sedimentary
Argillite / Mudstone
Argillaceous sedimentary rock
Hardness: 3-4 on the Mohs scale; Color: Varies widely, often gray, dark gray, or brownish; Luster: Dull or earthy; Crystal structure: Microcrystalline, composed of clay minerals and quartz; Cleavage: Often exhibits weak or no distinct cleavage, breaking into blocky fragments rather than thin sheets.
- Hardness
- 3-4 on the Mohs scale
- Color
- Varies widely, often gray, dark gray, or brownish
- Luster
- Dull or earthy
Identified More sedimentary →
Explore Argillite / Mudstone in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 3-4 on the Mohs scale; Color: Varies widely, often gray, dark gray, or brownish; Luster: Dull or earthy; Crystal structure: Microcrystalline, composed of clay minerals and quartz; Cleavage: Often exhibits weak or no distinct cleavage, breaking into blocky fragments rather than thin sheets.
Formation & geological history
Formed from the compaction and lithification of mud, silt, and clay-rich sediments over millions of years in quiet depositional environments such as deep marine basins, lakes, or floodplains.
Uses & applications
Used primarily in educational settings, geological studies, and occasionally carved into ornamental objects or native art (such as Haida argillite carvings).
Geological facts
Argillite is an intermediate rock type between soft clay shale and hard slate. It lacks the pronounced fissility (ability to split into thin layers) that characterizes typical shale.
Field identification & locations
Found worldwide in sedimentary sequences. Identification relies on its fine-grained texture, lack of visible crystals, and moderate hardness compared to softer shales or harder slates.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Sedimentary Sandstone
Arenite (Sedimentary Rock)
sedimentary
Fossiliferous Concretion
Septarian Concretion containing Brachiopods/Crinoids
fossil
Banded Siltstone / Mudstone Pebble
Siltstone / Argillite
sedimentary
Shale (or Mudstone/Siltstone)
Sedimentary Mudrock (SiO2, Al2O3 base with clay minerals)
sedimentary
Shale
Shale (clastic sedimentary rock)
sedimentary