Argillaceous Rocks Examples (Characteristics and Types)  

Many different types of rocks can be found worldwide, but scientists tend to focus on just the three main types:‌ ‌igneous,‌ ‌sedimentary,‌ ‌and‌ ‌metamorphic.

Upon further exploring rocks, you’ll discover subgroups of each group, one of which is‌ ‌argillaceous‌ ‌rocks.

This article will tell you everything you need to know about argillaceous rocks, including their characteristics and different types.

Argillaceous Rocks Examples (Characteristics and Types)  

What Are Argillaceous Rocks?

Argillaceous rocks are categorized as clastic sedimentary rocks.

They contain silt or clay particles less than 0.0625 millimeters in size.

There is 44-56 percent of these sedimentary rocks in the Earth’s crust, making them the most abundant sedimentary rock type.

Some of the stones that fit into the argillaceous include the following:

  • Shales
  • Argillites
  • Siltstones
  • Mudstones
  • Claystone

A shale is a type of laminated sedimentary rock that falls into the argillaceous category, as it contains silt and clay‌ ‌particles.

Seventy percent of Earth’s crust is composed of these rocks.

You will often find shales within the layers of limestone or sandstone rocks. ‌

These rocks are typically formed when gentle currents and compact carry mud, silt, and other sediments.

This often happens in environments such as basins of shallow seas, the deep-ocean floor, and river floodplains.

Although most shales form in expansive sheets of several meters of thickness, some develop in lenticular forms.

Argillites or argillaceous rocks are fine-grained sedimentary rocks composed mainly of clay.

There are a variety of silt-sized particles in argillaceous rocks made from lithified muds and oozes.

When the fissile layering characteristic of shale develops, the argillites grade into shale.

Siltstones‌ ‌are‌ ‌clastic sedimentary rocks composed mainly of ‌the‌ ‌mineral‌ ‌silt.

Despite its low permeability and porosity, siltstone is sometimes categorized as a tight gas reservoir rock.

Since the rock’s clay content is low, it is classified as a mudrock, and ‌its lack of fissility makes it distinguishable from shale.

It is best to taste a rock sample, see if the mineral is gritty, and determine if it is silt.

For a rock to be classified as claystone, the mineral must contain clay particles measuring less than 1/256 millimeters in diameter.

It also can not be laminated or easily split into thin layers.

When claystones are blocky and massive, they are sometimes classified as mudstones.

The terms claystones and mudstones are often used interchangeably to classify shale rocks.

Geologists sometimes refer to partially hardened muds as mudstones and call fully set material claystone.

Argillaceous Rocks Physical Characteristics

As argillaceous rocks are a group of different minerals, we’ll discuss each rock’s different physical characteristics.

Starting with shale rocks, the color will often depend on their composition make-up.

Often these rocks come in the shade of dark grey, green, red, yellowish-brown, or brown.

The texture of shale rocks is smooth due to finely grained clay minerals.

It ranks low on the hardness scale and is very brittle. ‌

Depending on the level of brittleness, thin layers or flat planes can break easily.

Argillites come in various shades of grey, but may also be found in a dark greenish color.

Natural roughness and dullness affect the rock’s opaque appearance.

However, the rock ‌can‌ ‌be‌ ‌polished.

Therefore, the argillite does not require much maintenance as‌ ‌other‌ ‌sedimentary‌ ‌rocks.

Siltstone comes in the colors of blueish-grey, olive green, and brown.

Sometimes you can find them in the colors of reddish-orange or black when stained. It ranks on the Mohs hardness scale between 6-7 and has high porosity.

Mudstones come in a variety of colors ranging from brown, green, and black to the shades of yellow, orange, and red.

The rocks have a clastic texture, and you can only see it under a microscope.

On the Mohs hardness scale, it ranks between 2 and 3. ‌

This material is also water-resistant, but it is not wind, acid, or scratch-resistant.

Like mudstones, claystone comes in a variety of different colors ranging from brown, black, yellow, and blue.

These rocks have a hardness between 3.5 and 4.0 on the Mohs scale, with a density of 2-2.9.

Common Appearance of Argillaceous Rocks

Argillaceous rocks can come in all shapes and sizes.

Although, it is most common to see these rocks in different shades of grey.

You may be able to find argillaceous rocks in big bulks connected to other sedimentary rocks.

For example, shales and siltstones are often seen like this.

The Chemical Composition of Argillaceous Rocks

Argillaceous rock’s chemical composition is mixed with many different elements.

For example, claystone often has the chemical composition of biotite, chlorite, pyrite, and muscovite.

The compound content of claystone is aluminum oxide, iron oxide, and silicon dioxide.

The formation of claystone takes place during the weather erosion of mudstone.

Mudstone has feldspar, micas, pyrite, and chlorite chemical composition.

The compound content is the same as claystone: containing aluminum oxide, NaCI, CaI, and iron oxide.

Formation of mudstone happens during chemical and mechanical weathering.

Siltstone’s chemical composition consists of clay minerals, feldspars, micas, and quartz.

While the composition is higher in clay minerals, within the slits of siltstone, feldspars, and quartz is high.

Siltstone compound content is silicon dioxide, iron oxide, CaO, MgO, and aluminum oxide.

Argillites contain clay minerals, quartz, feldspars, and quartz.

The compound content of argillites includes aluminum oxide, Na20, and K20.

Shales’ chemical composition is about the same as the other argillaceous rocks.

The rock contains feldspar, clay minerals, and quartz.

Where Can You Find Argillaceous Rocks?

You can find argillaceous rocks worldwide, but they are abundant within Azerbaijan and the South Caspian Basin.

The stones make up between 50-95% of the formation of the area.

In the United States, you can find many of these argillaceous rocks where water flows.

This means within river banks, lakes, and offshore beaches.

Wrapping Up

Argillaceous rocks break down into five groups.

This includes argillite, claystone, shale, siltstone, and mudstone.

If a rock contains mainly clay minerals, then more than likely, it’s an argillaceous rock.

But, to know the full details of rocks, you will have to examine them under a microscope to tell the difference.

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