- hornblende
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—hornblendic, adj./hawrn"blend'/, n.a dark-green to black mineral of the amphibole group, calcium magnesium iron and hydroxyl aluminosilicate.[1760-70; < G; see HORN, BLENDE]
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Any of a subgroup of amphibole minerals that are calcium-iron-magnesium-rich and monoclinic in crystal structure.Hornblende, whose generalized chemical formula is (Ca,Na)2(Mg,Fe,Al)5(Al,Si)8O22(OH)2, occurs widely in metamorphic and igneous rocks. Common hornblende is dark green to black in colour and usually found in middle-grade metamorphic rocks (formed under medium conditions of temperature and pressure). Such metamorphic rocks with abundant hornblende are called amphibolites.* * *
▪ mineralcalcium-rich amphibole mineral that is 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: 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 hornblende, see amphibole (table).Hornblende occurs widely in metamorphic and igneous rocks. Pargasite occurs in high-grade metamorphic rocks that tend to have more aluminum. Metamorphic rocks with abundant hornblende are called amphibolites (amphibolite).* * *
Universalium. 2010.
