study of minerals and the study of rocks is largely based.
A phase is a homogeneous substance that has a fixed
composition and uniform chemical and physical properties.
Only a mineral that displays no solid solution may there-
fore be considered a phase. Quartz (SiO
2
), for example, is
a low-temperature phase in the Si-O
2
(SiO
2
) system, and
kyanite (Al
2
SiO
5
) is a high-pressure phase in the Al
2
O
3
-
SiO
2
(Al
2
SiO
5
) system. The term phase region is used when
a mineral exhibits compositional variation, as in the solid-
solution series between forsterite and fayalite. A phase may
exist as a solid, liquid, or gas: H
2
O, for example, occurs in
the form of ice (solid), water (liquid), and steam (gas).
Equilibrium refers to the stable coexistence of two
or more phases and is established relative to time. If two
phases in a mixture of water and ice coexist so that the
amount of each is fixed indefinitely, they are said to be
in equilibrium. The minerals of some rocks have existed
together since their formation for periods of several
million years, yet one cannot always ascertain if these
rock constituents are in equilibrium or are still undergo-
ing changes.
A determining factor of the equilibrium state of min-
erals is the presence (or absence) of a reaction rim, which
is a region separating two or more minerals consisting of
the products of a reaction between them. The absence
of any observable reaction rims between minerals that
physically touch each other suggests that they were in
equilibrium at the time when the rock formed. Additional
chemical data regarding elemental distribution between