Sedimentary Rocks

What are clastic sedimentary rocks?

Clastic sedimentary rocks are produced by the compaction and cementation of rock and mineral fragments that were derived from weathering or preexisting rock. These fragments are called clasts. Clastic rocks are classified by clast size.

Describe crystalline texture.

Crystalline texture refers to rocks that have minerals which can be seen with the unaided eye. This may even include fossil fragments.

Describe the typical composition of each clast size.

Gravel-sized clasts are mostly rock fragments, sand-size clasts are mostly minerals (quartz), and mud-size clasts are mostly clay minerals.

Describe the two carbon-rich sedimentary rocks.

The two carbon-rich sedimentary rocks are peat and coal. Peat is made of compressed plant fibers. It is soft and breaks apart easily. Coal in dark brown to black with a dull to shiny luster. It may be brittle.

How are chemical rocks classified?

Chemical rocks are classified primarily by rock composition. The most common minerals in chemical rocks are calcite (limestone), quartz (chert), and gypsum and halite (evaporites).

What are chemically precipitated rocks?

Chemically precipitated rocks form in an aqueous environment. The precipitation occurs either directly, by inorganic processes, or by the actions of organisms (biochemical). Chemically precipitated rocks include: carbonate rocks (calcite), siliceous rocks (quartz), and evaporite rocks (gypsum and halite).

Describe oolitic texture.

Oolitic texture refers to rocks that contain sand-sized, spherical particles. These particles grow in layers as they roll in the water (pushed by currents), much like a pearl grows in layers inside a mollusk.

Describe dense texture.

Dense texture is fine-grained. The crystals cannot be seen with the unaided eye.

What is fissility?

Fissility is a characteristic of rocks composed of mud-size clasts, in which they have the tendency to break into thin layers. It originates from the alignment of sheet-like clay minerals, which imparts planes of weakness in the rock. Shale, for example, is typically fissile.

How are sedimentary rocks formed?

Sedimentary rocks are formed by the compaction and cementation of sediment, or by the chemical precipitation of minerals. They form at or near the surface of the earth.

What are the size classes used to describe clastic rocks?

The size classes used to describe clastic sedimentary rocks are, from smallest to largest: clay and silt (mud size); fine and coarse (sand size); pebbles, cobbles, and boulders (gravel size).

What are the three textures of chemical rocks?

The three textures of chemical rocks are crystalline, dense, and oolitic.

What are the three types of sedimentary rocks?

The three types of sedimentary rocks are clastic, chemically precipitated, and carbon-rich (organic).