Minerals In Thin Section


Formula	SiO₂

Optic class & sign	Uniaxial positive

Relief	Low (positive)

Refractive indices	n_o = 1.544

	n_e = 1.553



Birefringence (max.)	0.009



Sign of elongation	Length-slow, l (+) for crystals elongate in c; deformed crystals may have a variety of grain shapes showing either mainly length-slow or length-fast characteristics (or neither if c is at a high angle to the grains' long dimension), depending on the dominant deformation mechanism and slip system

Interference figure	Broad isogyre cross, grey to white interference colours; biaxial figures with 2V up to 20° may be observed if quartz is undulatory; quartz is optically active, but the rotation effect on polarized light waves parallel to the optic axis cannot be recognized in standard-thickness thin sections.

Colour / pleochroism	Colourless; colours of coloured varieties are rarely visible in standard thin section

Zoning


Form	Habit	Granular; hexagonal bipyramidal crystals in rhyolite and dacite; deformation-related elongate to discoid forms, also subgrain formation in metamorphic rocks; fibrous in fibre veins and spherulites

	Surface	Anhedral in plutonic and metamorphic rocks; euhedral or subhedral in felsic volcanics; euhedral in veins and cavities

Cleavage		None; unusual {101} and {100} cleavage is reported

Twinning		Common Dauphiné twins (c axis = twin axis) cannot be recognized in thin section due to identical indicatrix orientation; same applies to Brazil contact twins; rare Japan twins can be recognized as the twin plane is the pyramid face

Extinction		Undulatory extinction very common; sections ∥ c of euhedral crystals: straight extinction with respect to prism faces, symmetrical extinction with respect to pyramid faces


Reaction textures	Characteristic palisade-textured quartz coronas around partially transformed coesite in ultra-high pressure rocks

Alteration / decomposition	Chemically stable


Occurence	Ign	Felsic plutonic and volcanic rocks; silica-saturated mafic plutonic and volcanic rocks, pegmatites; characteristic quartz-feldspar intergrowths: myrmekite, granophyre, graphic intergrowth

	Met	Metamorphosed felsic rocks, metabasites, metapelites, metapsammites, metacherts

	Sed	Major constituent in detrital sediments and sedimentary rocks; cherts, authigenic as euhedral neoformed crystals or overgrowths on detrital quartz grains; cementing mineral in sandstones

	Hyd	Common vein mineral; precipitation in cavities, including amygdales of volcanic rocks

	Other


Distinctive properties	Low birefringence and relief, optic class, lack of cleavage and twinning, absence of alteration

Additional comments	Not compatible with silica-deficient minerals; α-quartz is the low-temperature structure, while β-quartz is the stable form of silica at high temperatures (e.g., magmatic crystallization; upper amphibolite and granulite facies in metamorphic environments). All quartz at ambient conditions is α-quartz as the displacive transformation between the α and β polymorphs is immediate. The former existence of β-quartz can only be recognized if crystals are euhedral or subhedral (e.g., short hexagonal bipyramids in volcanics indicate crystallization as β-quartz).

Quartz