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Augite thin section

About Augite
From Greek for luster, from the appearance of cleavage planes.
Augite hand-specimen
Formula: (Ca,Mg,Fe)2Si2O6
System: Monoclinic
Color: Brown-green, black, etc.
Lustre: Vitreous, Resinous, Dull
Hardness: 5½–6
Density: 3.19–3.56
Augite #1 thin section (hFOV 2mm)
Augite #2 thin section (hFOV 2mm)
Augite PPL properties
Relief: High positive
Habit/Form: Crystals are usually stubby prisms elongate along the c axis. Basal cross sections are four or eight sided and show the two cleavages at ~87°. Longitudinal sections are roughly rectangular and show only one cleavage direction. Augite also forms anhedral grains, or irregular masses that may enclose associated minerals. Overgrowths of hornblende are relatively common. Lamellae of exsolved orthopyroxene are commonly found parallel to {100} , whereas pigeonite lamellae are inclined somewhat to either P100} or {001}. The lamellae may be inclined up to 22 and 17° from (100) and (001) respectively for Mg-rich augite. Smaller inclinations are found in Fe-rich augite.
Color: Colorless, pale green, pale brown, green, greenish-yellow, pale yellow green, grayish green, violet. Zonal structure is sometimes present.
Pleochroism: Absent or weak; it is best shown in {100} sections.
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.
Augite XPL properties
Isotropy/Anisotropy: Anisotropic
Interference color: Order I-II; bright
Extinction angle: The maximum extinction angle of longitudinal sections varies from 35° to 45°. These sections have the maximum interference colors for the slide. Some varieties have a peculiar concentric wavy extinction known as the hourglass structure. Cross sections have parallel or symmetrical extinction depending upon whether {100} and {010} or {110} predominates.
Twins: Twins with {100} as twin-plane are common; these often appear as twin seams. Polysynthetic twins with {001} as twin-plane are occasionally found. Combined {100} twins with {001} polysynthetic twins give what is known as herringbone structure.
Uniaxial/Biaxial: Biaxial (+)
Optic axial angle (2V): 2V measured: 40 – 52°, 48 – 68°
Augite distinguishing features under the microscope
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  • 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