fissility (geology)
{{short description|Tendency of a rock to split along flat planes of weakness}}
In geology, fissility is the ability or tendency of a rock to split along flat planes of weakness (“parting surfaces”).{{Cite book|url=https://books.google.com/books?id=c0wPAAAAYAAJ|title=Principles of North American Pre-Cambrian Geology|last=Hise|first=Charles Richard Van|date=1896-01-01|publisher=U.S. Government Printing Office|language=en}} These planes of weakness are oriented parallel to stratification in sedimentary rocks.{{Cite book|url=https://books.google.com/books?id=VanQlSQdXRQC&pg=PA24|title=Structural Fabric in Deep Sea Drilling Project Cores from Forearcs|last=Moore|first=J. Casey|date=1986-01-01|publisher=Geological Society of America|isbn=978-0-8137-1166-9|page=24}} 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.{{Cite book|url=https://books.google.com/books?id=1ju0DAAAQBAJ|title=Sedimentary Petrology: An Introduction to the Origin of Sedimentary Rocks|last=Tucker|first=Maurice E.|date=2013-05-22|publisher=John Wiley & Sons|isbn=978-1118698907|language=en}} Planes of weakness are developed in metamorphic rocks by the recrystallization and growth of micaceous minerals.{{Cite journal|last=Ingram|first=Roy L.|date=1953-08-01|title=Fissility of Mudrocks|url=http://bulletin.geoscienceworld.org/content/64/8/869|journal=GSA Bulletin|language=en|volume=64|issue=8|pages=869–878|doi=10.1130/0016-7606(1953)64[869:FOM]2.0.CO;2|issn=0016-7606}} 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).{{Cite book|url=https://books.google.com/books?id=Gcgp5oLFrZMC|title=Sedimentology and Stratigraphy|last=Nichols|first=Gary|date=2013-04-30|publisher=John Wiley & Sons|isbn=9781118687772|language=en}} However, some professions, like drilling engineers, continue to use the terms shale and mudstone interchangeably.{{citation needed|date=February 2020}}