HORNBLENDE is a mem ber of the amphibole (q.v.) group of minerals. The name is now re stricted to the sesquioxide-bear ing members of the monoclinic amphiboles, hornblende occupying the same position in the amphi bole group that augite occupies in the pyroxene group. The horn blendes have a complex and vari able composition, their constitu tion being at present unsatisfac torily elucidated. In addition to the dominant molecules, they contain -- - variable amounts of and Na,O, the last probably as NaAI(SiO3),,, but the man ner in which the excess sesquioxides enter into their constitution is undecided. The crystals are usually prismatic with well-de veloped cleavages (no-no) at 56°. The colour is grey, green, brown or black according to the composition. Coloured varieties are usually strongly pleochroic. Some hornblendes contain sig nificant percentages of titanium—as in basaltic hornblende, while others rich in soda are known as alkali-amphiboles (see AMPHI