AEGIRINE

Aegirine is an inosilicate of the Pyroxene Supergroup. It is an uncommon mineral, whose occurrence is restricted to some types of rocks. It has no economic importance.

Aegirine forms a solid solution with augite. The middle term is aegirine-augite. “Acmite” was a term used for both green and brown aegirines with pointed ends; the name was discredited in 1988.

As impurities, Mn, Mg, V, Ti, Al, Ca, K, Ce and Zr can occur in aegirine. It has three varieties (high Na, high Ti and with V).

1. Characteristics

Crystal system: Monoclinic prismatic.

Color: Very dark green to greenish black, can be reddish brown to black.

Habit: Long prismatic with pointed ends.

Cleavage: {110} good. Striations // to the c axis.

Tenacity: Brittle

Twinning: Simple and lamellar on {100}

Fracture: Irregular.

Mohs Hardness: 6

Parting: On {100}.

Streak: Yellowish white.

Lustre: Vitreous, a bit resinous.

Diaphaneity: Transparent.

Density (g/cm³): 3.5 – 3.6

 

2. Geology and Deposits

Aegirine is typically found in alkaline igneous rocks (rich in Na and poor in silica) such as alkali granites, syenites, phonolites, basanites, clinopyroxenites, carbonatites and pegmatites. Also in rare rocks such as apatitolites and eudialitites.

It can also occur in metamorphic rocks of the blueschist facies such as riebeckite-schists and glaucophane-schists. It occurs in rocks of regional metamorphism such as schists, gneisses and banded iron formations (BIFs). By sodium metasomatism in granulites.

            It is autigenic mineral in some impure shales and limestones.

 

3. Mineral Associations

Occurs associated with feldspars (orthoclase, microcline, albite), feldspathoids (nepheline), amphiboles (riebeckite, arfvedsonite), astrophyllite, aenigmatite, apophyllite, serandite, analcime, eudyalite, rinkite, natrolite, quartz, rhodochrosite, zircon and catapleiite.

 

4. Transmitted Light Microscopy

Refraction indices: nα: 1.720 – 1.778          nβ: 1.740 – 1.819              nγ: 1.757 – 1.839

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Moderately to strongly pleochroic:

X = deep green;

Y = green grass;

Z = yellow-brown.

Color zoning (hourglass zoning!) is common, the edges are usually darker than the crystal core.

Relief: High to very high.

Cleavage: {110} perfect: these are two cleavages that intersect at angles of 87º and 93º in the basal sections (as in all pyroxenes!).

In the longitudinal sections, only one cleavage is observed.

Habits: Short prismatic to acicular. Granular, skeletal and poikiloblastic habits are possible. Grains with black margins and hourglass texture are possible.

Basal sections with 8 sides are typical.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence from 0.037 to 0.061, resulting in intense and strong colors of the 3rd and 4th order, which are often masked by the intense color of the mineral. Birefringence and extinction angle decrease with increasing iron content, until reaching aegirine-augite. 

Extinction: Low-angle oblique, from 0° to maximum 12°.

Can simulate parallel extinction!

Elongation sign: ES(-)

Twins: Simple and lamellar twins in {100}, common.

Zoning: Frequently zoned, visible in PPL and in XPL.

CONVERGENT LIGHT

Character: B(+) or B(-)

2V angle: 60-70º, varies with composition.

Alterations: does not alters easily. Alteration to arfvedsonite is possible.

May be confused with: other minerals with pleochroism in green and greenish colors.

Amphiboles often are elongated too, but show angles of 124 and 56º of the cleavages in the basal sections.

Augite and diopside may be similar, but have 2V larger angles.

Orthorhombic pyroxenes exhibit straight extinction.

 

Aegirine 01: image in PPL, showing one of its colors of pleochroism, deep moss green. Typical is also the long prismatic, almost acicular habit.
Aegirine 02: the same set of crystals as in the previous figure, also in PPL, rotated 90 degrees, now showing the second color of pleochroism, a yellow-brown.
Aegirine 01: the same set as in the two previous images, now in CPL and in the position of maximum illumination. The colors are strong, somewhat masked by the own strong color of aegirine.
Basal sections of aegirine in PPL, showing the two cleavage directions intersecting at ~90º, typical of pyroxenes.
Poikiloblastic aegirine in phonolite in PPL. They are skeletal crystals, with many inclusions, showing typical pleochroism, near-parallel extinction and strong interference colors.
Aegirine (yellow-green) in PPL. The long to acicular prismatic habit that aegirine can present is a habit much more typical of amphiboles than of pyroxenes
Zoned aegirine in a volcanic rock in PPL, with a basal 8-sided zone (brownish).

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of aegirine. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with aegirine, like magnetite and ilmenite.

Sample preparation: the polishing of aegirin is simple and of excellent quality, following the feldspars and feldspathoids that usually occur together. It’s a similar situation to augite, which is also easy to polish.

PLANE POLARIZED LIGHT – PPL

Reflection color: Light gray, a much lighter color than the color of feldspars, feldspathoids (and quartz, which does not occur associated). It is a similar situation to the augite, but the color is lighter than that of the augite.

Pleochroism: No.

Reflectivity: Low (< 10%).

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Distinct anisotropy, a little bit difficult to visualize due to the strong and dark internal reflections.

Internal reflections: Widespread in strong colors, with stronger shades where the crystals are thicker and lighter shades with finer crystals in the polished section. The color of the reflections is yellowish brown, caramel, with a hint of green. In very thick grains or which have other grains around them and below them, the reflections can be almost black.

May be confused with: other dark colored transparent minerals, mainly other pyroxenes and amphiboles.

Augite has darker internal reflections.

Tourmaline can have similar internal reflections, but the habits and the paragenesis are different.

Poikiloblastic aegirine in phonolite in PPL. Its reflection color is much lighter than the associated feldspars and feldspathoids.
Aegirine in CPL, with yellowish brown internal reflections, may be caramel, with a slight green hue. The thinner de mineral, the lighter the color.
Aegirine may have a long, almost acicular prismatic habit, more typical of amphiboles than pyroxenes. Image in CPL.
The color of the internal reflections of aegirine, in CPL, will depend on the thickness of the grain at the point considered (the finer, the lighter the reflections) and on the grains situated around and below it (the more grains around and below it, the darker the colors of the reflections).
Aegirine 02: aegirine in PPL in eudialyte-nepheline-syenite. aegirine is almost white, contrasting with the dark gray nephelines around it. At the top, very poorly polished alteration material.
Aegirine 02: In CPL, reflections in various shades of dark green are characteristic of aegirine, while nepheline has milky reflections and alteration material is yellowish.
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