In this article, you’ll learn what an arenaceous rock is, and about some common examples you are likely to come across.
Arenaceous Rocks Examples (Characteristics and Types)
What Is an Arenaceous Rock?
When talking about arenaceous rocks, we are referring to clastic sedimentary rocks that consist of sand or particles of a substance similar to sand.
The three main types of these rocks are quartzite, arkose, and greywacke.
Typically, arenaceous rocks (also known as arenites) are classified as such due to their particulate size, which ranges from 2 to 0.06 mm.
Silty samples change this to 0.004 mm.
Arenites tend to contain mostly carbonate particles (referred to as calcarenites) and are often categorized along with limestones.
With that said, the majority of arenaceous rocks are just referred to as sandstone.
What is a Quartzite rock?
Quartzite is a beautiful amalgamation of quartz, hematite and other mineral particles that have formed during tectonic compression that causes intense heat and pressure.
It is a hard metamorphic stone that was once pure quartz sandstone.
Quartzite is typically 95% quartz with other minerals making up the rest of its structure.
These days, quartzite is typically sought after for its silica content for the production of silicon, road works, and as a decorative material in stonework.
In the past, it was used for making tools due to its hardness and durability.
It is pretty easy to find quartzite in the wild as it consists of almost pure silica content which leaves almost no space for dirt to collect.
This means that you will usually find exposed ridges or stones on or near the surface with maybe a thin dusting of soil that has come to rest on top of it with little to no vegetation covering it.
Quartzite, due to the varying amount of hematite present, is often different tones of pink or red, but can also be found in other colours like blue, yellow, orange and green, depending on what other minerals are present in the sample.
There are a couple things to know when distinguishing whether what you have found is in fact quartzite or orthoquartzite.
Firstly, what is orthoquartzite? Well, that’s where it can be a challenge that often requires microscopic examination to determine, as the makeup of the two are so similar.
The main difference is that orthoquartzite has been subjected to much lower heat and pressure.
To be classified as a true quartzite, a sample must contain at least 80% quartz by volume.
- Quartzite is very hard and mostly composed of a mosaic of interlocking quartz crystals. Quartzite will cleanly fracture apart around the grains of the stone and is extremely resistant to chemical weathering.
- Orthoquartzite contains a thin mortar like texture in some spots that is visible under a microscope, and irregular coarse grains of varying sizes. The most notable (and testable in the field) difference is the way orthoquartzite fractures. It fractures across grains rather than around them.
What is an Arkose rock?
Arkose are clastic sedimentary sandstones that consist mostly of quartz and at least 25% feldspar grains.
These arenaceous rocks tend to be found close to granite deposits.
Due to its brittle nature, most of the feldspar is destroyed during the formation process of the rock, or chipped and rubbed away as it is moved along by the earth to its final destination.
Arkose is typically pink or grey in colour (depending on the feldspar the stone contains) and coarse in grain.
Some may associate arkose with red sandstone, which is not actually caused by the presence of feldspar, but rather the mineral hematite.
Hematite covers sand grains, giving them a reddish tone.
Identifying an arkose rock can be easily done by looking for the angular grains of feldspar, which can be spotted without any magnification or other visual aid, and by the coarse grains that make up the rock. It will feel very rough in your hands.
Arkose makes up about 15% of all sandstones, making it rather common to find when out rock hounding.
What is a Greywacke rock?
In its simplest description, greywacke is just a sedimentary mud mixture with rock fragments strewn throughout it, but there is so much more to it than that.
Geologists attribute the formation of Greywacke to submarine avalanches and turbidity currents.
These arenaceous stones are typically found on what is considered to be the edge of continental shelves, or at the base of mountain formations and at the bottom of ocean trenches.
Greywackes are an amalgamation of quartz, feldspar grains, small angular rocks, clay, carbonate and chlorite.
These rocks have also become known as muddy sandstone due to their common hues of grey, brown, yellow and black.
You can find Greywacke in thick or thin layers alongside shale and limestone.
While these rocks contain a wide variety of mineral compositions, the most common tend to be quartz, feldspar, chert, slate, gneiss and schist.
Greywacke is a material that has gone through significant tectonic movement, which often leaves it fractured and deformed.
Fossils, while not commonly found in greywacke formations, can be commonly found in thinner rock beds that are associated with them.
Greywacke is similar in appearance to basalt, but it often contains veins of quartz and other minerals that make it more easily identifiable.
Other minerals that can be found in Greywacke samples include epidote, garnet, tourmaline, pyrite and other trace minerals.
Greywackes are thought to make up 20-25% of the sandstone deposits in the world, predominantly in the Earth’s crust.
That’s a lot of dense information to take in all at once. Let’s break down what we’ve learned to the basics.
Arenaceous rocks are classified as sand-like in both appearance and texture, and contain clastic sedimentary rocks from three main categories, which include Quartzite, Arkose and Greywacke.
- Quartzite deposits are typically exposed and very weather resistant. Samples are usually pink or red, but can also be found in other colors like blue, yellow, orange and green, depending on the trace minerals present.
- Arkose deposits are usually found near granite, have a coarse texture and there are angular feldspar grains visible throughout. Typical colors include pink or grey.
- Greywacke is usually found at the base of mountain formations where tectonic movement has exposed it. It can be found near shale and limestone and typically contains a variety of materials including quartz, feldspar, clay, chert, slate, gneiss and schist. Other materials that can be found in greywackes include epidote, garnet, tourmaline, pyrite and other trace minerals.
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