HORNBLENDE

General Formula:
(Na,K)0-1Ca2(Mg,Fe2+,Fe3+,Al)5(Si,Al)8O22(OH)2
Sample: AUPI-37 System:
Monoclinic
Euhedral Hornblende Phenocrysts
Numerous hornblende phenocrysts, exhibiting a range of pleochroic colours, euhedral shapes and two cleavages which intersect at 56-124°:.
Field of View = 2.7 mm, plane light
Euhedral Hornblende Phenocrysts
Note that the interference colour exhibited by the individual hornblende grains is masked to some degree by the dark colour of the grain.
Field of view = 2.7 mm, crossed polars
Block diagram showing the relationship between the crystallographic axes and the indicatrix axes.

Optical Properties

Colour
Pleochroism
distinctly coloured, shades of green, yellow-green, blue-green and brown
X = light yellow, light yellow green, light blue green
Y = green, yellow green, gray-green, brown
Z = dark green, dark blue-green, dark gray-green, dark brown
Form found as slender prismatic to bladed crystals, with a 4 or 6 sided cross section which exhibit amphibole cleavage at 56 and 124°, also as anhedral irregular grains
Relief
RI
moderate to high
nalpha = 1.60-1.70
nbeta = 1.61-1.71
ngamma = 1.62-1.73
Cleavage amphibole cleavages on {110} intersect at 56-124° fragment shape is controlled by cleavage
Birefringence
Interference Colours
0.014-0.034
usually upper first or lower second order, but may be masked by mineral colour
Twinning simple and lamellar twins on {100} are not uncommon
Interference Figure
Optic Sign
2VX
biaxial
positive or negative
35 - 130°
Optic Orientation X^a = +3 to -19°, Y = b, Z^c = +12 to +34°, optic plane = (010)
basal sections exhibit symmetrical extinction with the slow ray parallel to the long diagonal between the cleavages, longitudinal sections are length slow
Composition exhibits a wide range of compositions Alteration may be altered to biotite, chlorite or other Fe-Mg silicates
Occurrence common mineral found in a variety of geological environments, i.e. in igneous, metamorphic and sedimentary rocks Distinguishing Features cleavage and grain shape, inclined extinction, pleochroism


Back to: