This document last updated on 12-Feb-2004
EENS 212 Petrology
Prof. Stephen A. Nelson Tulane University
Ternary Phase Diagrams
In our discussion of ternary phase diagrams, we will first review and then expanding on
material covered last semester in Earth Materials on binary and ternary phase diagrams. You
should therefore spend some time reviewing this material. Links are as follows -
http://www.tulane.edu/~sanelson/eens211/2compphasdiag.html or for the PDF version -
http://www.tulane.edu/~sanelson/eens211/2compphasdiag.pdf
and
http://www.tulane.edu/~sanelson/eens211/ternaryphdiag.htm or for the PDF version -
http://www.tulane.edu/~sanelson/eens211/ternaryphdiag.pdf
Bring both sets of lecture notes with you to class. After reviewing this material we will proceed
with the following discussion:
When presented with a complex phase diagram, the first thing one must do is understand what
phases can coexist during crystallization or melting, and what phases coexist in all possible
subsolidus assemblages. Remember that in a ternary system at constant pressure, the maximum
number of phases that can coexist is 4, and 4 phases can only exist at ternary invariant points
(F= C+1-P, if F=0, C=3, then P = 4). Knowing what phases must be present in the subsolidus
assemblage for any composition in the system is important, because it tells us where the
crystallization path will lead, i.e. to which of the invariant points in the system where 3 solid
phases and a liquid will coexist prior to the disappearance of the liquid phase. Univariant
curves, also called cotectics, are lines along which 3 phases coexist at constant pressure (F =1,
C=3, so P = 3). Such curves bound the primary phase fields (divariant fields), and by inspection
of the phases that exist on either side of the curves, one can tell which phases are crystallizing
as the liquid composition moves along the curve. Furthermore, in the absence of isotherms or
other temperature information, one can determine the down-temperature direction that a liquid
will move along such a curve. The method is as follows:
1. First determine all possible subsolidus
assemblages by drawing the
compositional triangles. These are also
called 3 phase triangles, since a maximum
of 3 phases can coexist in a ternary system
below the solidus. To do this, examine
each of the cotectics and determine what
solid phases are in equilibrium along the
curve. Next, draw a line between the
compositions of the two solid phases that
coexist. These lines are called Alkemade
Lines. For example, in Figure 1, the curve
separating the fields of X+L and XY + L,
indicates that X and XY solids coexist
along that boundary curve.
Ternar
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