About Tourmaline
(Mineral group name), from Singhalese turamali, a term originally applied to zircon and other gems by jewelers of Ceylon.
Tourmaline hand-specimen
Formula: (Na,Ca)(Mg,Li,Al,Fe2+)3Al6 (BO3)3Si6O18(OH)4
System: Trigonal
Color: Pale brown to dark-brown to brownish-black, also dark-yellow, blue
Lustre: Vitreous, Resinous
Hardness: 7
Density: 2.9–3.1
System: Trigonal
Color: Pale brown to dark-brown to brownish-black, also dark-yellow, blue
Lustre: Vitreous, Resinous
Hardness: 7
Density: 2.9–3.1
XPL
PPL
Tourmaline #1 thin section (hFOV 2mm)
XPL
PPL
Tourmaline #2 thin section (hFOV 2mm)
XPL
PPL
Tourmaline #3 thin section (hFOV 2mm)
XPL
PPL
Tourmaline #4 thin section (hFOV 2mm)
XPL
PPL
Tourmaline #5 thin section (hFOV 2mm)
XPL
PPL
Tourmaline #6 thin section (hFOV 2mm)
Tourmaline PPL properties
Relief: Moderate positive
Habit/Form: Tourmaline commonly forms euhedral, stubby columnar to acicular crystals that show a rounded triangular or crudely hexagonal cross section. Longitudinal sections are usually roughly rectangular. Acicular crystals may form radiating masses. Also found as anhedral grains or irregular masses.
Color: Schorl: pale yellow, pale brown, yellow-brown.
Dravite: colorless, yellowish, greenish, brownish, pale yellow.
Elbaite: colorless, yellow, olive green, purple, colorless, pink, pale green, pale blue, deep blue.
Tourmaline is often color-zoned.
Pleochroism: Intense
Cleavage: Absent but irregular fractures are common.
Habit/Form: Tourmaline commonly forms euhedral, stubby columnar to acicular crystals that show a rounded triangular or crudely hexagonal cross section. Longitudinal sections are usually roughly rectangular. Acicular crystals may form radiating masses. Also found as anhedral grains or irregular masses.
Color: Schorl: pale yellow, pale brown, yellow-brown.
Dravite: colorless, yellowish, greenish, brownish, pale yellow.
Elbaite: colorless, yellow, olive green, purple, colorless, pink, pale green, pale blue, deep blue.
Tourmaline is often color-zoned.
Pleochroism: Intense
Cleavage: Absent but irregular fractures are common.
Tourmaline XPL properties
Isotropy/Anisotropy: Anisotropic
Interference color: Order II yellow/green to low order III
Extinction angle: Parallel / 0° / straight in longitudial sections. Cross sections remain dark on rotation.
Twins: Absent
Uniaxial/Biaxial: Uniaxial (-)
Optic axial angle (2V): –
Interference color: Order II yellow/green to low order III
Extinction angle: Parallel / 0° / straight in longitudial sections. Cross sections remain dark on rotation.
Twins: Absent
Uniaxial/Biaxial: Uniaxial (-)
Optic axial angle (2V): –
Tourmaline 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
