Page 117 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS PYROXENE GROUP

start of crystallisation (T high) end of crystallisation Orthopyroxenes
Enstatite (En)- orthoferrosilite (Fs) orthorhombic
I MgSi0 3 -FeSi0 3
A Mg-rich Opx~ 1.03: 1:0.59 (En)
I
I pigeonite ---o>i
'
B Mg-rich augite----------~ ferroaugite ferrohedenbergite A complete sequence of orthopyroxenes is possible from 100% MgSi0 3
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C Mg-rich o li vi ne ~ hiatus fayalitic (Fe-rich olivine)----;.- (100% enstatite) to 100% FeSi0 3 {100% orthoferrosilite). Pure
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enstatite can be written En 100 Fs 0 and pure orthoferrosilite En 0 Fs, 00 •
Mg:Fe = 70:30 Mg: Fe = 35 : 65 Nowadays it is conventional to give the composition of an orthopyr-
oxene as the percentage of orthoferrosilite present. Thus Fs 24 means
--------- Mg: Fe ratio decreasing---------~
En 76 Fs,., or, to give its complete formula, {Mg 0 • 76 Fe 0 _,.)Si0 3 •
It should be noted that the Mg: Fe ratios given above may change c='( the figure shows opx Fs,. to Fs, 00
depending upon the amount of cations replacing Mg, Fe (and Ca) in the I which have a as acute bisectrix
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pyroxene structure. The above diagram can be represented as part of the 2V •. Opx of composition Fs 0 to Fs 13
cpx triangular diagram Figure 2.26: A, 8 and Care shown on the figure. have y as acute bisectrix 2V, but
OAP is the same
Key
Figure 2.26
Crystallisation Wollastonite A Ca-rich cpx
trends in cpx, B Ca-poor opx-pigeonile
opx and olivines. !rend
C olivine trend

En Fs
n. 1.650-1.768
np 1.653-1.770
Enstatite hiatus c Orthoferrosilite n , 1.658-1.788
Forsterite Fayalite 8 0.008-0.020
2V, = 60°-90° +ve (for opx of composition Fs 0 -Fs 13 )
2V. = 90°-50° - ve (for opx of composition Fs 13 -Fs 87 )
High Fe-opx {Fs 87- FS 100 ) are also positive with large 2V but these are not
present in igneous rocks, and only occur in some meteorites. Figure 2.27
gives details of 2V variation.
OAP is parallel to (010). It is occasionally shown as being parallel to
(100)
D = 3.21-3.96 (Fe-rich) H = 5-6
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