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Hornblende thin section

About Hornblende
From German Horn (= hornblenden (= to blind or deceive); and old German word for any dark prismatic mineral occurring in ores but containing no recoverable metal.
Hornblende hand-specimen
Formula: (Ca,Na,K)2-3(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2
System: Monoclinic
Color: Dark green to black, greenish-brown, more rarely lighter green
Lustre: Vitreous
Hardness: 5–6
Density: 3.10–3.36
Hornblende #1 thin section (hFOV 2mm)
Hornblende #2 thin section (hFOV 2mm)
Hornblende PPL properties
Relief: High positive
Habit/Form: Commonly found as slender prismatic to bladed crystals with a diamon-shaped cross section showing the amphibole cleavage at 56° and 124°. Also found as anhedral to strongly irregular grains, which in some cases may poikilitically enclose associated minerals. Large grains or fine fibrous masses of hornblende may mantle or replace pyroxene.
Color: Green or brown of various tones
Pleochroism: Very intense to absent
Cleavage: Good in two directions {110} – in two directions are at nearly obtuse angles (124° with 56°), and in one direction for longitudinal section. Sections parallel to (100) which display no cleavage since both cleavages are at too acute an angle to the section to be visible.
Hornblende XPL properties
Isotropy/Anisotropy: Anisotropic
Interference color: Order II green; bluish near extinction.
Extinction angle: The maximum extinction angle in longitudinal sections varies from about 12° to about 34°. In cross sections the extinction is symmetrical to the outlines or to cleavage traces.
Twins: Simple and lamellar twins on {100}
Uniaxial/Biaxial: Biaxial (+/-)
Optic axial angle (2V): 2V measured: 12 – 76°, calculated 30 – 62°
Hornblende 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