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Orthopyroxene

Orthopyroxene. By Dominic Papineau. The varieties of orthopyroxene. The enstatite series of isomorphs. Enstatite . Clinoenstatite. Bronzite. Hyperstene. Ferrohyperstene. Eulite. Orthoferrosilite. The physical properties of orthopyroxenes.

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Orthopyroxene

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  1. Orthopyroxene By Dominic Papineau

  2. The varieties of orthopyroxene The enstatite series of isomorphs Enstatite Clinoenstatite Bronzite Hyperstene Ferrohyperstene Eulite Orthoferrosilite

  3. The physical properties of orthopyroxenes Group: orthopyroxenes are inosilicates (single chain silicates) Luster: vitreous to pearly on cleavage faces Cleavage: good Hardness: 5.5 - 6 Specific gravity: 3.2 – 3.6 increasing with Fe content Crystal habit: massive, fibrous, or lamellar

  4. The chemical properties of orthopyroxenes Orthopyroxenes are solid solutions of MgSiO3 – (Mg,Fe)SiO3 Composition: Pure enstatite contains 40.0% MgO and 60.0% SiO2 Pure ferrosilite (very rare) contains 40.0% FeO and 60.0% SiO2 Pure ferrosilite is very rare because in most geologically observed temperature and pressure ranges, fayalite (Fe2SiO4) and quartz (SiO2) are more stable.

  5. The optical properties of orthopyroxene orthopyroxene is biaxial negative or positive 2VZ = 50-132° •  = 1.649 – 1.768 •  = 1.653 – 1.770 • = 1.657 – 1.788 • = 0.007 – 0.020 (usually first order yellow or lower interference colors) Weak pleochroism May be twinned

  6. Relation between the physical, chemical and optical properties of orthopyroxenes

  7. The crystallographic properties of orthopyroxene Crystal system: orthorhombic Point Group: 2/m 2/m 2/m Unit cell parameters: Z = 8 Space group: Pbca

  8. a b a b c c Enstatite and ferrosilite: the end members Silicon tetrahedron Magnesium polyhedron Oxygen atom Enstatite Silicon tetrahedron Iron polyhedron Oxygen atom Ferrosilite

  9. c b a The crystallographic properties of enstatite Crystal system: orthorhombic Point Group: 2/m 2/m 2/m Unit cell parameters: a = 18.22 Å b = 8.81 Å c = 5.17Å Z = 8 Space group: Pbca Magnesium polyhedron Silicon tetrahedron Oxygen atom

  10. a b c The crystallographic properties of ferrosilite Crystal system: orthorhombic Point Group: 2/m 2/m 2/m Unit cell parameters: a = 18.41 Å b = 9.07 Å c = 5.24Å Z = 8 Space group: Pbca Iron polyhedron Silicon tetrahedron Oxygen atom

  11. Crystallography of enstatite Cleavage Unit cell Enstatite structure (to scale) This is a high resolution TEM image of enstatite along the c-axis The white regions is the area between the M2 sites in the structure

  12. The thermodynamic properties of orthopyroxenes Thermal expansion of the three crystallographic axes of B1-9 orthopyroxene

  13. More thermodynamic properties of orthopyroxenes Percentage variation of mean interatomic distance with temperature M1-O distance (Mg) M2-O distance (Fe) Si-O distance Corrected for riding motion Corrected for noncorrelated thermal motion

  14. Metamorphic rocks Occurrences of Orthopyroxenes Igneous rocks In intrusive rocks like gabbro, norite, peridotite, diorite, and syenite, but also in extrusive rocks like basalts. In high temperature metamorphic rocks (granulite facies) Associated minerals include clinopyroxenes, amphiboles, olivine, feldspars, garnets, etc.

  15. Locations on Earth: Extraterrestrial locations: Occurrences of orthopyroxenes United States (NY, Co, NC, Pa, Md, Az), Canada (Labrador), India, Sri Lanka, Finland, Tanzania, etc. Iron and stony iron meteorite, and thus presumably on asteroids (very common) On the Moon (very common) On Mars (apparently very common) From the Kapoeta meteorite From lunar highland Apollo samples From Pathfinder analysis

  16. (Mg, Fe)SiO4 MgSiO3 1/2O2 Feo + + Enstatite chondrites Chondrite meteorites represents the oldest rocks found: from 4.4 to 4.56 billion years old Enstatite chondrite meteorites are characterized by the presence of orthopyroxene, but without olivine, because: Their Fe content occurs as a metallic alloy

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