The Dynamic Earth @ National Museum of Natural History

Main Menu > Rocks and Mining > Rocks Tell Stories > 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)]

		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.

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

		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.

		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.

		Here are granite’s minerals in their relative proportions.

		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.

		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.

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

		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.