A mineral group; characterized by double chains of silica tetrahedra having the composition A_0-1B₂Y₅Z₈O₂₂(OH,F,Cl) , where (A=Ca,Na,K,Pb,B), (B=Ca,Fe,Li,Mg,Mn,Na), (Y=Al,Cr,Fe,Mg,Mn,Ti), and (Z=Al,Be,Si,Ti); in the orthorhombic or monoclinic crystal systems, including actinolite, anthophyllite, arfvedsonite, cummingtonite, hornblende, richterite, glaucophane, grunerite, anthophyllite, riebeckite, tremolite, and others. All display a diagnostic prismatic cleavage in two directions parallel to crystal faces and intersecting at angles of about 54 degrees and 124 degrees . Some members may be asbestiform.