Page 126 - A Practical Introduction to Optical Mineralogy

SILICATE MINERAlS
PYROXENE GROUP
Aegirine (acmite) NaFe'+Si 2 0 . 1.099 : 1:0.601 , .B = 106°49'
HABIT Elongate prismatic crystals are most common.
Aegirine-augite (Na,Ca)(Fe,Fe'+,Mg)Si20 • axial ratio variable CLEAVAGE Similar to other pyroxenes.
RELIEF High to very high.
*BIREFRINGENCE High but third order interference colours are usually masked by strong
Aegirine
greenish colour which persists under crossed polars.
although prismatic. aegirine INTERFERENCE 2V data given in Fig. 2.28.
possesses steep pyramidal FIGU RE
faces and may appear lozenge
shaped in some sections

5( )
)
-2~
~
)
IW-...-1------i--l-- -- --- b = ~ -- v v
I-- 1---
~ 2Va
5( )
1.84
1.82
~ ~
~
1.8( ) ~
~
~ 1.76 -- '-.... ~
c 1.78
.2
n p
Aegirine Aegirine-augite u t-- r-.. """ ~
~
n. = 1.750-1.776 n. = 1.700-1.750 ~ ............... ~ ........ I'-.
np = 1.780-1.820 np = 1.710-1.780 0 1.7 4 !'-...
X ~ I'-,
n > = 1.800-1.836 n > = 1.730-1.800 '0 ~ :..
"'
8 = 0.040-0.06 8 = 0.03-0.05 c: 1.7 2 '~ ",
2V. = 40°-60° - ve 2V. = 70°-90° - ve " " '
1.7 ()
OAP is parallel to {010) 2V > = 90°-70° +ve ~~
D = 3.55-3.60 H = 6 OAP is parallel to {010) 1.6 8 '~
D = 3.40-3.55 H = 6
1 6 6
*COLO U R Aegirine is strongly coloured in shades of green. · 100 90 80 70 60 50 40 30 20 10 ()
Aegirine-augite is also coloured green in thin section. NaFe'+ (SiO,), Ca(Mg.Fe.Al)I(Si.A \)0., 1,
* PLEOCHROISM Aegirine is strongly pleochroic, with a emerald green, .B deep green and I I I I
>------ aegirine ---- (:l~girine-augi t~ augite
y brownish green. Aegirine-augite is also pleochroic with a similar I I
scheme to that of aegirine, but usually has a more yellowish colour
throughout. Figure 2.28 Variation of the c·a, 2V angle, and indices of refraction in the
aegirine- augite series (data from Deer, Howie & Zussman 1962).
114
115