Minerals In Thin Section


Formula	ZrSiO₄

Optic class & sign	Uniaxial positive

Relief	Very high

Refractive indices	n_o = 1.922 -1.960

	n_e = 1.961 -2.015

	High Hf lowers refractive indices; metamict zircon 1.8

Birefringence (max.)	0.042 - 0.065

	Metamict zircon is essentially isotropic

Sign of elongation	Length-slow, l (+)

Interference figure	Only obtainable from large enough crystals, then good isogyre cross with isochromes over two or three interference colour orders

Colour / pleochroism	Colourless to pale brown

Zoning


Form	Habit	Bipyramidal-prismatic crystals elongate in c; simple four-sided crystals; crystals with a multitude of faces tend to show ellipsoidal to near-spherical shapes; crystal size is commonly very small

	Surface	Euhedral crystals are common; zircon resorption in magmas can obliterate crystal faces

Cleavage		{110}, {111}, poor; commonly not observed in thin section

Twinning		Uncommon

Extinction		Straight to prism faces in sections ∥ c


Reaction textures

Alteration / decomposition	Resistant to alteration and weathering


Occurence	Ign	Common accessory mineral in felsic igneous rocks; less common in mafic rocks; silica-undersaturated rocks with sufficient Zr contain baddeleyite instead of zircon; large zircon crystals can occur in pegmatites and kimberlites

	Met	Metamorphic rocks derived from clastic sediments or igneous rocks commonly contain zircons inherited from these source materials, and, at medium to high grade in particular, newly grown zircon

	Sed	Detrital sediments (heavy mineral fraction); accessory mineral in silt- and sandstones

	Hyd

	Other


Distinctive properties	Characteristic form, very high relief, bright interference colours

Additional comments	Concentric growth zoning may be observed in thin section; α-radiation damage from radiogenic isotopes concentrated in zircon create pleochroic halos in some ferro-magnesian minerals

Zircon