Page 112 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS PYROXENE GROUP

between the high Ca-bearing members (> 25% CaSiO,), which are The remaining pyroxenes of importance cannot be fitted into
always monoclinic, and the low Ca-bearing members ( < 15% CaSiO,). Figure 2.20 since they include sodium- and aluminium-bearing end
In the latter group the iron-rich members (> 30% FeSi0 3 ) are mono- members. Figure 2.21 shows two further systems to include the Na and
clinic at high temperatures (pigeonites) but invert to an orthorhombic AI members. In Figure 2.21a the central tie line from Figure 2.20,
form ( opx) at low temperatures. The magnesium-rich members above CaMgSiO,-CaFeSi0 3 (50 mol%) is used as the base of the
( < 30% FeSiO,) are orthorhombic at all temperatures. Orthopyrox- triangle, and the other apex is NaFe •Si0 3 . In Figure 2.21b the tie line
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enes can exist with up to 5 mol% CaSiO, in the structure. CaMgSiO,-CaFe •Si0 3 , which represents diopside-hedenbergite or the
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diopside solid solution series (written as diopside,, or diss), now re-
(a) aegirine NaFc3+ presents only one corner of the system, and the Na-bearing pyroxene
NaFeHSiO, another. The third apex of the triangle is the phase
NaAISiO, or jadeite. The main mineral phases occurring are depicted in
the two figures. The shaded area in Figure 2.21a means that a continu-
aegirine ous sequence of change from diss to aegirine (NaFeHSi0 3 ) does not
exist. Similarly it will be observed from Figure 2.21b that the two main
Al-bearing phases jadeite (NaAISiO,) and omphacite (a cpx containing
Ca-rich aegirine
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Na,Al,Fe3+ and Mg,Fe •) are both isolated phases, quite separated from
the other end member in the system.

aegirine- Di"

Di ,~
diopside CaMg hedenbergite CaFe
Di" Ca, Mg, Fe
(b)

omphacite

jadeite

Jadeite NaAI Aegirine NaFe-'+
Figure 2.22 Extinction angles for clinopyroxenes. Ranges of c·y (maximum
Figure 2.21 (a) Composition diagram for Na-pyroxenes (b) composition diag-
extinction angles) for several common pyroxene minerals. Note that c·a extinc-
ram for Na- and Al-pyroxenes.
tion angles for aegirine/aegirine-augite vary from oo to - 20°.
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