1. Orthopyroxene By Dominic Papineau

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

3. The physical properties of orthopyroxenes 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 Group: orthopyroxenes are inosilicates (single chain silicates)

4. The chemical properties of orthopyroxenes Orthopyroxenes are solid solutions of MgSiO 3 – (Mg,Fe)SiO 3 Composition: Pure enstatite contains 40.0% MgO and 60.0% SiO 2 Pure ferrosilite (very rare) contains 40.0% FeO and 60.0% SiO 2 Pure ferrosilite is very rare because in most geologically observed temperature and pressure ranges, fayalite (Fe 2 SiO 4 ) and quartz (SiO 2 ) are more stable.

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

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 Space group: Pbca Unit cell parameters: Z = 8

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

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

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

11. Crystallography of enstatite 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 Unit cell Cleavage Enstatite structure (to scale)

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

16. Enstatite chondrites Enstatite chondrite meteorites are characterized by the presence of orthopyroxene, but without olivine, because: (Mg, Fe)SiO 4 MgSiO 3 Fe o 1/2O 2 ++ Their Fe content occurs as a metallic alloy Chondrite meteorites represents the oldest rocks found: from 4.4 to 4.56 billion years old