Page 114 - A Practical Introduction to Optical Mineralogy

PYROXENE GROUP
SILICATE MINERALS
Figure2.25
Figure 2.23
Exsolution
Exsolution
lamellae parallel
lamellae parallel
to (001).
to (100).
"....
(010) face (010) face
cools, and the excess calcium is exsolved as clinopyroxene lamellae
The 'normal' pyroxenes which are represented in Figure 2.20 are parallel to the (100) plane (Fig. 2.23).
essential constituents of the basic calc-alkaline igneous rocks. These As crystallisation proceeds the ratio Mg: Fe in the liquid decreases.
pyroxenes may occur in some ultrabasic and intermediate igneous rock When the ratio reaches the value Mg: Fe = 70: 30, orthopyroxene is
types; and some pyroxenes in this system may occur in high temperature replaced by a Ca-poor clinopyroxene '(pigeonite). This Ca-poor
regional and thermal metamorphic rocks. clinopyroxene may also contain an excess of calcium at high tempera-
The Na-bearing pyroxenes primarily occur in alkaline igneous rocks ture, which will be exsolved as Ca-rich clinopyroxene lamellae as the
of various types, and the Al-bearing pyroxenes occur in rocks pigeonite cools. These exsolution lamellae are parallel to the (001)
(metamorphic or igneous) in which high pressures and temperatures plane of the monoclinic pigeonite. As cooling proceeds, theCa-depleted
have been operative. pigeonite cools through an 'inversion' curve, below which it changes (or
The individual pyroxene minerals (or series) are discussed separately, inverts) to orthopyroxene (Fig. 2.24). The final result is a crystal of
but Figure 2.22 gives extinction angles for all the pyroxene minerals. orthopyroxene containing lamellae of Ca-rich clinopyroxene parallel to
the monoclinic (001) plane of the original pigeonite (Fig. 2.25).
Exsolution lamellae Ca-rich clinopyroxenes also contain lamellae of exsolved pyroxene
In many slowly-cooled pyroxenes, especially orthopyroxenes and parallel to (100) and (001). The lamellae which are parallel to (100) are
augites, lamellae occur which have a definite crystallographic orienta- exsolved orthopyroxene, whereas the lamellae parallel to the (001)
tion. These are not twin lamellae but exsolved sheets with a different plane represent exsolved pigeonite. If cooling proceeds through the
composition from the host mineral. inversion curve, the pigeonite lamellae will invert to orthopyroxene,
Orthopyroxenes may contain lamellae parallel to the (100) plane. producing a second set of lamellae parallel to (100). Recent research
These actually areCa-rich clinopyroxene lamellae; the opx first crystal- work has shown that the crystallographic orientation of these lamellae,
lises at a high temperature with some calcium in the structure, then as well as their chemical composition, may be much more complex than
originally was thought.

Crystallisation trends
Two Mg-rich pyroxenes may first crystallise from a basic magma, a
Ca-poor orthopyroxene and a Ca-rich augite. As crystallisation pro-
ceeds, both pyroxenes become more Fe-rich until the Mg: Fe ratio
reaches 70: 30, at which point pigeonite replaces orthopyroxene as the
Ca-poor pyroxene crystallising. Both clinopyroxenes become increas-
ingly Fe-rich as crystallisation continues. In many intrusions crystallis-
ation ceases at this point, but if fractionation is very marked, only one
0 pyroxene (a Ca-rich ferroaugite) crystallises when the Mg: Fe ratio
drops below 35:65. Finally, in extreme cases, crystallisation continues
MgSi03 FeSi0 3
enstatite ferrosilite until there appears a Ca-rich ferrohedenbergite containing no Mg.
Mol per cent Diagrammatically the crystallisation sequence, including olivine, can be
depicted as follows:
Figure 2.24 Inversion curve and phase diagram for pyroxenes.
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