Pyroxene

The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica crystals with the general formula XY(Si,Al)2O6 where X and 
Y represents metal ions. 
Although aluminium substitutes extensively for silicon in silicates such as feldspars and amphiboles, the substitution occurs only to a limited extent in most pyroxenes.
Pyroxenite is an igneous rock made up of minerals from the pyroxene group.