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Quartz

 

Mineral structure: Framework silicate

Common mineral in Granites, Pelites (Schists, Gneisses), Sandstones, Quartzites

Rock-forming minerals associated with Quartz: Plagioclase, K-Feldspar, Muscovite, Biotite

Quartz in schist

Analyser out Analyser in
Quartz in schist (Analyser out) Quartz in schist (Analyser in)
Note: low relief and low order interference colours, quartz appears as a background colourless mineral with the analyser out.

Key optical features of quartz in thin section:

Colourless (clear and unaltered)
No cleavage
Low relief
Low order interference colours may show strained extinction
No preferred crystal shape

Common minerals that might be confused with quartz and occur in similar rocks:
Plagioclase and K-Feldspar (commonly twinned, commonly slightly altered),
Calcite (High relief, high interference colours, twinned, cleavage)

Polycrystalline chert (fine grained quartz) clast in conglomerate

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Polycrystalline chert (fine grained quartz) clast in conglomerate (Analyser out) Polycrystalline chert (fine grained quartz) clast in conglomerate (Analyser in)

Lack of alteration in comparison to plagioclase

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Lack of alteration in comparison to plagioclase (Analyser out) Lack of alteration in comparison to plagioclase (Analyser in)

Quartz has no alteration products – very stable mineral. But is commonly found in veins, develops quartz overgrowths and often contains lines of fluid inclusions trapped on annealed fractures.

Analyser out  
Lack of alteration in comparison to plagioclase (Analyser out) Lack of alteration in comparison to plagioclase (Analyser out)
Note: contrast with dusty alteration of feldspar

Trails of fluid inclusions in quartz

Analyser in Analyser out (High magnification)
Trails of fluid inclusions in quartz (Analyser in) Trails of fluid inclusions in quartz (Analyser out High magnification
Note: Small vapour bubbles within high magnification inclusions

Quartz shape

Interstitial quartz in granite

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Interstitial quartz in granite (Analyser out) Interstitial quartz in granite (Analyser in)
Note: Late crystallisation of quartz filling gaps between early plagioclase and amphibole

Rounded detrital quartz grains with overgrowths in sandstone

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Rounded detrital quartz grains with overgrowths in sandstone (Analyser out) Rounded detrital quartz grains with overgrowths in sandstone (Analyser in)
Note:Overgrowths in crystallographic continuity with host grain

Metamorphic recrystallised quartz

Analyser in Analyser in
Metamorphic recrystallised quartz (Analyser in) Metamorphic recrystallised quartz (Analyser in)
Note: Granoblastic texture with relatively straight grain boundaries meeting at ca. 120 o

Quartz strained extinction and recrystallisation

Analyser in Analyser in
Quartz strained extinction and recrystallisation (Analyser in) Quartz strained extinction and recrystallisation (Analyser in)
Analyser in Analyser in
Quartz strained extinction and recrystallisation (Analyser in) Quartz strained extinction and recrystallisation (Analyser in)
Note: Strained quartz (left hand images) individual subgrains of the mineral lattice are rotated into slightly different orientations. Adjacent images (right hand images) quartz is largely unstrained and composed of smaller individual recrystallised grains.

Take the Quartz test
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