Fissility (geology)

In geology, fissility is the ability or tendency of a rock to split along flat planes of weakness (“parting surfaces”).^[1] These planes of weakness are oriented parallel to stratification in sedimentary rocks.^[2] Fissility is differentiated from scaly fabric in hand sample by the parting surfaces’ continuously parallel orientations to each other and to stratification. Fissility is distinguished from scaly fabric in thin section by the well-developed orientation of platy minerals such as mica. Fissility is the result of sedimentary or metamorphic processes.

Planes of weakness are developed in sedimentary rocks such as shale or mudstone by clay particles aligning during compaction.^[3] Planes of weakness are developed in metamorphic rocks by the recrystallization and growth of micaceous minerals.^[4] A rock's fissility can be degraded in numerous ways during the geologic process, including clay particles flocculating into a random fabric before compaction, bioturbation during compaction, and weathering during and after uplift. The effect of bioturbation has been documented well in shale cores sampled: past variable critical depths where burrowing organisms can no longer survive, shale fissility will become more pervasive and better defined.

Fissility is used by some geologists as the defining characteristic which separates mudstone (no fissility) from shale (fissile).^[5] However, some professions, like drilling engineers, continue to use the terms shale and mudstone interchangeably.^{[citation needed]}

References

^ Hise, Charles Richard Van (1896-01-01). Principles of North American Pre-Cambrian Geology. U.S. Government Printing Office.
^ Moore, J. Casey (1986-01-01). Structural Fabric in Deep Sea Drilling Project Cores from Forearcs. Geological Society of America. p. 24. ISBN 978-0-8137-1166-9.
^ Tucker, Maurice E. (2013-05-22). Sedimentary Petrology: An Introduction to the Origin of Sedimentary Rocks. John Wiley & Sons. ISBN 978-1118698907.
^ Ingram, Roy L. (1953-08-01). "Fissility of Mudrocks". GSA Bulletin. 64 (8): 869–878. doi:10.1130/0016-7606(1953)64[869:FOM]2.0.CO;2. ISSN 0016-7606.
^ Nichols, Gary (2013-04-30). Sedimentology and Stratigraphy. John Wiley & Sons. ISBN 9781118687772.

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[1] Hise, Charles Richard Van (1896-01-01). Principles of North American Pre-Cambrian Geology. U.S. Government Printing Office.

[2] Moore, J. Casey (1986-01-01). Structural Fabric in Deep Sea Drilling Project Cores from Forearcs. Geological Society of America. p. 24. ISBN 978-0-8137-1166-9.

[3] Tucker, Maurice E. (2013-05-22). Sedimentary Petrology: An Introduction to the Origin of Sedimentary Rocks. John Wiley & Sons. ISBN 978-1118698907.

[4] Ingram, Roy L. (1953-08-01). "Fissility of Mudrocks". GSA Bulletin. 64 (8): 869–878. doi:10.1130/0016-7606(1953)64[869:FOM]2.0.CO;2. ISSN 0016-7606.

[5] Nichols, Gary (2013-04-30). Sedimentology and Stratigraphy. John Wiley & Sons. ISBN 9781118687772.

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v t Structural geology
Underlying theory	Deformation mechanism Lamé's stress ellipsoid Mohr–Coulomb theory Mohr's circle Overburden pressure Rock mechanics Strain Strain partitioning Stress Stress field Tension
Measurement conventions	Brunton compass Inclinometer Orthographic projection Rake Stereographic projection Strike and dip
Large-scale tectonics	Accretionary wedge Allochthon Autochthon Back-arc basin Continental collision Convergent boundary Décollement Divergent boundary Extensional tectonics Fault block Fenster Fold and thrust belt Fold mountains Foreland basin Graben Half-graben Horse Horst Horst and graben Intra-arc basin Inversion Klippe List of tectonic plate interactions Mélange Mountain formation Nappe Obduction Orogeny Passive margin Plate tectonics Pull-apart basin Rift valley Rift Saddle Strike-slip tectonics Structural basin Suture Syneclise Terrane Thick-skinned deformation Thin-skinned deformation Thrust tectonics
Fracturing	Columnar jointing Dike Exfoliation joint Fissure Joint Vein
Faulting	Asperity Cataclasite Cataclastic rock Detachment fault Disturbance Fault friction Fault mechanics Fault scarp Fault trace Thrust fault Transfer zone Transform fault
Foliation and lineation	Cleavage Compaction Crenulation Fissility Oblique foliation Pressure solution Rock microstructure Slickenside Stylolite Tectonic phase Tectonite
Folding	Anticline Chevron Detachment fold Dome Homocline Monocline Syncline Vergence
Boudinage	Competence
Kinematic analysis	3D fold evolution Paleostress inversion Paleostress Section restoration
Shear zone	Mylonite Pure shear Shear
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Fissility (geology)

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