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Is the Word “Breccia” Used in Other Ways?
Geologists have been very generous in their use of the word “breccia.” It is common to hear the term used when referring to a rock or rock debris made up of angular fragments. Although it is mainly used for rocks of sedimentary origin, it can be used for other types of rocks. A few more…
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What Color is Breccia?
Breccia can be any color. The color of the matrix or cement along with the color of the angular rock fragments determine its color. Breccia can be a colorful rock, as shown in the photos on this page.
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What is Breccia’s Composition?
Breccia has many compositions. Its composition is mainly determined by the rock and mineral material that the angular fragments were produced from. The climate of the source area can also influence composition. Most breccias are a mix of rock fragments and mineral grains. The type of rock that the fragments were produced from is often used as an adjective…
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How Does Breccia Differ From Conglomerate?
Breccia and conglomerate are very similar rocks. They are both clastic sedimentary rocks composed of particles larger than two millimeters in diameter. The difference is in the shape of the large particles. In breccia the large particles are angular in shape, but in conglomerate the particles are rounded. The particle shape reveals a difference in how far…
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How Does Breccia Form?
Breccia forms where broken, angular fragments of rock or mineral debris accumulate. One of the most common locations for breccia formation is at the base of an outcrop where mechanical weathering debris accumulates. Another is in stream deposits a short distance from the outcrop or on an alluvial fan. Some breccias form from debris flow deposits. The…
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What is Breccia?
Breccia is a term most often used for clastic sedimentary rocks that are composed of large angular fragments (over two millimeters in diameter). The spaces between the large angular fragments are filled with a matrix of smaller particles and a mineral cement that binds the rock together.
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Gneiss in the Classroom
Small rock and mineral specimens about one inch in size are usually adequate for student examination and identification. However, many rock units, identified as gneiss in the field, have bands that are thicker than one inch. If samples of these rock units are broken into one-inch pieces, many of them will be too small to…
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Commercial Terminology
Don’t be surprised if you see gneiss labeled as “granite” at a cabinet shop or monument company. In the dimension stone trade, any rock with visible, interlocking grains of feldspar is considered to be “granite” in that industry. Seeing gneiss, gabbro, labradorite, diorite, and other types of rock marketed as “granite” disturbs many geologists. However, this long-time practice of…
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Uses of Gneiss
Gneiss usually does not split along planes of weakness like most other metamorphic rocks. This allows contractors to use gneiss as a crushed stone in road construction, building site preparation, and landscaping projects. Some gneiss is durable enough to perform well as a dimension stone. These rocks are sawn or sheared into blocks and slabs used in…
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Composition and Texture of Gneiss
Although gneiss is not defined by its composition, most specimens have bands of feldspar and quartz grains in an interlocking texture. These bands are usually light in color and alternate with bands of darker-colored minerals with platy or elongate habits. The dark minerals sometimes exhibit an orientation determined by the pressures of metamorphism. Some specimens of gneiss contain distinctive…