Hornblende


Hornblende is a complex inosilicate series of minerals. Hornblende is not a recognized mineral, in its own right but the name is used as a general or field term, to refer to a dark amphibole. It is an isomorphous mixture of three molecules; a calcium-iron-magnesium silicate, an aluminium-iron-magnesium silicate and an iron-magnesium silicate. Manganese, titanium and sodium are sometimes present. Fluorine often substitutes for the hydroxyl in the structure. The general formula can be given as (Ca,Na)<sub>2-3</sub>(Mg,Fe,Al)<sub>5</sub>(Al,Si)<sub>8</sub>O<sub>22</sub>(OH,F)<sub>2</sub>. Hornblende has a hardness of 5–6, a specific gravity of 2.9–3.4 and is typically an opaque green, greenish-brown, brown or black colour. Its cleavage angles are at 56 and 124 degrees. It is most often confused with the minerals augite and biotite mica, both of which are black and can be found in granite and in charnockite.