OLIVINE

General Formula:
(Fe,Mg)₂SiO₄

Sample: PT-96

System:
Orthorhombic

Euhedral to subhedral olivine phenocrysts, exhibiting irregular fractures, in a plagioclase microlite matrix

Field of View = 4.0 mm, plane light

Olivine phenocrysts exhibiting 2^nd to 3^rd order interference colours
Field of View = 4.0 mm, Crossed polars

Block diagram showing the relationship between the crystallographic axes and the indicatrix axes.

Colour
Pleochroism

usually colourless, darker colours correspond to higher iron content
non pleochroic

Form

generally subequant anhedral grains or aggregates in intrusive and metamorphic rocks. Equidimensional or elongated euhedral grains in volcanics

Relief
RI

high positive
n_a = 1.636-1.827
n_b = 1.651-1.869
n_g = 1.669-1.879

Cleavage

not observed

Birefringence
Interference Colours

0.033-0.052
up to third order

Twinning

not common

Interference Figure
Optic Sign
2V

biaxial
positive or negative
46-98°

Optic Orientation

elongate grains have parallel extinction and may be either length fast or slow

Composition

minor substitution of Mn, Zn, Ca, Ni, Cr or Al for Fe and Mg

Alteration

commonly alters to iddingsite and chlorophaeite, which are really mixtures of various minerals which cannot be identified, and serpentine. Alteration progresses from the edge and along cracks

Occurrence

pure Fo (Mg-rich) is restricted to metamorphosed carbonates, intermediate Fe-Mg olivine is common in mafic and ultramafic igneous rocks, Fe-rich olivine occurs in felsic rocks.

Distinguishing Features

high birefringence, distinctive fracturing, lack of cleavage, and alteration products.