Apelite (from Ancient Greekπηλός (pēlós)'clay, earth')[3] ormetapelite is ametamorphosed fine-grainedsedimentary rock, i.e.mudstone orsiltstone. The term was earlier used by geologists to describe a clay-rich, fine-grainedclasticsediment or sedimentary rock, i.e. mud or a mudstone, the metamorphosed version of which would technically have been ametapelite. It was equivalent to the now little-usedLatin-derived termlutite.[4][5][6] Asemipelite is defined in part as having similar chemical composition but being of acrystalloblastic nature.[7]
Pettijohn (1975)[8] gives the following descriptive terms based on grain size, avoiding the use of terms such asclay orargillaceous which carry an implication of chemical composition. The Ancient Greek terms are more commonly used for metamorphosed rocks, and the Latin for unmetamorphosed:
In the late 1800s and early 1900s, George Barrow discovered the classic Barrovian-type metamorphic sequence in the southeasternScottish Highlands.[9][10] It represents a common type of regional peliticorogenic metamorphism. He observed that as a pelitic rock undergoes higher pressures and temperatures, its mineral assemblage changes from predominantlychlorite tobiotite togarnet tostaurolite tokyanite tosillimanite. This later turned out to be overly simplistic.
^Wei, Chunjing; Powell, Roger; Clarke, Gordon (2004). "Calculated phase equilibria for low‐ and medium‐pressure metapelites in the KFMASH and KMnFMASH systems".Journal of Metamorphic Geology.22 (5):495–508.doi:10.1111/j.1525-1314.2004.00530.x.S2CID128393826.
^Potter, P.E., J.B. Maynard, and P.J. Depetris (2005)Muds and Mudstones. New York, New York, Springer. 279 pp.ISBN978-3-540-22157-9
^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.ISBN0-922152-76-4
