amphiboles

A large group of rock-forming metasilicate minerals. They have a structure of silicate tetrahedra linked to form double endless chains, in contrast to the single chains of the pyroxenes, to which they are closely related. They are present in many igneous and metamorphic rocks. The amphiboles show a wide range of compositional variation but conform to the general formula: X_2–3Y₅Z₈O₂₂(OH)₂, where X=Ca, Na, K, Mg, or Fe²⁺; Y=Mg, Fe²⁺, Fe³⁺, Al, Ti, or Mn; and Z=Si or Al. The hydroxyl ions may be replaced by F, Cl, or O. Most amphiboles are monoclinic, including: cummingtonite, (Mg,Fe²⁺)₇(Si₈O₂₂)(OH)₂; tremolite, Ca₂Mg₅(Si₈O₂₂)(OH,F)₂; actinolite, Ca₂(Mg,Fe²⁺)₅(Si₈O₂₂)(OH,F)₂; hornblende, NaCa₂(Mg,Fe²⁺,Fe³⁺,Al)₅((Si,Al)₈O₂₂)(OH,F)₂; edenite, NaCa₂(Mg,Fe²⁺)₅(Si₇AlO₂₂)(OH,F)₂; and riebeckite, Na₂,Fe₃ ²⁺(Si₈O₂₂)(OH,F)₂. Anthophyllite, (Mg,Fe²⁺)₇(Si₈O₂₂)(OH,F)₂, and gedrite, (Mg,Fe²⁺)₆Al(Si,Al)₈O₂₂)(OH,F)₂, are orthorhombic amphiboles.