Arkose is a clastic or detrital sedimentary rock that is characterized by being rich in feldspar minerals, among them the most common are orthoclase, microcline and plagioclase. This sedimentary rock is usually derived from the physical weathering and erosion of nearby granitic rocks, so it is generally coarse-grained and can have pink to gray colors depending on the minerals it contains.
Finally it can be mentioned that arkose It is also known as feldspathic sandstone or arcose sandstone (from the group of sandstones) all in the group of sandstones rich in feldspars, at first sight it can be confused with granite.
Some properties
Some important properties of this sedimentary rock are shown in a table below.
| Rock type | detrital sedimentary |
| Source | detrital sedimentary |
| Texture | clastic; Medium grain (0.06 – 2 mm) |
| Composition | Feldspars and Quartz |
| Color | Pink, reddish brown, reddish gray |
| geological environment | alluvial fans, fluvial |
Types of arkoses
In the Pettijohn classification of sandstones, the different types of feldspar-rich sandstones called arcosas. It can be deduced that there are three types of arkose: lithic arkose, the arkose itself and the subarcose, all of them are within the group of arcosic sandstones or feldspathic sandstones.
The rocks rich in feldspars, that is to say, the arkose as such, are considered those sedimentary rocks rich in feldspars that, in terms of total composition, exceed 25% of the mineral content.
Arkose = rock with more than 25% feldspars.
In addition, it must be considered that the percentage of matrix must be less than 15%.
Arcosa proper
The arcosa as such, it is considered a rock with more than 25% feldspars (orthoclase, microcline and plagioclase), it can also have rock fragments and a high percentage of quartz, in this case the rock fragments usually have less than 5% of the content of the rock.
Lithic arkose
This type of arkose is characterized by having between 5% and 10% lithic fragments (rock pieces) and more than 25% feldspars (orthoclase, microcline, and plagioclase). Quartz can also dominate with even 50% of its total mineral composition.
Subarcosa
Basically, the subarcosa is a quartz-ripple rock, but it has between 5% and 25% of feldspars, so it is considered a sedimentary rock that is moderately rich in feldspars.
Origin, formation and sedimentary environment
Taking into account that the arcosa It is a rock rich in feldspar minerals, mainly orthoclase and microcline, so it must be analyzed from which types of rocks it can come from, obviously the source rocks will be equally rich in feldspars.
So the best candidates to form the arkose is the physical weathering and erosion of granitic rocks, gneiss and even the arkose themselves.
Basically, when these source rocks are lifted or exposed to the environment, they wear away, and all the sediments rich in feldspars are transported mainly by fluvial environments, which is why arkoses are related to alluvial fans and fluvial environments, very common in braided rivers..
The uplift or exposure of the source rocks rich in feldspars can occur due to the action of geological faults.
Distinctive features of arkose
Mineral composition
In the arkose sandstone rock the dominant minerals are quartz and feldspar minerals.
Regarding quartz, this is usually the dominant mineral in the rock, however, feldspar (orthoclase and microcline) usually appear to make up at least 25% of the mineral composition of this rock.
In addition, it should be mentioned that the arkose also usually has parts of some types of rocks in its composition depending on where it comes from.
Other notable minerals that make up arkose are micas, amphiboles, rock fragments, clay minerals, heavy minerals, etc. Regarding the micas (biotites, muscovite and also chlorites) they tend to be larger than other grains and are found parallel to the stratification. Clay minerals (mainly kaolinite) presumably come from interperism and chemical weathering of feldspar grains.
Although arkose contains micaceous and clay minerals, these are generally not taken into account unless they exceed 5% of its composition.
Texture
The arkosa has a detrital texture with a grain size that enters the group of sands between 0.06mm and 2mm, that is to say that if you observe with a magnifying glass you will even be able to define the minerals and pieces of rock that make up this sedimentary rock..
Arkose colors
This rock, because it has a high content of feldspars, usually appears with a pinkish color similar to granite. In fact, at first glance it can sometimes be confused with granite, however, it must be differentiated taking into account the texture that in the case of the arcosa is clastic.
In addition, if the rock matrix is clayey, this can give it reddish tones when there is a high iron content, or greenish tones when there is a high chlorite and biotite content, yellow tones and sometimes whitish due to other types of clays.
Uses
The arcosa does not usually have very interesting uses, however we highlight the following:
- It can be used as low to medium quality ballast on second and third order highways, on railway tracks, and as a foundation for country houses.
- If the arkosa has a texture similar to granite and has sufficient strength, it can be used in the construction industry to create tiles and coverings for walls, floors and sculptures.