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A pelite (Ancient Greek pelos, clay) is a metamorphosed fine-grained sedimentary rock, i.e. mudstone or siltstone. The term was earlier used by geologists to describe a clay-rich, fine-grained clastic sediment or sedimentary rock, i.e. mud or a mudstone, the metamorphosed version of which would technically have been a metapelite. It was equivalent to the now little-used Latin-derived term lutite.[1][2][3] A semipelite is defined in part as having similar chemical composition but being of a crystalloblastic nature.[4]

The term pelite is not to be confused with pilite, a rarely used name for an altered olivine that has been partially pseudomorphically replaced by an assemblage of carbonate–chlorite–actinolite and can be identified only in a thin section.

Pettijohn (1975)[5] gives the following descriptive terms based on grain size, avoiding the use of terms such as clay or argillaceous which carry an implication of chemical composition. The Ancient Greek terms are more commonly used for metamorphosed rocks, and the Latin for unmetamorphosed:

Descriptive size terms
Texture Common Ancient Greek Latin
Coarse gravel(ly) psephite (psephitic) rudite (rudaceous)
Medium sand(y) psammite (psammitic) arenite (arenaceous)
Fine clay(ey) pelite (pelitic) lutite (lutaceous)


  1. ^ Potter, P.E., J.B. Maynard, and P.J. Depetris (2005) Muds and Mudstones. New York, New York, Springer. 279 pp. ISBN 978-3-540-22157-9
  2. ^ Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) Glossary of Geology (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. ISBN 0-922152-76-4
  3. ^ Whitten, D.G.A. (1972). The Penguin Dictionary of Geology. London: Penguin Books. p. 342. ISBN 0140510494.
  4. ^ BGS rock classification scheme
  5. ^ Pettijohn, F. J. (1975), Sedimentary Rocks, Harper & Row, ISBN 0-06-045191-2.