a grain of sediment, silt, sand, gravel, etc., especially as a constituent fragment of a clastic rock formation, as distinguished from a chemical or biogenic component of such a formation.

1950–55; probably back formation from clastic Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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American Heritage
Science Dictionary
clast   (klāst)  Pronunciation Key 
A rock fragment or grain resulting from the breakdown of larger rocks.

clastic adjective (klās'tĭk)
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Large tan block is clast of sandstone in conglomerate.
Glacial deposits, containing all clast sizes from boulders to mud, partially mantle the rocks.
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