Hornblende

Monoclinic  Images    
   
   
  Formula (Na,K)0-1Ca2(Mg,Fe2+Al,Fe3+)5(Si6-7.5Al2-0.5O22(OH)2
  Optic class & sign Biaxial positive or negative
  Optical orientation X near a, Y = b, Z near c
  Optical plane (010)
  Relief Moderate to high
  Refractive indices nx = 1.611 -1.705
ny = 1.615 -1.729
nz = 1.630 -1.730
n increases with increasing Fe; substitution of F for OH decreases n. Oxidation increases refractive indices up to max values of nX = 1.72, nY = 1.78, nZ = 1.80 for oxyhornblende.
  Birefringence   (max.)0.015  - 0.034
   Δn increases with increasing Fe; oxidation increases Δn up to a max value of 0.083 for oxyhornblende. In strongly coloured varieties, interference colours may be masked by mineral colour.
  Optic Angle 2Vx = 10 - 90°
2Vz = 90 - 56°
  Sign of elongation Length-slow, l (+)
  Interference figure Highly variable 2V across the compositional range. Interference colours may be masked in intensely coloured hornblende varieties. Most hornblende compositions are optically negative; exceptions are Mg-rich pargasite and tschermakite.
  Colour /   pleochroism Commonly distinctly coloured and pleochroic in shades of green, blue green or brown
Pleochroism: X light green to pale yellow, Y green, yellow-green or yellow-brown to brown, Z dark green to blue-green, or brown to red-brown; Absorption Z ≥ Y > X. Oxyhornblende is brown and shows strong pleochroism and absorption Z > Y > X.
  Zoning 

  Form  Habit Columnar, long-prismatic, granular; 4-sided diamond-shaped or 6-sided cross-sections Ʇ c; also as irregular-shaped reaction rims; resorption effects are common in oxyhornblende
  Surface Euhedral to anhedral
  Cleavage 2 sets {110} perfect, at 124 and 56° (seen in sections Ʇ c). In prismatic sections, the traces of the two principal cleavage sets are parallel. Parting on {100} and {001} may occur.
  Twinning Simple or multiple on {100}
  Extinction Inclined in prismatic sections, cɅZ = 12 - 34°; close to zero in oxyhornblende; symmetrical to {110} cleavage and prism faces, straight to {010} faces in sections Ʇ c.

  Reaction textures  Late-magmatic or retrograde-metamorphic replacement of pyroxenes by hornblende
  Alteration /   decomposition Chlorite, biotite; oxyhornblende: aggregates of pyroxene, biotite, plagioclase and Fe-oxides

  Occurence     Ign Intermediate to felsic plutonic and volcanic rocks; late magmatic in basic plutonic rocks; ultramafic rocks. Oxyhornblende phenocrysts in volcanic and sub-volcanic rocks such as trachyte, basalt, andesite, latite.
  Met Medium- to high-grade metabasites, major component in amphibolites; calcic schists and gneisses, calcsilicate rocks, skarn, impure marble
  Sed 
  Hyd 
  Other 

  Distinctive   properties               Habit, including characteristic cross-sections ⊥ c, either diamond-shaped or six-sided combination of {110} and {010} faces; characteristic amphibole cleavage, colour and pleochroism, Δn, n, small extinction angle cɅZ.
The large compositional variety as well as a considerable variation of optical properties prevents the determination of hornblende composition based on optical properties alone. The hornblende group entails the Mg-Fe series magnesio-hornblende – ferro-hornblende, tschermakite – ferro-tschermakite, edenite – ferro-edenite, pargasite – ferro-pargasite, magnesio-hastingsite – hastingsite, as well as a tremolite – ferro-actinolite component. The special significance of the latter series in low-grade metabasites, ultramafic rocks and calcareous rocks warrants a separate consideration, though.
nz - Δn diagram: The fields shown for the various solid solution series of the hornblende group are based on Tables 11 to 15 in Deer, Howie & Zussman, Vol. 2B (1997). The Mg-Fe hornblende, tschermakite and pargasite series overlap strongly and cannot be shown as separate fields. For clarity, no nx data have been plotted.
  Additional   comments Oxyhornblende (Na,K)0-1Ca2(Mg,Fe2+,Fe3+,Al,Ti)5(Si,Al)8O22(O,OH)2 is not a recognised mineral name, but the term has its merits in microscopic petrography. Optical properties are transitional to common hornblende, depending on the extent of Fe oxidation and O replacing OH for charge balance.