Hornblende Ca2(Mg,Fe,Al)5(OH)2[(Si,Al)4O11]2
Optical Properties:
Biaxial negative; high 2V (50-85 degrees); strong pleochroism
in greens and browns; 2 cleavages at 56 degrees; moderate birefringence
(second order colors, often masked by the mineral's strong color. Hornblende's
strong pleochroism and inclined extinction are its most distinctive characteristics.
Occurrence:
Hornblende is common in many intrusive igneous rocks ranging from granite
to gabbro; it is also present in medium grade metamorphosed mafic rocks.
Hornblende occurs in some volcanic rocks, but it is less abundant in extrusive
rocks than in intrusive rocks.
Alteration:
Hornblende commonly alters to biotite
or chlorite.
The photographs above show hornblende in crossed polars (left) and in plane
polarized light (right photo). The black grains are an opaque mineral (magnetite).
The dark reddish brown mineral in the left photo is biotite.
The photos above (left in plane light, right under crossed polarizers) show
dark hornblende in the upper left corner, and dark biotite
in the lower right, with plagioclase in a
diorite.