Chemical Sediments
Particle Shape
Outline
Clastic sedimentary rocks are classified according to the grain size of the particles or clasts that are cemented together to form the rock.
| SEDIMENT PARTICLE | DESCRIPTION | ROCK NAME |
|---|---|---|
| Gravel | Rounded rock fragments. | CONGLOMERATE |
| Gravel | Angular rock fragments. | BRECCIA |
| Sand | Quartz predominant, visible grains, often thickly bedded, depositional structures such as cross-bedding common. | SANDSTONE |
| Sand | Sandstone with more than 25% feldspar grains. | ARKOSE |
| Silt | Quartz predominant, grains barely visible, gritty feel. | SILTSTONE |
| Clay | Thick beds >1cm blocky, fine mud, no particles discernable, may show polygonal cracks, composed predominantly of clay minerals and very fine quartz. | MUDSTONE |
| Clay | Laminated mudstone, fissile, splits into thin sheets. | SHALE |
The strength of sementation is often an important characteristic in engineering terms. Well-cemented quartz sandstones can be very strong mechanically, whereas friable uncemented sandstones are relatively weak rocks. Siltstonea, mudstones and shales are usually weak rocks because of the dominance of platey clay minerals that provided little frictional resistance.
Conglomerates and sandstones have relatively high volumes of voids (pores) and are economically important as aquifers for water supply and reservoir rocks for gas and petroleum.
Chemical Sediments
Particle Shape
Outline