Mineral Information Institute

Background | Name | Sources | Uses | Alternative Sources

Amphibolite is a dark, heavy, metamorphic rock composed mostly of the mineral amphibole. Amphibolites have very little to no quartz. “Amphibole” refers not to a single mineral, but to a group of minerals. Most belong to the monoclinic crystal system, but some belong to the orthorhombic crystal system. They are silicate minerals containing SiO4 molecules. The SiO4 groups are connected to each other in double chains.

Rocks that are composed mainly of amphibole minerals are found in both metamorphic and igneous environments. Geologists restrict the term amphibolite to metamorphic rocks composed of amphibole. In most instances the specific amphibole mineral is hornblende. By contrast, geologists often refer to igneous rock with amphibole as hornblendite. However, those who work with rock as a construction material usually refer to all rock types composed of amphibole as “amphibolite.” Based on this industrial application of the term “amphibolite” as both metamorphic and igneous in origin, the textures of amphibolite can be either roughly laminated if metamorphic or granular if igneous.

The original rock that is metamorphosed (called the protolith) into amphibolite is often igneous basalt or gabbro. However, the sedimentary rock called marl can also be metamorphosed into amphibolite. “Marl” is mudstone that has a certain amount of calcium carbonate (lime) mixed in it. Geologists have also discovered that some sediments derived from volcanic rock can also be metamorphosed into amphibolite.

The name amphibolite comes from the Greek word amphibolos which means ambiguous, a reference to the fact that the amphibole minerals are easily mistaken for other dark-colored minerals (especially the group of minerals called the pyroxenes).

Name

Sources

Amphibolite is relatively common. It is found in regions that have been affected by regional metamorphism. Amphibolite is found throughout the Appalachian Mountain chain. For example, significant quantities of amphibolite are found in the Gore Mountain region of the Adirondack Mountains in New York State. This is a particularly interesting deposit because the amphibolite contains large nodules of deep red garnet that has been mined for use in sand papers and other abrasives applications. They are also found in the Great Smoky Mountains National Park on the Tennessee-North Carolina border. Other states along the Appalachian Mountains producing amphibolite are Maine, Connecticut, Pennsylvania and North Carolina.

Uses

Amphibolite is very hard and takes a high polish. The combination of its ability to be polished, its dark color and its texture have made amphibolite a popular dimension stone in construction. It is used as paving stones and as a veneer or facing on buildings (both for interior and exterior use).

It is also used as crushed stone for the usual crushed stone applications such as road and railroad bed construction. In this application it is used locally, near the source of the amphibolite. This reduces the cost of transporting non-native stone in from other sources.
Gemologists and lapidary workers have discovered that some amphibolite rock produces a shimmer effect when it is polished. They use rounded and polished pieces of amphibolite for various pieces of jewelry.

Substitutes andAlternative Sources
There are nearly limitless alternatives for the various crushed stone applications for which amphibolite is occasionally used. Any type of rock, local or imported, that can be readily quarried, crushed and transported can replace amphibolite. In the United States, limestone and granite together represent over 80% of all the crushed rock consumed annually. As noted above, amphibolite is used locally where it is easily quarried, reducing the costs of transporting rock in from other regions.

There are many options to amphibolite as dimension stone. Marble, granite, and quartzite, for instance, can all be polished and used as facing on the interior and exterior of buildings. In some environments even sandstone can be used for building construction. In the end, amphibolite is chosen for the particular color, texture and overall look it gives to a building. Substitutes that provide a similar look include plastics and some varieties of other dark rock like dark granite.

By Darryl Powell

Darryl Powell is a 1984 graduate of the University of Rochester, Rochester, New York with a B.S. in Geological Sciences. For six years Mr. Powell was an Adjunct Professor at Finger Lakes Community College in Canandaigua, New York, where he taught “Introduction to Physical and Historical Geology.” He creates Earth Science educational kits for NeoSCI, Inc., a science education resource company based in Rochester, New York. He also writes Earth Science materials for NewPath Learning, Inc. based in Victor, New York. Mr. Powell is the founder of Diamond Dan Publications, a small company focused on creative activity books and activity-based learning games about rocks and minerals, based in Manchester, New York.