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Glauconite thin section

About Glauconite
From Greek for blue, because of its color.
Glauconite hand-specimen
Formula: (K,Na)(Mg,Fe2+,Fe3+)(Fe3+,Al)(Si,Al)4O10(OH)2
System: Monoclinic
Color: Blue-green
Lustre: Dull, Earthy
Hardness: 2
Density: 2.4–2.95
Glauconite #1 thin section (hFOV 2mm)
Glauconite PPL properties
Relief: Moderate positive
Habit/Form: Glauconite forms small pellets or granules in sedimentary rocks, or casts of foraminifera or other microfossils. The pellets and casts usually consist of aggregates of fine, irregular flakes. A variety called celoadonite is essentially the same but forms small radiating masses in vesicles in volcanic rocks.
Color: Green, yellow-green, or olive-green. Brown when altered. Aluminous varieties may be nearly colorless.
Pleochroism: Weak to moderate (from yellow to green).
Cleavage: Very good in one direction {001}; in basal sections (z-axis) have no cleavage.
Glauconite XPL properties
Isotropy/Anisotropy: Anisotropic
Interference color: Order II green to order II red; basal sections are almost isotropic.
Extinction angle: 0 – 3°; basal sections are almost isotropic, without cleavage traces.
Twins: Absent
Uniaxial/Biaxial: Biaxial (-)
Optic axial angle (2V): 2V calculated: 20 – 24°
Glauconite 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