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TITLE: Three Rocks and Their Minerals


Each of these rocks—granite, gneiss, and sandstone—is made mostly of these three common mineral types: mica, quartz, and feldspar. How can three rocks made of the same basic ingredients look so different? The minerals came together in varied ways. By studying a rock’s minerals, scientists uncover clues to its origins and history. In this exhibit, so can you.

[Photo: Granite, gneiss, and sandstone with their five most common minerals: plagioclase and orthoclase (feldspar), quartz, and muscovite and biotite (mica)]

 


 

[Illustration: Two continents colliding, burying and heating a sandstone.]

Gneiss
Gneiss is a metamorphic rock—one that was transformed from older rocks by heat and pressure within the Earth. Two continents collided 1.7 billion years ago, burying and heating a sandstone. Its minerals recrystallized and separated into alternating layers. Molten granite invaded this gneiss, forming a vein that was later deformed into a zigzag shape.

 

[Pie Chart of gneiss minerals: orthoclase, biotite, plagioclase, and quartz.]

Here are the gneiss’s minerals in their relative proportions.

 

[Photo: Closeup of gneiss revealing minerals: orthoclase, biotite, plagioclase, and quartz.]

This thin slice of gneiss has been magnified 30 times. Using cross-polarized light on the slice reveals mineral features you can’t see using plane-polarized light.

 


 

[Illustration: Molten rock cooling underground.]

Granite
Granite is an igneous rock—one that solidified from a hot, molten state. The crystals in this granite formed when molten rock cooled underground 1.1 billion years ago. They grew until they bumped into neighboring crystals. As the granite slowly cooled, the crystals grew fairly large.

 

[Pie Chart of gneiss minerals: orthoclase, biotite, plagioclase, and quartz.]

Here are granite’s minerals in their relative proportions.

 

[Photo: Closeup of granite revealing minerals: orthoclase, biotite, plagioclase, and quartz.]

This thin slice of the granite has been magnified 30 times. Using cross-polarized light on the slice reveals mineral features you can’t see using plane-polarized light.

 


 

[Illustration: Sand and pebbles being carried downhill by a river and deposited in layers.  Shows points of future sandstone development.]

Sandstone
Sandstone is a sedimentary rock—one that formed from sediments deposited at Earth’s surface by water, ice, or wind. The sand and pebbles that make up this sandstone are fragments that came from older rocks. The fragments were carried downhill by rivers and streams 300 million years ago, deposited in layers, then bound together by natural cement.

 

[Pie Chart of sandstone minerals: orthoclase, muscovite, plagioclase, and quartz.]

Here are the sandstone’s minerals in their relative proportions.

 

[Photo: Closeup of sandstone revealing minerals: orthoclase, muscovite, plagioclase, and quartz.]

This thin slice of the sandstone has been magnified 30 times. Using cross-polarized light on the slice reveals mineral features you can’t see using plane-polarized light.


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Smithsonian National Museum of Natural History Department of Mineral Sciences website Credits