Pyroxene
Pyroxene (abbreviated to Px) is a common silicate mineral found in meteorites and made typically of five elements magnesium (Mg), iron (Fe), calcium (Ca), silicon (Si) and oxygen (O) – with the chemical formula for meteorites (Mg,Fe,Ca)₂Si₂O₆.
Basic properties of pyroxenes
Pyroxene is a major group of rock-forming silicate minerals very common in both Earth’s mantle and many meteorites. They are built from single chains of SiO₄ tetrahedra (unlike olivine’s isolated tetrahedra). This chain structure gives their name inosilicates. The general formula XY(Si,Al)₂O₆ where:
- X = calcium, sodium, iron, or magnesium
- Y = magnesium, iron, aluminum, or titanium
Pyroxenes form a solid solution with the most important pyroxene end-members:
- Enstatite (MgSiO₃) → magnesium-rich pyroxene
- Ferrosilite (FeSiO₃) → iron-rich pyroxene
- Diopside (CaMgSi₂O₆) → calcium-magnesium pyroxene
- Hedenbergite (CaFeSi₂O₆) → calcium-iron pyroxene
Appearance
- Usually dark green to black in igneous rocks.
- Hardness: ~5–7 on Mohs scale.
- Two good cleavages at nearly 90°, which helps distinguish them from amphiboles.
Importance in geology & meteoritics
- Earth: Pyroxenes are abundant in basalts, gabbros, and peridotites — key rocks of the crust and mantle.
- Meteorites: Found in many stony meteorites (especially chondrites and achondrites) and used to infer cooling history and parent-body processes.
- Planetary science: Detection of pyroxenes on the Moon, Mars, and asteroids (via spectroscopy) helps identify crust/mantle composition.
A simple comparison chart: olivine vs. pyroxene
| Feature | Olivine | Pyroxene |
|---|---|---|
| Chemical formula | (Mg,Fe)₂SiO₄ | XY(Si,Al)₂O₆ |
| Key elements | Mg, Fe, Si, O | Mg, Fe, Ca, Al, Si, O |
| Silicate type | Nesosilicate (isolated SiO₄ tetrahedra) | Inosilicate (single chains of SiO₄ tetrahedra) |
| Crystal system | Orthorhombic | Mostly monoclinic, some orthorhombic |
| Common end-members | Forsterite (Mg₂SiO₄), Fayalite (Fe₂SiO₄) | Enstatite (MgSiO₃), Ferrosilite (FeSiO₃), Diopside (CaMgSi₂O₆), Hedenbergite (CaFeSi₂O₆) |
| Color | Olive-green, yellow to brown | Dark green, brown, black |
| Cleavage | Poor (not prominent) | Two directions at ~90° (distinct) |
| Occurrence on Earth | Mantle rocks (peridotite), basalts, gabbros | Basalts, gabbros, peridotites |
| Occurrence in meteorites | Chondrites, pallasites | Chondrites, achondrites |
| Gem variety | Peridot | (Not typically used as gems) |