Pyroxene Group

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Pyroxenes are formed either direct or by the reaction of olivine with quartz.

  • It has the general formula W(X,Y)Z2O6,
    • where W = Ca, Na
    • X = Mg, Fe+2, Mn, Li
    • Y = Al, Fe+3, Ti
    • Z = Si, Al
  • Pyroxenes are belongs to the Single Chain Ionosilicate Group.
  • Pyroxenes are crystallizes in Orthorhombic system [orthopyroxene], in monoclinic system [clinopyroxene].
  • They have characteristic prismatic cleavage angle 87 and 93 degree.

Structure

Structurally Pyroxenes are single chain inosilicates, where silicate tetrahedra are linked in a single chain. One oxygen of each tetrahedra is shared with the neighboring tetrahedra to form a long chain.

Each chain is separated by cations like iron, magnesium, calcium etc.

The result in Si : O ratio of 1 : 3, and the net charge per silicon atom is (–2). In pyroxene, the one divalent cation (2+) per tetrahedron balances that –2 charge.

All pyroxenes show perfect {110} cleavage.  When viewed looking down the c-crystallographic axis, the cleavages intersect at near 90o angles (the angles are actually 92 – 93o and 87 – 880)

Important minerals –

  • Orthopyroxene –
    • Enstatite – (MgSiO3)
    • Hypersthene – (Mg, Fe)SiO3
    • Ferrosillite – (FeSiO3)
  • Clinopyroxene –
    • Diopside – CaMgSi2O6
    • Hederbergite – CaFeSi2O6
    • Augite – (Ca, Mg, Fe+2, Fe+3, Al)2 (Si, Al)2 O6
    • Pigeonite – (Ca, Mg, Fe)2 Si2O6
  • Other pyroxenes –
    • Aegirine (Acmite) – NaFeSi2O6
    • Jadeite – NaAlSi2O6
    • Spodumene – LiAlSi2O6
    • Johannsenite – CaMnSi2O6

Physical properties –

  • Color – nearly black to green
  • Form – prismatic
  • Streak – colorless or grayish black
  • Lusture – Sub-vitreous, hypresthene shows metallic lusture
  • Cleavage – two set 87 and 93 degree
  • Fracture – uneven
  • Hardness – 5-6.5
  • Sp. Gravity – 3.2-3.5

Optical Properties

Orthopyroxene

  1. Color – Pale green
  2. Habit – subhedral to euhedral
  3. Cleavage – perfect 87-90 degree
  4. Paleochroism – paleochroic [except in enstatite]
  5. RI – high [Biaxial +ve]
  6. Cross nicol – anisotropic
  7. Interference color – 1st order gray
  8. Extinction – parallel

Clinopyroxene

  1. Color – pale green
  2. Habit – subhedral to euhedral
  3. Cleavage – perfect 87-90 degree
  4. Paleochroism – Paleochroic
  5. RI – higher than canada balsam
  6. Cross nicol – anisotropic
  7. Interference color – 2nd order
  8. Extinction – Inclined at 45 degree

Alteration – Orthopyroxene alter to serpentine or fibrous amphibole. Augite alters to chlorite/ hornblende.

Varities –

  • Diallage – Translucent and is fibrous augite.
  • Omphacite – It is a member fo clinopyroxene intermediate between Jadeite and Augite. Foliated diopside, found in Eclogite facies.
  • Kunzite – A gem variety of Spodumene
  • Hiddenite – Emrald green spodumene. gem variety.

Occurances –

  • Opx occurs in basic and ultrabasic rocks as well as high grade metamorphic rocks. It is present in Charnokite and Norite.
  • Cpx occurs in ultrabasic rock such as peridotites and basic rock like gabbro and basalt.
  • In metamorphic Limestones we find diopside, hedenbergite.
  • Aegirine occurs in soda rich igneous rocks such as nepheline syenite.
  • Hypresthene occurs in charnokite.
  • Spodumene foun din pegmatite and is good source of Li.

REFERENCES –

  • https://open.maricopa.edu/physicalgeology/chapter/2-4-silicate-minerals/
  • https://www2.tulane.edu/~sanelson/eens211/inosilicates.htm
  • https://www.alexstrekeisen.it/english/pluto/pyroxene.php
  • https://www.sciencedirect.com/topics/earth-and-planetary-sciences/clinopyroxene

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