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Aegirine (Acmite) thin section

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About Aegirine (Acmite)
Aegirine – for Aegir, the Scandinavian god of the sea, the mineral being reported first in Norway. Acmite – from Greek for point, in reference to its pointed crystal habit.
Aegirine (Acmite) hand-specimen
Formula: NaFe3+Si2O6
System: Monoclinic
Color: Dark green to greenish black, reddish brown, black
Lustre: Vitreous
Hardness: 6
Density: 3.5–3.6
XPL
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PPL
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Aegirine #1 thin section (hFOV 2mm)
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Aegirine #2 thin section (hFOV 2mm)
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Aegirine (Acmite) PPL properties
Relief: Very High positive
Habit/Form: Aegirine form stubby to quite elongate prisms. Eight-sided cross sections show both cleavages at about 87°. Longitudinal sections are elongate to roughly rectangular and show one direction of cleavage. The brown varieties may have acute or sharply pointed terminations. In the process of fenitization, fibrous aegirine may form at the expense of biotite or hornblende. Spongy intergrowths of riebeckite and aegirine may be found.
Color: Generally brown to green, but also black, emerald-green, grass-green, deep green, yellow, brownish-green, yellowish brown
Pleochroism: Intense pleochrosim from bright green to yellow-green and brown-green. The acmite variety of aegirine is weakly pleochroic in pale brown and yellow.
Cleavage: Good in two directions {110} (at right angles to the c-axis – on (001)) – the clivage angle in two directions are at nearly right angles 87° and 93°.
Prismatic sections parallel to (100) or (010) to which both cleavages are at too acute an angle to be always visible, especially in slightly thick sections.
Aegirine (Acmite) XPL properties
Isotropy/Anisotropy: Anisotropic
Interference color: Order I-II brilliant; or anomalous blue/brown
Extinction angle: Very slightly inclined; the maximum extinction angle in longitudinal sections is very small: from 2° to 10° (in some crystals you can have 15-20° extinction angle). {100} sections show parallel extinction.
Twins: Simple and lamellar twinning on {100} and {001}
Uniaxial/Biaxial: Biaxial (-)
Optic axial angle (2V): 2V measured: 60 – 90°, calculated: 68 – 84°
Aegirine (Acmite) distinguishing features under the microscope
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References
  • Deer, W. A., Howie, R. A., & Zussman, J. (2013). An introduction to the rock-forming minerals (pp. 498). Mineralogical Society of Great Britain and Ireland, London.
  • mindat.org – The Mineral Database