Amphibole (hornblende)

(Na,K)_0-1Ca₂(Mg,Fe²⁺,Fe³⁺,Al)₅Si_6-7.5Al_2-0.5O₂₂(OH)₂

Chain silicate

Crystal system: monoclinic (+) (-)

Colour in hand specimen: black 
Cleavage: two at ~56° 
Hardness: 5-6

Colour in plane polarised light: pale green, green, light yellow-brown to brown 
Pleochroism: yes 
Relief: moderate 
Birefringence: ~0.02; 2^nd order purple 
Extinction: inclined 
Twinning: simple, lamellar, common

M210 Amphibole, hornblende 
Bilen, Bohemia 
Single, black prismatic crystal. 
At the ends of the crystal two sets of cleavage planes are visible, intersecting at ~56°

M211 Amphibolite 
Isle of Coll, Inner Hebrides 
A metamorphic rock consisting almost entirely of crystals of the amphibole hornblende 
Black 
Brilliant lustre 
Two sets of cleavage planes intersect at ~56°

L102 Hornblende biotite granite 
L120 Granodiorite

The amphibole group encompasses a wide range of compositions. Even the hornblendes, which are a sub-group of the amphiboles, encompass a range of compositions (see mineral formula, above). The properties above refer to hornblende.