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Calcite

Mineral structure: Carbonate

Common mineral in Limestones, Marbles, Calc-silicates, Veins

Rock-forming minerals associated with Calcite: various Ca-rich silicates (e.g. plagioclase and amphibole), quartz in sedimentary rocks

Calcite in Marble

Analyser out Analyser in
Calcite in Marble (Analyser out) Calcite in Marble (Analyser in)

Key optical features of calcite in thin section:

Colourless (Cloudy)
Most sections show two intersecting cleavages
High – Low relief (changes on rotation of stage)
High order interference colours

Common minerals that might be confused with calcite and occur in similar rocks:
Quartz (No cleavage, low relief, low interference colours extinction),

Calcite in limestone

Analyser out Analyser in
Calcite in Limestone (Analyser out) Calcite in Limestone (Analyser in)

Calcite shape

Biogenic calcite in limestone with calcite cement between bioclasts

Analyser out Analyser in
Biogenic calcite in limestone with calcite cement between bioclasts (Analyser out) er in)
Analyser in  
Biogenic calcite in limestone with calcite cement between bioclasts (Analyser in)
Biogenic calcite in limestone with calcite cement between bioclasts Biogenic calcite in limestone with calcite cement between bioclasts

Calcite cleavage and twinning in marble

Analyser out Analyser in
Calcite cleavage and twinning in marble (Analyser out) Calcite cleavage and twinning in marble (Analyser in)
Note: Granoblastic texture. Most calcite grains show two sets of cleavage traces.

Calcite interference colours in calc-silicates

Analyser in Analyser in
Calcite interference colours in calc-silicates (Analyser in) Calcite interference colours in calc-silicates (Analyser in)
Note: Very high interference colours typically merge to produce golden-brown colour

Calcite relief change in quartz-rich calc-silicate

Analyser out Analyser out (stage rotated by 90 o)
Calcite relief change in quartz-rich calc-silicate (Analyser out) Calcite relief change in quartz-rich calc-silicate (Analyser out Stage rotated 90)
Note: Relief of calcite changes form low to high when stage is rotated.

Take the Calcite test
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