Short notes on Physical properties and uses of Olivine

Olivine includes a group of related minerals, which form a complete solid solution series from pure forsterite (Mg₂Si0₄) to pure fayalite (Fe”Si0₄). The composition of different members is given in.

Where: FO = first-rate (Mg₂Si0₄) and Fa = fayalite (Fe^Si0₄). Figures as subscript indicate the composition in percent. For example Fo_{3 ()} Fa is equal to forsterite 30% and fayalite 70%. From the above table it is evident that the composition of each member of the olivine group is variable within certain limits.

Physical properties:

(i) Form: Crystallised varieties are prismatic in habit. Massive and compact grains are common.

(ii) Colour: Forsterite is colourless, white or green and Fayalite is green to yellow. Other members are green, pale green, olive-green, grayish-green, brownish.

(iii) Lustre: Vitreous.

(iv) Streak: White.

(v) Hardness: 6-7.

(vi) Cleavage: Poor.

(vii) Fracture: Conchoidal.

(viii) Specific gravity: 3.2 – 4.3.

(ix) Diaphaneity: Transparent to translucent.

Use: Olivine is used as refractory material for the casting industry. Peridot, the transparent green variety of olivine is used as gemstone.