Hypersthene

Hypersthene (Gr. hyper, 'above,' sthenos, 'strength'; so called to distinguish it from Hornblende, q.v., with which it was formerly confounded), a rock-forming mineral which crystallises in orthorhombic forms. It is an anhydrous magnesian silicate, containing a large percentage (15-24) of ferrous oxide with very little alumina. It is generally dark green or raven-black in colour, but has a pearly or metallic lustre when fractured across the cleavage-planes. This is due to the presence of very numerous minute brown scales of some foreign substance, which are arranged in lines along these planes. This mineral occasionally occurs massive, like hornblende, as in the island of St Paul on the Labrador coast. It is met with also as an occasional constituent of some eruptive igneous rocks, as in certain andesites and porphyrites, and in plutonic rocks, such as gabbro.