pyroxene

pyroxene
Robert M. Lavinsky · CC BY-SA 3.0 · source
Name	Pyroxene
Category	Inosilicate
Formula	(Ca,Mg,Fe,Na)(Mg,Fe,Al)(Si,Al)2O6
Crystal system	Monoclinic or Orthorhombic
Color	Green, brown, black, colorless
Habit	Prismatic, stubby crystals, granular, massive
Cleavage	Two nearly at 90°
Fracture	Uneven to conchoidal
Hardness	5–6 (Mohs)
Luster	Vitreous to dull
Streak	White to gray
Gravity	3.2–3.8

pyroxene Pyroxenes are a major group of inosilicate minerals that form essential components of mafic and ultramafic rocks and occur in a wide range of igneous, metamorphic, and planetary materials. They play a central role in petrology, geochemistry, and planetary science by recording magmatic differentiation, mantle processes, and thermal histories. Studies of pyroxene inform models of mantle convection, magma genesis, and planetary differentiation, linking field investigations with experimental petrology and geophysical observations.

Introduction

Pyroxenes occur worldwide in igneous provinces such as the Deccan Traps, Siberian Traps, Columbia River Basalt Group, and the Ontong Java Plateau, and in metamorphic terrains like the Himalaya, Alps, Appalachian Mountains, and Scandinavian Caledonides. They are also abundant in extraterrestrial materials recovered from the Moon, Mars, Eagle Station meteorite, and many Lunar meteorite finds, and are studied in context with institutions such as the Smithsonian Institution, Natural History Museum, London, and California Institute of Technology. Pyroxene-bearing lithologies are key targets in programs led by agencies including NASA, European Space Agency, Japan Aerospace Exploration Agency, and Russian Federal Space Agency.

Composition and Crystal Structure

The pyroxene structure is built from single chains of silica tetrahedra, with cation sites occupied by calcium, magnesium, iron, aluminum, and sodium—details investigated in experimental work at laboratories like Carnegie Institution for Science, Geological Survey of Canada, Scripps Institution of Oceanography, and Lamont–Doherty Earth Observatory. Crystallographically, pyroxenes adopt monoclinic clinopyroxene or orthorhombic orthopyroxene symmetries; notable endmembers include Enstatite, Diopside, Hedenbergite, Augite, Jadeite, and Aegirine. Substitution mechanisms such as coupled Na+Al for Ca+Si and Fe^3+–Mg exchange are central to thermodynamic models developed at University of California, Berkeley, Massachusetts Institute of Technology, and ETH Zurich. High-pressure phase transitions relevant to the lower mantle have been explored in connection with Wadati–Benioff zone studies and shock experiments at facilities like the Diamond Light Source and Advanced Photon Source.

Classification and Mineral Groups

Pyroxenes are classified into orthopyroxene and clinopyroxene series, with further divisions including the diopside-hedenbergite series, enstatite-ferrosilite series, and jadeite-aegirine series—taxonomy refined by committees such as the International Mineralogical Association and researchers from the British Geological Survey. Type localities and historical descriptions link to collectors and institutions like the Russell Society, Natural History Museum, London, and pioneering geologists such as Gustav Rose, James Dwight Dana, and William Nicol. Mineralogists use nomenclature schemes appearing in monographs from publishers including Cambridge University Press, Springer Nature, and Elsevier.

Physical and Optical Properties

Pyroxenes exhibit prismatic habit and two prominent cleavages near 90°, with hardness around 5–6 and densities that vary with composition—properties cataloged in databases maintained by the United States Geological Survey and the Mineralogical Society of America. Optical characteristics such as birefringence, pleochroism, and extinction angles are used in thin-section petrography at institutions like University of Oxford, University of Cambridge, and Imperial College London. Spectroscopic signatures across visible, infrared, and Mössbauer spectra have been exploited by teams involved with missions such as Mars Reconnaissance Orbiter, Viking program, Lunar Reconnaissance Orbiter, and analytical facilities including Max Planck Institute for Mineralogy.

Occurrence and Geological Significance

Pyroxenes are dominant in peridotites, gabbros, basalts, and many granulites, linking them to tectonic settings like mid-ocean ridges (Mid-Atlantic Ridge), island arcs (Izu–Bonin–Mariana Arc), continental flood basalts, and ophiolites such as the Semail Ophiolite. Their presence in kimberlites informs diamond exploration led by companies and institutions like De Beers Group and the GIA, and mantle xenolith studies at locations like Oman, Kaapvaal Craton, and the North American Craton illuminate lithospheric evolution explored by researchers at Columbia University and University of Texas at Austin.

Formation and Paragenesis

Experimental petrology, including work at Woods Hole Oceanographic Institution and University of Tokyo, constrains pyroxene crystallization from basaltic to ultramafic melts, polymorphic reactions during prograde metamorphism in orogenic belts like the Zagros Mountains and Andes, and reaction rims in contact metamorphism adjacent to plutons such as those in the Sierra Nevada (United States). Reaction textures with olivine, plagioclase, amphibole, and spinel record pressure–temperature paths interpreted using geothermobarometry protocols developed by research groups at Arizona State University and University of Melbourne.

Industrial Uses and Economic Importance

Pyroxene-bearing rocks are sources of commodities and strategic minerals, influencing exploration by firms such as BHP, Rio Tinto, and Anglo American. Clinopyroxene compositions can host chromium and nickel ores in cumulate deposits like the Bushveld Complex and Norilsk-Talnakh province; jadeite-bearing pyroxenes underpin gemstone markets centered in regions including Myanmar and trade hubs such as Hong Kong. Pyroxene studies support construction material assessment, ceramic raw-material evaluation in industries connected with Siemens and BASF, and planetary resource assessments for programs like NASA Artemis.

Category:Minerals