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Something just clicked in my head today. Rocks, by the definition I was taught, are composed of minerals which in turn are by definition not organic, so they are or were not alive.

However. Coal, bitumen, and shale are all made of organic matter. Why are they classified under sedimentary carbonaceous rocks?

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  • $\begingroup$ Re not being alive, consider limestone & diatomite, composed of parts of once-living creatures. $\endgroup$ – jamesqf Jun 28 at 4:35
  • $\begingroup$ No-one has pointed out that not all coal is sedimentary : Anthracite is metamorphic. $\endgroup$ – Spencer Jun 28 at 5:59
  • $\begingroup$ the chemocal formula of coal is mineral not organic. limestones and shells and diatoms are arguably more organic than coal. google.com/… $\endgroup$ – com.prehensible Jun 30 at 4:53
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I'm not sure why you are including shale in your question, unless you are referring to carbonaceous shale, which in not coal, but shale with carbon throughout the matrix of the shale.

Shale is an argillaceous rock, and argillaceous rocks are "detrital sedimentary rocks" [1]. They are composed of clays and the bedding plane within the shales is "due to the orientation of the clay minerals" [1]. Clearly a sedimentary rock.

Coal however, as you state in your question is derived from organic material - vegetation. The deposition of the vegetation material is analogous to the deposition of minerals that formed sedimentary rocks.

Both materials, vegetation and minerals, were transported to a location. In the case of minerals the transportation may have been a long or short distance and carried out by wind, water or gravity (mudslides). Vegetation tends to travel shorter distances than minerals but like the minerals it has traveled and it accumulates.

Over time the vegetation compacts, is heated, undergoes chemical reactions and experiences a very slow form of metamorphosis and petrification.

Being composed of carbon, coal forms a carbonaceous deposit. Having been transported and accumulated in a single deposit it is sedimentary. Having undergone metamorphosis and petrification it is a rock. Consequently it is reasonable to classify coal as a carbonaceous sedimentary rock.

[1] D. G. A Whitten with J. R. V. Brooks, Penguin Dictionary of Geology, 1979

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  • $\begingroup$ It is pragmatic and useful as you point out to consider coal a sedimentary rock. All coal will contain a small fraction of mineral matter and most if not all shale will contain a fraction of organic matter. It is a continuum. $\endgroup$ – haresfur Oct 13 '15 at 0:29
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Rocks are not necessarily made out of minerals. Yes, most of them are but it's not a requirement. Also, not every collection of minerals is a rock. For instance, sand is not a rock.

This is not limited to rocks of biogenic origin. Take for example obsidian, or volcanic glass. In some cases there is not a single mineral in the rock, yet it is a rock.

My own personal opinion about definitions (especially in geology) is that occasionally they're useless. What happens if you have a borderline case that falls out of the strict definition? Remember - names and definitions are an artificial thing that we, humans, invented. Nature doesn't always play by our rules.

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  • $\begingroup$ My definition of a rock is that it's one or more mineraloids hardened together. That allows for amorphous silicates to be rocks, at least. $\endgroup$ – person27 Apr 14 at 18:04
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Coal as a sedimentary rock:

Sedimentary rocks are formed from solid debris and dissolve minerals matter produced by mechanical and chemical breakdown of pre-existing rocks or in some cases from skeletal material of dead plants and animals.

Source: C.D. Gribble, 2012: Rutley's Elements of Mineralogy. 27th ed. Springer Netherlands, 496pp

The sediment in an organic sediment rock is made of fossils.

Plants remain squashed deep underground over millions of years make an organic sedimentary rock called coal.Coal doesn't look like it is made of sediments.It is also often difficult to seee the plant fossils within it because they have become so compacted over time,the less stable materials have left and all that remains is carbon.

Source: https://answers.yahoo.com/question/index?qid=20091201222522AAFc6hi

And coal is carbonaceous material.

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Of the three categories of rocks, sedimentary, igneous and metamorphic, sedimentary is the closest type that coal could fit into. Coal was not derived from molten material, so is not igneous by definition. It was not transformed under temperature and pressure from a pre-existing rock into a metamorphic rock. Coal was lithified by compressive forces from loose material at relatively low temperature. This is most similar to how sedimentary rock is formed, which is generally the compaction and cementation of inorganic material. The primary difference between coal and other sedimentary rocks is that by definition "rock" is an aggregate of minerals (minerals being inorganic solid material of specific chemical formula and crystaline structure), whereas coal is made of plant material primarily. Plants are not minerals, but geologists have made an exception for coal, since it is a natural earth solid material that is best characterized as a rock versus anything else we could classify it as, such as life, water, or air.

INTRODUCTION TO TYPES AND CLASSIFICATION OF ROCKS by Geoffrey Mibei Geothermal Development Company

This purely organic sedimentary rock that forms mainly from plant debris. The plant debris usually accumulates in a swamp environment. Coal is combustible and is often mined for use as a fuel.

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