Pyroxene
The name pyroxene comes from Ancient Greek words meaning fire and stranger. Early geologists found these crystals embedded in volcanic glass during eruptions. They assumed the minerals were unwanted impurities trapped inside the molten rock. This misunderstanding led to a label that translates roughly to fire stranger. Scientists later realized these minerals formed early before lava erupted into the air. The crystals are simply early-forming minerals rather than accidental intruders. Modern researchers now understand their role in igneous and metamorphic rocks.
Silica tetrahedra form parallel chains within every pyroxene crystal structure. Each silicon ion sits surrounded by four oxygen ions creating a tetrahedron shape. Two of those oxygen ions connect to neighboring silicon atoms along the chain. All tetrahedra point in the same direction throughout the mineral lattice. Oxygen ions on the narrower face become apical oxygen ions. Pairs of single chains bind together on their apical sides using Y cations. These pairs resemble I-beams in engineering terminology. Additional X cations bond the outer faces of these beams to neighbors. This binding remains relatively weak compared to other silicate structures. The weakness creates characteristic cleavage planes where the mineral splits easily.
Twenty mineral names exist today according to the International Mineralogical Association. One hundred five previous names have been discarded over time. Cations occupy specific sites labeled X, Y, or T within the crystal. Silicon fills the tetrahedral site first followed by aluminum and iron(III). Magnesium calcium and iron dominate both the X and Y positions. Sodium enters the structure with a charge of 1 plus requiring extra positive charges elsewhere. Jadeite forms when sodium combines with aluminum on the Y site. Omphacite appears when sodium pyroxenes contain more than 20 mol percent calcium magnesium or iron(II). Coupled substitutions allow different ion combinations to maintain electrical neutrality. The Tschermak substitution places a 3 plus ion into both the Y and T sites simultaneously.
Pyroxenes split into two main groups based on crystal symmetry. Clinopyroxenes crystallize in the monoclinic system while orthopyroxenes form orthorhombic crystals. Enstatite exists as three polymorphs including orthoenstatite protoenstatite and clinoenstatite. Increasing calcium content prevents formation of orthorhobic phases like pigeonite. Pigeonite only crystallizes in the monoclinic system regardless of other factors. A miscibility gap separates pigeonite from augite compositions between 15 and 25 mol percent calcium. No complete solid solution exists for calcium contents exceeding 45 mol percent. The divide between augite and diopside-hedenbergite occurs at greater than 45 mol percent calcium. Pyroxene with over 50 mol percent calcium remains impossible due to structural constraints.
The upper mantle of Earth contains mainly olivine and pyroxene minerals. Basalt andesite and gabbro rocks consist largely of pyroxene and feldspar. Green peridot olivine dominates xenoliths found within basalt host rocks at San Carlos Indian Reservation. Black orthopyroxene crystals appear alongside rare grass-green diopside grains in these samples. Meteorite ALH84001 from Mars consists mostly of pyroxene minerals. Curiosity rover analysis revealed pyroxenes alongside feldspar and olivine in Martian soil. These minerals form essential components of planetary crusts throughout the solar system. Their abundance makes them critical subjects for understanding geological history.
Common questions
What is the origin of the name pyroxene?
The name pyroxene comes from Ancient Greek words meaning fire and stranger. Early geologists found these crystals embedded in volcanic glass during eruptions and assumed they were unwanted impurities trapped inside molten rock.
How do silica tetrahedra form within pyroxene crystal structures?
Silica tetrahedra form parallel chains where each silicon ion sits surrounded by four oxygen ions creating a tetrahedron shape. Two of those oxygen ions connect to neighboring silicon atoms along the chain while all tetrahedra point in the same direction throughout the mineral lattice.
Which minerals are classified as pyroxenes today according to the International Mineralogical Association?
Twenty mineral names exist today according to the International Mineralogical Association while one hundred five previous names have been discarded over time. Jadeite forms when sodium combines with aluminum on the Y site and omphacite appears when sodium pyroxenes contain more than 20 mol percent calcium magnesium or iron(II).
What distinguishes clinopyroxenes from orthopyroxenes based on crystal symmetry?
Clinopyroxenes crystallize in the monoclinic system while orthopyroxenes form orthorhombic crystals. Enstatite exists as three polymorphs including orthoenstatite protoenstatite and clinoenstatite and pigeonite only crystallizes in the monoclinic system regardless of other factors.
Where do pyroxene minerals occur within Earth and Mars geological formations?
The upper mantle of Earth contains mainly olivine and pyroxene minerals while basalt andesite and gabbro rocks consist largely of pyroxene and feldspar. Meteorite ALH84001 from Mars consists mostly of pyroxene minerals and Curiosity rover analysis revealed pyroxenes alongside feldspar and olivine in Martian soil.