<P> Amphiboles crystallize into two crystal systems , monoclinic and orthorhombic . In chemical composition and general characteristics they are similar to the pyroxenes . The chief differences from pyroxenes are that ( i ) amphiboles contain essential hydroxyl ( OH ) or halogen ( F , Cl ) and ( ii ) the basic structure is a double chain of tetrahedra ( as opposed to the single chain structure of pyroxene ) . Most apparent , in hand specimens , is that amphiboles form oblique cleavage planes ( at around 120 degrees ) , whereas pyroxenes have cleavage angles of approximately 90 degrees . Amphiboles are also specifically less dense than the corresponding pyroxenes . In optical characteristics , many amphiboles are distinguished by their stronger pleochroism and by the smaller angle of extinction ( Z angle c ) on the plane of symmetry . Amphiboles are the primary constituent of amphibolites . </P>
<P> Amphiboles crystallize into two crystal systems , monoclinic and orthorhombic . In chemical composition and general characteristics they are similar to the pyroxenes . The chief differences from pyroxenes are that ( i ) amphiboles contain essential hydroxyl ( OH ) or halogen ( F , Cl ) and ( ii ) the basic structure is a double chain of tetrahedra ( as opposed to the single chain structure of pyroxene ) . Most apparent , in hand specimens , is that amphiboles form oblique cleavage planes ( at around 120 degrees ) , whereas pyroxenes have cleavage angles of approximately 90 degrees . Amphiboles are also specifically less dense than the corresponding pyroxenes . In optical characteristics , many amphiboles are distinguished by their stronger pleochroism and by the smaller angle of extinction ( Z angle c ) on the plane of symmetry . Amphiboles are the primary constituent of amphibolites . </P>
the most common member of the amphibole group
the most common member of the amphibole group
Amphiboles
Amphiboles