ZIRCON

General Formula:
ZrSiO4
Sample: GB 12 System:
Tetragonal
Zircon
Euhedral zircon grain within a granitic gneiss. Note the highly fractured nature of the grain and the very strong relief.
Field of View 1 mm, plane light
Zircon
The same image as that on the left, the high interference colours of the zircon grain are not readily evident in this image,
Field of View 1 mm, crossed polars
Block diagram showing the relationship between the crystallographic axes and the indicatrix axes.

Optical Properties

Colour
Pleochroism
colourless to pale brown
weakly pleochroic
Form euhedral to sudhedral tetragonal crystals with pyramidal terminations
Relief
RI
very high positive
nw = 1.920-1.960
ne = 1.967-2.015
Cleavage not usually seen in thin section
Birefringence
Interference Colours
0.036-0.065
up to third or fourth order
Twinning not twinned
Interference Figure
Optic Sign
2V
uniaxial
positive
Optic Orientation elongate grains are length slow with parallel extinction
Composition significant Hf for Zr and minor U and Th Alteration does not readily alter
Occurrence common accessory mineral in felsic rocks and less common in mafic rocks, common in metamorphic rocks derived from clastic sediments, common detritial mineral Distinguishing Features small, high-relief grains with bright interference colours


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