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A rock that is a mechanical mixture of different minerals and/or rock fragments (clasts).
A breccia may also be distinguished by the origin of its clasts: (Type of breccia whose clasts are composed of a single (mono-) rock type, possibly all from a single rock unit (e.g., L6 with L6). Monomict breccias are rare on the Moon because meteoroid impacts tend to mix different kinds of rocks. The example is a terrestrial granite breccia. Image Source:: monogenetic or monolithologic, and General term for all breccias that are neither monomict nor dimict. Modified from image source: http://www.saharamet.com/meteorite/gallery/HED/index.html.: polygenetic or polylithologic). The proportions of these fragments within the unbrecciated material can vary significantly. Many meteorites categorized as breccias were produced due to collisional processes like hypervelocity impacts. An excellent paper on the subject can be found by clicking this link where the excerpt originates:
Impacts at speeds above ~20 m/s broke rock, while hypervelocity impacts left shock damage and formed breccias from fragments of earlier rocks. The study of meteoritic breccias contributes significantly to our understanding of early The Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids. processes of Accumulation of smaller objects into progressively larger bodies in the solar nebula leading to the eventual formation of asteroids, planetesimals and planets. The earliest accretion of the smallest particles was due to Van der Waals and electromagnetic forces. Further accretion continued by relatively low-velocity collisions of smaller bodies in the Click on Term to Read More, A process by which a generally homogeneous chondritic body containing mostly metal, silicates and sulfides will melt and form distinct (differentiated) layers of different densities. When the melting process continues for a long enough period of time, the once chondritic body will re-partition into layers of different composition including Click on Term to Read More, and surface (Mixture of unconsolidated rocky fragments, soil, dust and other fine granular particles blanketing the surface of a body lacking an atmosphere. Regolith is the product of "gardening" by repeated meteorite impacts, and thermal processes (such as repeated heating and cooling cycles). Click on Term to Read More) evolution, and also provides unique information about the primordial, chemical, and mineralogical characteristics of the accreted components themselves (Bischoff and Stöffler, 1992).
Note that the definition for a breccia used by meteoriticists is only loosely correlated to the definition used by terrestrial geologists.
There are other geologic processes beyond collisional processes that produce breccias including tectonic, Igneous rock that forms from cooling magma on the surface of a planet or asteroid., sedimentary, and collapse. On Earth, breccias and conglomerates can appear similar but in breccias the large clasts are mostly angular in shape, while in conglomerates the clasts are nearly all rounded, evidence of prolonged tumbling in running water or moving waves in the past. In the case of “lunar” meteorwrongs, a clear indicator of its terrestrial origin is the rounded clasts.
Breccias other than impact-melt breccias are lithifiied by impacts that caused limited shock induced grain boundary melting cementing the rock fragments together (Kieffer, 1975; Ashworth and Barber, 1976; Bischoff et al., 1983). Ashworth and Barber (1976) and Bischoff et al. (1983) showed that ordinary chondritic regolith breccias experienced limited shock-induced grainboundary melting. This melt is important for consolidating loose debris into brecciated rock. Some porous lunar regolith breccias may have been lithified by a thermal welding process (McKay et al., 1989), and carbonaceous Chondrite meteorites are the most common accounting for 83.6% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), free Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More by growth of secondary phases. Thermal annealing after mixing of fragments and lithification may lead to a recrystallized Fine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents. Click on Term to Read More between large fragments as in the case of LL chondritic fragmental breccias (Jäckel and Bischoff, 1998).