Identification

Exploring Rocks: Color

The colors seen in thin sections are caused by polarized light interacting with minerals. Color depends on the chemical composition of the minerals, their atomic structure, and the orientation of their grains to the light path. Observing colors in several different ways reveals optical properties that help to identify the minerals.


 

Light filtered through one polarizer becomes plane-polarized, showing minerals in their natural colors. Here, red-brown biotite is bordered by lighter brown grains of hornblende. Different orientations of the grains cause the different shades of color. Garnet (upper right) and the feldspar surrounding it are nearly colorless.

 


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Olivine metagabbro, Essex County, New York
Plane polarized light
 

Other colors appear when a second polarizer, with its polarizing plane at right angles to the first, is added to the light path between the thin section and the camera. Most images in Splendor in Stone were made with crossed polarizers. This arrangement creates the bright colors, called interference colors.

 


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Olivine metagabbro, Essex County, New York
Plane polarized light
 

A gypsum plate inserted in the light path adds its interference color to (or subtracts it from) those of the minerals in the thin section.

 


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Olivine metagabbro, Essex County, New York
Plane polarized light


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