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Mudrocks are a family of fine-grained siliciclastic sedimentary rocks deriving from the consolidation of clay- and silt-sized sediment (< 0.0625 mm), collectively known as mud. Mudrocks are also known as pelites or lutites, based on their Greek and Latin root names. The distinction of silt (0.004 – 0.0625 mm) from clay (< 0.004 mm) is possible only with laboratory or microscopy techniques. In terms of composition, mudrocks contain very fine-grained quartz and feldspar (constituting the silt-sized particles) and clay minerals such a kaolinite, montmorillonite, and illite, cemented by iron oxides or carbonatic cement. As other clastic sedimentary rocks, the name of mudrocks derives from the grain size of the corresponding loose sediment. There are several classifications, but in general, following Prothero (1996), we can classify a mudrock as:
siltstone, if they contain more than 2/3 silt over clay
mudstone, when they contain between 1/3 and 2/3 clay with the rest is silt
claystone, when clay constitutes more than 2/3 of the total.

In the field, we cannot distinguish silt from clay with the hand lens or the unaided eye. Thus, we can loosely classify these rocks as mudrocks. However, we can touch them to obtain a rough estimate of their grain size. According to Prothero (1996), siltstones are gritty on the touch, whereas mudstones and claystones feel loamier. Claystone is slick.

mudrock deposits
The layering and different erosion of these laminated mudrocks indicates the presence of layers with different grain size. Glacial sediments of Lake Missoula, W Montana, USA. Photo © Matt Affolter via wikipedia.

Siliciclastic sedimentary rocks
• quartz
• feldspar
• clay minerals
• iron oxides
• carbonates

• siltstone
• claystone
• mudstone
• shale

mud layers in sandy sediment
The human eye can easily recognize particles of sand, whereas grains of mud (the dull, homogeneous layers) are indistinguishable for us, due to their very tiny grain size. The ‘ball’ of mud in sand is a rip-up clast. Pliocene deposits, Volterra, Pisa, Italy. The spatula is about 20 cm. Photo © Samuele Papeschi/GW.
mudrock and shale diagram
Mudrocks are stacks of more or less oriented platy clay minerals, like piles of cards. Deformation and compaction parallelize these ‘cards’ producing shales. Graphics: Samuele Papeschi/GW.

Fissility and shales
Mudrocks tend to break along roughly parallel planes. The presence of clay minerals determines this property, called fissility. Clay minerals are phyllosilicates with a platy habit and they tend to align parallel to one another during deposition. Consequently, mudrocks break more easily parallel to the oriented clay minerals. In mudrocks, however, clay minerals are only poorly or partially oriented, resulting in the tendency of mudrocks to break as elongated, irregular blocks along conchoidal fractures. Compaction, diagenesis, and deformation can reorganize clay minerals in strong parallelism with each other producing strongly fissile rocks, known as shales. Differently from other mudrocks, a shale has a strong tendency to break along nearly parallel surfaces, forming thin flat chips.

red mudrocks
Fractured red mudrocks. Fractures are strongly irregular, since clay minerals are still poorly organized in the sediment. Ragged Reef Formation, Cumberland Basin, Nova Scotia, Canada. Photo © Michael C. Rygel via Wikimedia Commons.
Casanova shale
Shales are very fissile and break into flat, parallel chips. Casanova Complex, Casanova (Rovegno), Genova, Italy. Photo © Samuele Papeschi/GW.

From shales to slates
Shales are very fissile (foliated) and similar to slates, very low-grade metamorphic rocks that derive from the metamorphism of shales. In terms of composition, slates consists of very fine-grained metamorphic minerals such as muscovite and chlorite, which have replaced the original clay minerals that largely constitute shales. These mineralogical differences are not visible in the field, but, on touch, shales feel loamy due to the clay minerals, whereas slates are hard. Furthermore, shales appear dull and do not reflect light, while slates show a very weak reflectivity.

Rio Marina slate
Outcrops of slate. At outcrop scale, they are very similar to shales, but these rocks experienced very low-grade metamorphism. Rio Marina Schist, Monte Arco, island of Elba, Italy.

Bennett, R. H., O’Brien, N. R., & Hulbert, M. H. (1991). Determinants of clay and shale microfabric signatures: processes and mechanisms. In Microstructure of Fine-Grained Sediments (pp. 5-32). Springer, New York, NY.
Chamley, H. (1989). Clay minerals. In Clay Sedimentology (pp. 3-20). Springer, Berlin, Heidelberg.
Folk, R. L. (1980). Petrology of sedimentary rocks. Hemphill publishing company.
Millot, G. (2013). Geology of clays: weathering· sedimentology· geochemistry. Springer Science & Business Media.
O’Brien, N. R., & Slatt, R. M. (2012). Argillaceous rock atlas. Springer Science & Business Media.
Potter, P. E., Maynard, J. B., & Depetris, P. J. (2005). Mud and mudstones: Introduction and overview. Springer Science & Business Media.
Weaver, C. E. (1989). Clays, muds, and shales. Elsevier.

See also – Shale

Detrital and Authigenic Minerals
Sedimentary Structures
Sedimentary Rocks


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