The Pyroxene Minerals

Pyroxenes are abundant primary minerals in basalt, peridotite and other mafic igneous rocks. Some also are metamorphic minerals in high-grade rocks. Their basic structure is chains of silica tetrahedra with metal ions (cations) in two different sites between the chains. The general pyroxene formula is XYSi₂O₆, where X is Ca, Na, Fe⁺² or Mg and Y is Al, Fe⁺³ or Mg. The calcium-magnesium-iron pyroxenes balance Ca, Mg and Fe in the X and Y roles, and the sodium pyroxenes balance Na with Al or Fe⁺³. The pyroxenoid minerals are also single-chain silicates, but the chains are kinked to fit more difficult cation blends.

Pyroxenes are usually identified in the field by their nearly square, 87°/93° cleavage, as opposed to the similar amphiboles with their 56°/124° cleavage.

Geologists with lab equipment find the pyroxenes rich in information about a rock's history. In the field, usually the most you can do is note dark-green or black minerals with Mohs hardness of 5 or 6 and two good cleavages at right angles and call it "pyroxene." The square cleavage is the main way to tell pyroxenes from amphiboles; pyroxenes also form stubbier crystals.

The last two pages in this gallery are classification diagrams for the pyroxenes.