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Breccia

Rock composed of sharp-angled fragments embedded in a fine-grained matrix.

A clastic rock composed of angular gravel-size fragments; the consolidated equivalent of rubble. The designation gravel-size refers to a mean particle diameter greater than 0.08 in. (2 mm), which means that 50% or more of the particles (by volume) are this size or larger. Various classifications specify different values for the degree of angularity. One system specifies angular or subangular fragments (roundness ≤0.25), whereas another restricts the term breccia to aggregates with angular fragments (roundness ≤0.10).

Breccia is formed in a very similar fashion to conglomerate. The difference between the two rocks is that breccia's rock fragments are very sharp and angular. These rock fragments have not been transported by water, wind, or glaciers long enough to be rounded and smoothed like in the conglomerate. The cementing agents silica, calcite, and iron oxides are the same as in conglomerate.

Breccia is a detrital sedimentary rock composed mostly of large, angular clasts. Breccia forms as streams transport grains, or clasts, only a short distance before depositing them within a stream channel or valley. Because of this short distance of transportation, the clasts have experienced relatively few collision, and so still possess their original sharp edges and corners produced when they weathered from rocks in their source area. Breccia can also form from landslide masses that became fragmented as they moved down slope. Once such a mass stablizes and solidifies, it can form a breccia containing huge, boulder-sized fragments.

Sedimentary breccias, also known as sharpstone conglomerates, are significant because the angularity of their fragments indicates either proximity to the source or transportation by a mechanism that does not cause significant rounding of the fragments. Examples of the first condition are talus breccia formed at the base of a scarp, and reef breccia deposited adjacent to a reef margin. Transport mechanisms that can preserve the angularity of clasts over significant distances include debris flows, slumps, and glacial transport, although rounded fragments may also be carried. All of these mechanisms incorporate a large proportion of fine sediment in the transporting medium, which effectively cushions interparticle collisions and inhibits rounding.

Geologically, most breccias are either lithified rubble -- e.g.,lithified sediments or pyroclastic deposits that consist to a noteworthy extent of pebble- to boulder-sized angular fragments -- or were formed by crushing and grinding within fault zones. These breccias are generally referred to as sedimentary breccias and cataclastic (or fault) breccias, respectively. However, many breccias, including those used as gemrocks, have been formed as the result of other processes -- e.g., desiccation, which involves cracking as the result of dehydration or drying an subsequent filling of the cracks to give the overall mass a coherence. Indeed, in his "Dictionary of Rocks" (1985), my good friend Dick Mitchell (1929-1989), to whose memory I dedicated "Minerals of Virginia, 1990," has 62 entries (more than two pages!) of names that he found applied to breccias; most relate to genesis, a few are only descriptive.

In any case, it seems very likely that breccias other than those listed under the OTHER NAMES subheading have been or in the future will be used at least locally as the rough for fashioning ornamental and curio stones. And, whatever their origins or compositions, I suspect several will be given so-to-speak marketplace names rather than the names on Mittchell's list. Also, I suspect that even though most breccias used as gemrocks will be chosen because they are considered attractive, some will be fashioned and marketed because of their potential appeal as conversation pieces -- e.g., impact breccias.

See also: Pebble.

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