Types of Sedimentary Rocks

There are three main types of sedimentary rocks that differ based on their composition and process of formation.

1. Clastic Sedimentary Rocks (also called detrital sedimentary rocks)

Mechanical weathering physically splits or breaks previously formed rocks into smaller fragments called clasts. These clasts may be individual minerals or rock fragments, but they may also include fragments of hard organic materials such as shells. Clasts may range in size from boulders to microscopic particles most commonly known as clay.

After weathering creates the clasts they are transported by wind, water, ice or gravity to a new location where they are physically deposited. The resulting clastic sedimentary rock is composed of fragments of pre-existing rocks.

2. Chemical Sedimentary Rocks

In chemical weathering, minerals of existing rocks of any type are dissolved primarily by water.  The dissolved ions are transported to a new location where they come out of solution (a process called chemical precipitation) to form new chemical sedimentary rocks composed of interlocking mineral crystals. Some of these sediments have mineral grains that are visible to the eye, but often they are so fine-grained that the crystals may not be visible even with a microscope.

3. Biochemical Sedimentary Rocks

Biochemical sedimentary rocks are a subset of chemical sedimentary rocks.  They occur when biological organisms cause minerals to chemically precipitate from solution.  They can range from fine-grained mud on the deep ocean floor created by the microscopic shells of floating plankton that settle out from the ocean surface, all the way up to massive structures such as coral reefs formed by the support frameworks of coral colonies.

How Loose Sediments Become Sedimentary Rocks

A loose or unconsolidated sediment is one where the individual grains (clasts, minerals) of the sediment are separate from each other.  Air or water will occupy the pore space between the clasts when they are first deposited.  Sediments undergo lithification, which is the process of changing an unconsolidated sediment to a material that is considered a sedimentary rock.  There are three main processes:

1. Consolidation occurs as water (or air) is expelled from a sediment as it compresses under the weight of the sediments accumulating above, and the pore spaces between grains become smaller.

2. Cementation refers to the chemical precipitation of new minerals between pre-existing grains which helps to bind or cement the grains together. The most common cements are calcite (CaCO₃) and silica (quartz, SiO₂), but hematite (Fe₂O₃) and limonite [FeO(OH)] may also occur.

3. Re-crystallization may occur, in which the existing minerals that make up the sedimentary grains re-form their shapes to interlock together.  There is no change in the mineralogy, just the edges of the minerals are able to reform in new shapes that lock together better than before.

Once consolidated, cemented, and possibly re-crystallized, a sediment has become a sedimentary rock.

Mineral Composition

Determination of whether a rock is clastic, chemical or biochemical can start with identifying the mineral composition of the rock.  Some primary or original minerals occur mainly in clastic sedimentary rocks (e.g. quartz, feldspar) whereas others are exclusive to chemical/biochemical sedimentary rocks (e.g. gypsum, halite).  If the rock has been cemented together, then the composition of the cement can sometimes be indicative of the rock category.  Silica (quartz) and calcite are the most common cementing minerals in clastic rocks whereas calcite is the most common cement in chemical and biochemical sedimentary rocks.

Silicates

The clastic sedimentary rocks we shall examine first are based upon silicate minerals.  You will need to be able to identify two of these silicate minerals: quartz and feldspar.  Use these “non-technical” descriptions to guide you, along with help from the T.A.

Quartz: translucent, sometimes totally clear, but usually with a bit of light grayish colour, hard, does not have nice linear or planar sides, or a nice regular shape, but usually has a more irregular surface.

Feldspars: typically white or pink, opaque, hard, sometimes have linear or planar (flat) faces.

Non Silicates

These minerals are all covered in more detail in the chemical/biochemical rocks section.

Carbonates: The chemical and biochemical rocks are based upon carbonates.  A simple test using dilute hydrochloric acid (HCl) described here helps you tell carbonates (calcite, dolomite) apart from other minerals.

Gypsum, Halite:  These two minerals are also covered in the chemical sedimentary rocks.  They have similar appearances with a typically a translucent cloudy grey colour in big samples, or white in small samples, with visible linear or planar crystal faces.  Sometimes these planar features are visible inside the crystals.  Halite is familiar to you as table salt.

Hematite: This is an iron oxide mineral that is a reddish-brown rust-like colour.  It gives a reddish brown tint to many types of sediment that are formed in air where oxygen is present.

Want to Know More About Minerals?

This material is for additional background only. You will not be tested on it.

Minerals are described using:

• Colour
• Luster: the appearance of a mineral in normal light. (e.g., metallic, non-metallic, dull, pearly, brilliant, resinous, glassy, waxy)
• Hardness (H): A number from 1 to 10 that describes the hardness of a mineral relative to 10 index minerals that make up the Mohs hardness scale.
• Cleavage: the number and orientation of flat planes along which the mineral breaks.
• Fracture: how a mineral breaks when it breaks in a non-planar fashion not along a cleavage plane.
• Form: the shape a crystal will form when it grows in an unrestricted space.
• Twinning: the internal structure of some minerals changes across a plane and becomes a mirror image.  The externally visible crystal form will mirror across the twinning plane.