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.
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.
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.
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Sandatlas.org – Shale