Phyllonite
Strongly sheared rock with phyllite-like sheen or fine foliation produced by mylonitisation of an initially coarser rock.
Language and route: English scientific blend of phyllite and mylonite, whose deeper Greek elements concern leaves and milling. English scientific blend of phyllite and mylonite, whose deeper Greek elements concern leaves and milling
Literal gloss: phyllite-like mylonite
The name and its history
Historical structural-geology classification; no verified original coiner · twentieth-century usage
- What does the word literally mean?
- Immediate linguistic pathway: English scientific blend of phyllite and mylonite, whose deeper Greek elements concern leaves and milling. Relevant original form: phyllite + mylonite (phyllite + mylonite); conventional gloss ‘phyllite-like mylonite’. The linguistic gloss should not be mistaken for the original scientist’s stated motive unless directly demonstrated.
- What did the original author mean?
- A metamorphic rock could acquire a phyllitic appearance by tectonic grain-size reduction, misleading a lithologic classification based on visual aspect.
- Why was the name needed?
- The term was needed to distinguish deformation-produced phyllite-like rock from the actual low-grade metamorphic rock phyllite.
- What evidence supported the original concept?
- Fine micaceous foliation, shear bands, relict coarser grains and structural position along zones of intense strain.
- How has geology changed the concept?
- Modern tectonic studies distinguish processes and protoliths; phyllonite does not itself imply simple retrograde metamorphism or a particular age.
Further historical context
A metamorphic rock could acquire a phyllitic appearance by tectonic grain-size reduction, misleading a lithologic classification based on visual aspect.
The term was needed to distinguish deformation-produced phyllite-like rock from the actual low-grade metamorphic rock phyllite.
Fine micaceous foliation, shear bands, relict coarser grains and structural position along zones of intense strain.
Modern tectonic studies distinguish processes and protoliths; phyllonite does not itself imply simple retrograde metamorphism or a particular age.
Editorial qualification
No original naming page identified; sources are operational modern descriptions and retrospective usage.
Related names
Sources
- USGS thesaurus — Phyllonite — Current phyllonite descriptive definition under tectonite
- USGS glossary and bibliography of cataclastic rocks — Historical terminology for cataclasite, phyllonite and mylonite; full page not independently inspected
- BGS Northern Highlands: mylonite definition and field evidence — Distinguishes original mill analogy from predominantly ductile recrystallisation