(Na,K)0-1Ca2(Mg,Fe2+,Fe3+,Al)5Si6-7.5Al2-0.5O22(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; 2nd 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°
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.
(Na,K)0-1Ca2(Mg,Fe2+,Fe3+,Al)5Si6-7.5Al2-0.5O22(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; 2nd 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°
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.