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 Fo3 () 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.
(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.