amphiboles

amphiboles A group of minerals possessing double chains of silicon—oxygen [SiO₄] tetrahedra with a composition of [Si₄O₁₁]_n running parallel to the crystallographic axis; i.e. parallel to the prism zone of a crystal. The double chains are held together by monovalent, divalent, or trivalent cations, of which Na⁺, Ca²⁺, Mg²⁺, Fe²⁺, Al³⁺, and Fe³⁺ are the most important; hydroxyl ions also occur. There are three main groups of amphibole minerals: (a) calcium-poor amphiboles with the general formula X₂Y₅[Z₄O₁₁]₂(OH,F)₂, where X = Mg or Fe²⁺, Y = Mg, Fe²⁺, Fe³⁺, Al³⁺, etc., and Z = Si or Al; (b) the calcium-rich amphiboles with the general formula AX₂Y₅[Z₄O₁₁]₂(OH,F)₂, where A = Na, X = Ca, Y = Mg, Fe²⁺, Fe³⁺, Al, etc., and Z = Si or Al; and (c) the alkali amphiboles in which Na > Ca and with the general formula AX₂Y₅[Z₄O₁₁]₂(OH,F)₂, where A = Na or K, X = Na (or Na and Ca), Y = Mg, Fe²⁺, Fe³⁺, Al, etc., and Z = Si or Al. Calcium-poor amphiboles include the orthorhombic amphiboles (called the orthoamphiboles) and include anthophyllite and gedrite, but the other two groups are monoclinic and include the common hornblendes tremolite and actinolite, as well as the sodium-rich varieties such as glaucophane and riebeckite. Amphiboles are common rock-forming silicate minerals that occur in intermediate and alkaline igneous rocks and also in many regional metamorphic rock types.