Hornblende is a member of the amphibole group of more complex silicates, in which the tetrahedra are linked to form a continuous chain twice the width of the pyroxene chains. For this reason they are often referred to as double chain silicates. Like the pyroxenes, they can be subdivided into those with orthorhombic symmetry (orthoamphiboles) and those with monoclinic symmetry (clinoamphiboles). Hornblende is the most common of the clinoamphiboles.
Hornblende is commonly found in metamorphic rocks such as schists and gneisses, and igneous rocks such as diorites and dacites.
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| hornblende |
Chemical composition - Ca 2(Mg, Fe, Al) 5(Al, Si) 8O 22(OH) 2
Hardness - 5-6
Specific gravity - 2.9-3.4
Transparency - Opaque
Colour - Dark green to black
Streak - Dark green
Lustre - Vitreous
Cleavage/fracture - Imperfect in two directions at 56° and 124° / uneven
Crystal habit/mode of occurrence - Prismatic, acicular, fibrous / massive, granular
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