This is a group of rock-forming silicates. The minerals are olive- green or brown in colour. All these minerals crystallize in orthorhombic system.
1. Atomic structure:
These are a neso silicate which essentially consists of a series of isolated Si04 tetrahedra which are linked by means of metal cations.
2. Chemical composition:
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The group includes minerals which may be represented by the formula R2Si04, where R=Mg or Fe. The members of this group belong to a continuous series of solid- solutions between Forsterite Mg2Si04 and Fayalite Fe2Si04. Common olivine is intermediate between them with excess Mg and the formula is represented as (Mg, Fe)2Si04.
Various members are there in the isomorphous series, in which the end members are forsterite and fayalite, xiz.
4. Optical properties:
(i) Colourless and non-pleochroic. (ii) Ref. index. High positive relief, High Ref. index, (iii) Bi-refringence. Strong 0’037. (iv) Extinction. Straight.
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5. Varieties:
The gem-quality of olivines is referred to as period.
6. Alterations:
Olivine commonly alters to serpentine (antigorite) and secondary iron-oxide. In basaltic rocks, the alteration of the outer iron-rich rim of olivine produce a brownish red mineral called ‘Iddingsite’.
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7. Occurrence:
It characterises the ultra-basic igneous rocks as dunites, peridotites serpentinites and basic rocks like norite, gabbro, dolerite, basalt, etc. The common associates are chromite, spinel, pyrope etc. Olivine and quartz (primary) never occur together.
Forsterite is formed by dedolomitisation or contact metamor- phism of magnesium rich sedimentary rocks as dolomitic limestone rich in silica.