Smith College - Geology 222b - Petrology

Petrographic Data File

Calcite
Property
Value
Comments
FormulaCaCO3 
Crystal System hexagonal, 3 2/m 
Crystal Habitdrusy, pisolitic, stalatitic, stalagmitic, prismatic, acicular, tabular; scalenohedrons & rhombohedrons most common; may be contact twinned; may also be cryptocrystalline or coarsely crystalline  
Hardness 3 on cleavage, 2.5 on {001}  
Cleavageperfect rhombohedral rhombohedral cleavage is a distinguishing characteristic in thin section.
Luster vitreous  
Fracture & Tenacityconchoidal, brittle  
Color

In hand sample: colorless, white, or gray in most cases; can be pink, green, yellow or other colors

In thin section: colorless

  
Optic Signuniaxial (-)   
Optic Orientation difficult to determine due to high birefringence because calcite is uniaxial c must equal Z
Refractive Indices
epsilon = 
omega = 
1.486
1.658
 
Max Birefringence0.172 
Extinction symmetrical to cleavage traces when crystal is in an extinct position, birefringent calcite dust formed by grinding is visible.
Distinguishing Features

Hand sample: effervesces in dilute hydrochloric acid without powedering, can fluoresce under UV light, single clear crystals exibit birefringence ('double refraction').

Thin section: high order colors under crossed polars, very high relief that changes with rotation, lower refractive index than dolomite, simple twinning and lamellar twins on some crystal faces, rhobohedral cleavage. Another difference from dolomite is that dolomite contains cloudy inclusions with iron oxides.

OccurrenceOccurs in almost all rock types. It is the primary mineral in both marble and limestone. It occurs with dolomite, quartz, gypsum, barite, fluorite, pyrite, sphalerite, galena, chalcopyrite, danburite, stilbite, malachite, and azurite.
Editors Elizabeth Sklute (MHC '06), Sarah Stephen ('15J), Carly Peltier (MHC '15), Emily Padova ('15)

 
Photomicrograph of calcite under plane polarized light using 10X magnification. Calcite is almost indistinguishable from other minerals under plane polarized light.
Photomicrograph of calcite under cross polarized light using 10X magnification. Note how the twins stand out agains the extinct crystal and the crystal is specled with 'birefringent dust'.
Photomicrograph of calcite from sample 46-6a at 20x. The pale, high order interference colors of calcite under crossed polars are evident.
Photomicrograph of calcite from sample Ak92-23k at 10x. Simple twinning is shown.
Photomicrograph of calcite showing lamellar twinning in plane polarized light. Click on the image to see a larger image showing the same crystal in cross polarized light.
A photomicrograph of calcite showing lamellar twinning in plane polarized light. Calcite forms these deformation twins as a result relatively low mechanical stress. Click on the image to see a larger image showing the same crystal in cross polarized light.
Photomicrograph of calcite in plane polarized light and (in rollover) crossed polars. High order birefringence colors typical for calcite are visible. Under crossed polars twinning is clearly visible in two directions.

Returnto Petrography Index