Claystone
Fine-grained clastic sedimentary rock (clay minerals)
A very fine-grained sedimentary rock made mostly of clay minerals, smooth to the touch and lacking the gritty feel of siltstone.
- Mohs hardness
- 1-3
- Color
- Gray, brown, red, green, or buff
- Type
- sedimentary
Got a rock like this?
Identify any rock from a photo, free.
Overview
Claystone is a fine-grained clastic sedimentary rock composed predominantly of clay-sized particles (finer than 0.004 mm), mostly clay minerals such as kaolinite, illite, and smectite. It is the finest-grained member of the mudrock family.
It has a smooth, almost greasy feel with no grittiness, distinguishing it from siltstone. Like mudstone, it is non-fissile and breaks into blocky pieces rather than thin sheets.
Claystone is soft, dull, and earthy, and often becomes plastic and sticky when wet because of its clay content. Some clays swell dramatically with water, which has major engineering implications.
Formation & geology
Claystone forms from the slow settling of the finest sediment — clay particles — in the calmest water environments. Typical settings include deep lakes, quiet lagoons, distal floodplains, and deep, still marine basins where currents are negligible.
The clay minerals themselves form mainly from the chemical weathering of feldspars and other silicates on land, then wash into basins as suspended fines.
Over time the clay is buried, compacted, and slightly cemented into rock. Because clay particles are so tiny and platy, the resulting claystone is dense, smooth, and low in permeability, which makes it an excellent seal in geology.
How to identify it
The key clue is the very smooth, fine texture — claystone feels smooth (not gritty) when rubbed or bitten, and grains are completely invisible to the naked eye.
It is soft (Mohs ~1-3), can be scratched with a fingernail or knife, gives a strong earthy smell when dampened, and may become sticky or plastic when wet. It is blocky, not fissile.
Look-alikes: Siltstone (gritty feel), mudstone (mixed clay and silt, slightly less smooth), and shale (fissile, splits into layers). Claystone's hallmarks are extreme fineness, smoothness, and clay plasticity.
Uses & significance
Claystone and the clays it yields are raw materials for bricks, tiles, pottery, ceramics, and cement. Specific clays serve industrial roles: kaolinite for paper and porcelain, bentonite (smectite) for drilling muds and sealants, and various clays as fillers and absorbents.
In engineering, claystone is important as a low-permeability barrier — used to line landfills and ponds — but swelling clays can cause foundation and slope-stability problems.
Claystone is too soft for jewelry or carving, but it is geologically valuable for preserving fossils and recording quiet depositional environments and past climates.
Frequently asked questions
What is the difference between claystone and siltstone?
Claystone is made of finer clay particles and feels smooth, while siltstone has slightly coarser silt grains and feels gritty when rubbed or bitten.
Is claystone the same as mudstone?
Claystone is dominated by clay-sized particles, while mudstone is a broader term that includes both clay and silt. Claystone is the finer end member.
Why does claystone get sticky when wet?
Its abundant clay minerals absorb water and become plastic and sticky. Some clays, like smectite, even swell significantly when wet.
What is claystone used for?
It is used to make bricks, tiles, ceramics, and cement, and as a low-permeability liner for landfills and ponds because clay blocks water flow.
Claystone guides
In-depth guides for identifying, valuing, and understanding Claystone.
