OLIVINE

General Formula:
(Fe,Mg)2SiO4		 Sample: PT-96 System:
Orthorhombic
TITLE
DESCRIPTION
SCALE		 TITLE
DESCRIPTION
SCALE
 Block diagram showing the relationship between the crystallographic axes and the indicatrix axes.

Optical Properties

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
nalpha = 1.636-1.827
nbeta = 1.651-1.869
ngamma = 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 Alterationcommonly 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
Occurrencepure 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.