Here in Perú , we call those rocks ; lutita = shale , siltstone = limonita , but as far as I know we don't use the terms claystone and mudstone in spanish terms.

Clastic sedimentary rocks are classified by size of the sediment particles making up the rock. Particle size descriptions like sand, silt, and clay have specific meaning in geology and engineering. (see chart below).

Shales, mudstones and claystones are rock types that are very similar to each other.

  • Siltstone - greater than half of the composition is silt-sized particles.
  • Claystone - greater than half of the composition is clay-sized particles.
  • Mudstone - hardened mud; a mix of silt and clay sized particles.

The difference between mudstone and shale is that mudstones break into blocky pieces whereas shales break into thin chips with roughly parallel tops and bottoms.

The terms shale and claystone are sometimes used interchangeably.

References: Mudstones and shales Fun rock charts on pinterest

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  • My only quibble is that during a summer job a long time ago, with my freshman physical geography course, I couldn't convince a mining engineer that clay (soil) was about particle size rather than clay minerals. Mightn't it be necessary to call something with the right particle size but no clay minerals mudstone? – Spencer Feb 20 '17 at 20:37
  • most silicate rock when reduced to the appropriate size become clay due to interactions with water, clay minerals and particle size are inescapably linked in soils. Keep in mind the above chart only applies to CLASTIC rock, grains size by itself is not enough becasue without water it does not become clay in a chemical sense, but waterless conditions are so rare on earth that for all intents and purposes, grains that size in SOIL can be assumed to be clay minerals. – John Dec 9 '17 at 16:15
  • Small grained sediment (like volcanic tuff) can exist without being clay. But once they are weathered and incorporated into soil they will be transformed into clay very quickly. so for soil if it is small enough it is basically always clay but for other cases size alone is not enough. – John Dec 9 '17 at 16:17

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