About Spinel
Apparently from the Latin spinella (= little thorn), in allusion to the spine-shaped octahedral crystals.
Spinel hand-specimen
Formula: MgAl2O4
System: Cubic (Isometric)
Color: Black, blue, red, violet
Lustre: Vitreous
Hardness: 7½–8
Density: 3.6–4.1
System: Cubic (Isometric)
Color: Black, blue, red, violet
Lustre: Vitreous
Hardness: 7½–8
Density: 3.6–4.1
XPL
PPL
Spinel #1 thin section (hFOV 2mm)
XPL
PPL
Spinel #2 thin section (hFOV 2mm)
![[thumb]](http://microckscopic.ro/wp-content/uploads/spinel_1_04-1-150x150.jpg)
![[thumb]](http://microckscopic.ro/wp-content/uploads/spinel_5_01-1-150x150.jpg)
Spinel PPL properties
Relief: High positive
Habit/Form: Crystals are usually octahedrons yielding triangular, square, or diamond-shaped cross sections in thin section. Subequant anhedral grains also are common.
Color: Colorless, pale gray, pale gray-green, pale green, pale blue, pale pink
Pleochroism: Anomalous pleochroism is sometimes observed.
Cleavage: Absent
Habit/Form: Crystals are usually octahedrons yielding triangular, square, or diamond-shaped cross sections in thin section. Subequant anhedral grains also are common.
Color: Colorless, pale gray, pale gray-green, pale green, pale blue, pale pink
Pleochroism: Anomalous pleochroism is sometimes observed.
Cleavage: Absent
Spinel XPL properties
Isotropy/Anisotropy: Isotropic
Interference color: It is one of the few isometric minerals that is invariably isotropic.
Extinction angle: –
Twins: Twinning according to the spinel law with {111} as twin-plane is rather common, but it does not usually show in the slide.
Uniaxial/Biaxial: Isotropic
Optic axial angle (2V): –
Interference color: It is one of the few isometric minerals that is invariably isotropic.
Extinction angle: –
Twins: Twinning according to the spinel law with {111} as twin-plane is rather common, but it does not usually show in the slide.
Uniaxial/Biaxial: Isotropic
Optic axial angle (2V): –
Spinel 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
