Here are your brief notes on Pyroxenes

These rocks forming silicates contain the Si₂0₆ single chain structure (inosilicates). These are anhydrous silicates of Mg and Fe and thus are predominantly found in ferromagnesian rocks, i.e., in basic and ultra basic rocks. The following minerals have been included in the pyroxene group:

1. Orthorhombic:

(a) Enstatite

(b) Bronzite

(c) Hypersthene

(d) Ferrohypersthene

(e) Eulite

(f) Ferrosilite

(A) 2. Monoclinic:

(a) Clino-enstatite

(b) Diopside

(c) Sahlite

(d) Hedenbergite

(e) Augite

(f) Pigeonite

(B) Alkali-pyroxenes:

(n) Aegirine

(b) Jadeite

(c) Spodumene

(d) Johannsenite

3. Triclinic:

They are not related structurally to the pyro­xenes, although chemically they have identical formulae. They have a single chain of linked Si0₄ tetrahedra, which is not the simple chain of the pyroxenes. They are commonly known as “pyroxenoids”, and include minerals like Wollastonite (CaSi0₃), Pectolite [Ca₂NaH(SiO)₃] and Rhodonite (MnSi0₃), Bustamite [MnCa(Si0₃)j].

Physical Properties of Pyroxenes:

These are usually prismatic crystals.

1. Colour:

Nearly black or green of various shades.

2. Lustre Vitreous to subvitreous:

Hypersthene shows a kind of metallic-pearly lusture termed ‘Schillerisation’.

3. Cleavage:

2 sets, prismatic at angles 87° and 93°.

4. Hardness:

5 to 6.

5. Sp. Gravity:

Low to moderate.

6. Twinning:

Contact twins in case of monoclinic members. Optical Characteristics

(a) Orthepyroxenes:

(i) Green and pleochroic (except in enstatites).

(ii) Ref. index. High. Optically +ve.

(iii) Interference colour. 1st order,

(iv) Extinction. Parallel.

(b) Clinopyroxenes:

(i) Colourless to pale green and pleochroic.

(ii) Ref. index. Higher than Canada Balsam,

(iii) Int. colour. 2nd order,

(iv) Extinction. Inclined, 45°.

Varieties:

(i) Diallage:

Translucent and is fibrous augite.

(ii) Omphacite:

Foliated diopside, found in Eclogite.

(iii) Kunzite:

Gem-variety of spodumene.

(iv) Hiddenite:

Emrald green spodumene (gem-variety).

Occurrence:

Mostly found in basic rocks like Gabbro and their hypabbyssal and volcanic equivalent, ultrabasic rocks like-peridotite, pyroxenite also contain predominantly pyroxenes. Hypersthene is characteristic of norites and charnockites. Spodumen occurs as large crystals in pegmatites.