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Chalcedony thin section

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About Chalcedony
From Chalcedon or Calchedon, an ancient maritime city of Bithynia, on the Sea of Marmara, Asia Minor.
Chalcedony hand-specimen
Formula: !SiO2
System: Trigonal (Quartz) + Monoclinic (Moganite)
Color: Colorless, white, gray, blue, any color due to embedded minerals, brown, bronze-yellow
Lustre: Waxy, Dull
Hardness: 6½–7
Density: 2.55–2.66
XPL
XPL
XPL
PPL
PPL
Chalcedony #1 thin section (hFOV 2mm)
XPL
XPL
XPL
PPL
PPL
Chalcedony #2 thin section (hFOV 2mm)
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Chalcedony PPL properties
Relief: Low positive
Habit/Form: The quartz fibers that make up chalcedony are typically aligned in a parallel or spherulitic fashion and have a feathery appearance. Chalcedony may form the cement in clastic sediments or may form colloform or encrusting masses. The fibers are usually elongate perpendicular to the c axis along one of the a crystal axes, and are twisted. Fibers elongate parallel to the c axis also are occasionally observed.
Color: Colorless to pale brown; may be almost opaque due to finely disseminated oxides
Pleochroism:
Cleavage:
Chalcedony XPL properties
Isotropy/Anisotropy: Anisotropic
Interference color: Order I gray and white; some chert may be sensibly isotropic, due to extremely fine grain size.
Extinction angle: Extinction is parallel, but fibers are sometimes twisted causing only sections of the fibers to be extinct at any one time.
Twins: The variety often shows a radial or mosaic structure between crossed polars, where fibrous the extinction is parallel to the length of the fibers.
Uniaxial/Biaxial: Uniaxial (+)
Optic axial angle (2V):
Chalcedony 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