Minerals of pyroxene group

Pyroxene forms an important group of rock minerals. They have a general formula XYSi₂O_6, where X is diavalent caution mainly Ca, Fe, Mg, while Y is monovalent cation like Na, Li and Trivalent caution Al, Fe. In some cases, Al may partly replace silicon. The common pyroxenes are Ca, Mg, and Fe silicates. They are characterized by two cleavages, which intersect almost at a right angle. Pyroxenes are broadly divided into two groups: (i) ortho-pyroxene, and (ii) clinopyroxenes.

3. Orthorhombic Pyroxene/ Orthopyroxenes

These pyroxenes crystallize in the orthorhombic system and contain very little calcium. The ”enstatite” (Mg SiO3) and ”hypersthene” (Mg, Fe)SiO3 belong to this group.

List of Orthorhombic Pyroxene/ Orthpyroxene:

• Enstatite, Mg2Si2O6
• Bronzite, intermediate between enstatite and hypersthene
• Hypersthene, (Mg,Fe)SiO3
• Eulite, intermediate between hypersthene and ferrosilite
• Ferrosilite, Fe2Si2O6
• Donpeacorite, (MgMn)MgSi2O6
• Nchwaningite, Mn²⁺
  2SiO3(OH)2·(H2O)

Physical Properties of Orthopyroxene Group: 

• Crystal system: Orthorhombic.
• Cleavage: Good
• Specific gravity: 3.2 -3.6.
• Luster: Vitreous
• Colours: greyish, yellowish, and greenish.
• Habit: Massive or lamellar.

Occurrence of Orthopyroxene Mineral:

Magnesium-rich orthopyroxenes are common constituents of peridotites,  gabbros, norites, and basalts. They are found associated with augite, olivine and plagioclase. The orthopyroxenes alter to serpentineous products or fibrous amphibole.

Diopside: CaMgSi2O

Hedenbergite: CaFeSi2O6

2. Monoclinic Pyroxene/ Clinopyroxene

These pyroxenes crystallize in the monoclinic system and contain either calcium or Na, Al, Fe (ferric) or Li. ”Diopside”, ”hedenbergite” and ”augite” are the important members of this group.

Augite:  (Ca, Na) (Mg, Fe, Al) (Al, Si)2 O6.

Enstatite: MgSio3

Hypersthen : (Mg, Fe) Sio3

List of Clinopyroxene minerals: 

• Clinopyroxenes (monoclinic)
• Aegirine, NaFe³⁺Si2O6
• Augite, (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6
• Clinoenstatite, MgSiO3
• Diopside, CaMgSi2O6
• Esseneite, CaFe³⁺[AlSiO6]
• Hedenbergite, CaFe²⁺Si2O6
• Jadeite, Na(Al,Fe³⁺)Si2O6
• Jervisite, (Na,Ca,Fe²⁺)(Sc,Mg,Fe²⁺)Si2O6
• Johannsenite, CaMn²⁺Si2O6
• Kanoite, Mn²⁺(Mg,Mn²⁺)Si2O6
• Kosmochlor, NaCrSi2O6
• Namansilite, NaMn³⁺Si2O6
• Natalyite, NaV³⁺Si2O6
• Omphacite, (Ca,Na)(Mg,Fe²⁺,Al)Si2O6
• Petedunnite, Ca(Zn,Mn²⁺,Mg,Fe²⁺)Si2O6
• Pigeonite, (Ca,Mg,Fe)(Mg,Fe)Si2O6
• Spodumene, LiAl(SiO3)2

Physical Properties of Clinophyroxenes:

• Crystal Form: Monoclinic
• Cleavage: at 87° and 93° imperfect cleavage.
• Hardness: 5-6
• Specific Gravity: 3.2 – 3.3
• Luster: Vitreous
• Colour: White to light green in diopside, deepens with increase of iron. Augite is black.
• Habit: Crystal shows square or eight-sided cross-section. Also granular, massive, and lamellar.

Occurrence of Clinophyrxene:

Diopside and hedenbergite occur in metamorphic rocks. Augite occurs in gabbros, basalts, diorites and some syenites.

Classification of Pyroxene on the basis of their chemical composition

On the basis of their chemical composition, Pyroxenes can be classified into magnesium-iron pyroxenes, calcium pyroxenes, calcium-sodium pyroxenes, sodium pyroxenes, and lithium pyroxenes.

Uses of Pyroxene Minerals

Pyroxene has not an industrial level usage, but still some of the common uses of the pyroxene minerals are as under.

• Pyroxene minerals are used as gem materials if they are colourful and attractive. All pyroxenes are not used as gem materials. For instance, Jadeite, diopside, and spodumene are the only pyroxene minerals, which are used as gem materials.
• Some rocks are used as crushed and dimension stones, which have an inculstion of Pyroxeme minerals.
• Lithium can be extracted from a variety of pyroxene namely Spodumene, which consists of lithium aluminium inosilicate, LiAl(SiO₃)₂, and is a source of lithium. The cheapest source of lithium is Spodumene. Lithium can be used in heat-resistant ceramics, medication, and batteries.
• Hiddenite is a variety of pyroxene, which is enriched with Chromium.
• Nephrite and Jade minerals are the varieties of pyroxene which are used in carvings.
• Tourmaline is a Borone bearing mineral, which has some pyroelectric and piezoelectric properties:
  • Pyroelectric: crystal + heat – develops charge:
  • Causes crystals to collect dust in display cases
  • Piezoelectric (they generate an electric charge when mechanic stress is applied on the opposite ends.).