Smith College - Geology 222b - Petrology

Petrographic Data File

Aegirine (Acmite)
Property
Value
Comments
Formula NaFe3+Si2O6 for acmite

(Na,Ca)(Fe3+, Fe2+, Mg, Al)Si2O6 for aegerine-augite series

wide range of composition, but usually replacement is NaFe3+---->Ca(Mg, Fe2+); complete series to augite
Crystal System monoclinic beta = 105o-107o
Crystal Habit    stubby to elongate prisms
Cleavage at 87o and 93o, visible on {110}  
Twinning   simple and lamellar on {100}
Color/Pleochroism generally brown to green, but also black, yellow-brown, pale green or yellow-green highly pleochroic from bright green to yellow-green and brown-green

color zonation common; rim usually darker than core

Relief high positive  
Optic Sign biaxial (-) for acmite

biaxial (-) or (+) for aegerine-augite series

 
2V 60-70o  for acmite

70-110o for aegerine-augite series

variation with composition shown in Figure 13.8 below (Nesse 1998: 212)
Optic Orientation acmite
Y=b
X^c =+10-0o
Z^a =+7-+17o
o.p.= (010)

aegerine-augite
Y=b
X^c = -10-20o
Z^a = 16-36o
o.p. = (010)

length fast
Refractive Indices
Birefringence
alpha =
beta =
gamma =
birefringence (delta) =
acmite
1.75-1.78
1.78-1.82
1.80-1.84
0.40-0.60


aegerine-augite
1.70-1.76
1.71-1.80
1.73-1.81
0.28-0.50

variation with composition shown in Figure 13.8 below (Nesse 1998: 212)

2nd to 3rd order interference colors, but often masked by mineral's color

Extinction very slightly inclined, 0-10o  
Dispersion red > violet .
Distinguishing Features Cleavage distinguishes it from amphiboles.   (Amphiboles have cleavage at 56o and 124o and are length-slow.)

Color, pleochroism, small extinction angle, length-fast character and high refractive indices and birefringence distinguish it from other pyroxenes.

Occurrence Most often found in Na-rich and quartz-poor igneous rocks - such as alkali granite, syenite, nepheline-syenite - and in glaucophane- and riebeckite-bearing schists.
Associated minerals include orthoclase, feldspathoids and Na-rich amphiboles.
Editors Amber Kaup (00), updated by Lindsey Able and Laurel Mutti (01)

 
Thin section photograph of aegirine crystal, surrounded by perthite and quartz, in plane light.    Note characteristic green-brown color. (Sample JB8)
Same thin section as above, in cross polarized light.   Note second order interference colors.   (Aegirine can display up to third order colors.)
Thin section photograph of aegirine with perthite and nepheline in plane light.   Note pyroxene cleavage (roughly 90 degrees) in lower left of crystal.   This sample (WS23) is a nepheline-sodalite syenite from near Red Hill, NH.
Same thin section as above, in cross polarized light.    Observe that the green-brown color of the mineral somewhat masks the interference colors. 
WWW Images U.C.L.A. Petrographic Workshop

 

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