HORNBLENDE

any of a subgroup of amphibole (q.v.) minerals that are calcium-rich and monoclinic in crystal structure. Hornblende's generalized chemical formula is (Ca,Na)2(Mg,Fe,Al)5(Al,Si)8O22 (OH)2. The four end-members and the cation content of their respective compositions are as follows: common hornblende, Ca2(Mg4Al) (Si7Al); tschermakite, Ca2(Mg3Al2)(Si6Al2); edenite, NaCa2(Mg)5(Si7Al); pargasite, NaCa2 (Mg4Al)(Si6Al2). Extensive solid solution occurs, and each end-member has iron-rich equivalents; minor elements, including manganese, titanium, chromium, potassium, fluorine, and yttrium, are usually present. Hornblendes exhibit typical amphibole structures; these are based on double tetrahedral chains between which four metal sites are located. Tetrahedral substitution of aluminum for silicon is common and provides charge balance for trivalent and univalent substitution in the metal sites. For detailed physical properties of common hornblende, see amphibole (table). Hornblende occurs widely in metamorphic and igneous rocks. Common hornblende is usually found in middle-grade metamorphic rocks, whereas tschermakite and pargasite occur in high-grade metamorphic rocks that tend to have more aluminum. Such metamorphic rocks with abundant hornblende are called amphibolites.