Limestone is a carbonate sedimentary rock that consists predominantly of calcite [CaCO3]. Limestones are the commonest rocks that contain non-silicate minerals as primary components and, even if they represent only a fraction of all sedimentary rocks (about 20 – 25%), their study is fundamental to understand past environments, climate, and the evolution of life. Limestones also represent the primary reservoir rocks for oil and natural gas. They are subject to dissolution in acidic waters and this is why caves and other karstic features commonly develop in limestones, making them important aquifers.
Calcite (and its polymorph aragonite) is slightly soluble in water, where it dissociates to Ca2+ and (CO3)2-, following the reaction:
Ca2+ + (CO3)2- ⇌ CaCO3
The solubility of calcium carbonate increases in acidic waters, containing dissolved atmospheric CO2 in the form of carbonic acid (H2CO3). CO2 dissolves in water following the reaction:
CO2(gas) + H2O(liquid) ⇌ H2CO3
H2CO3 ⇌ H+ + HCO3–
HCO3– ⇌ H+ + CO32-
Several parameters control these reactions. In first order, CO2 – and hence, carbonates – are less soluble at high temperature. Indeed, modern carbonate sediments form mostly in tropical seas. Higher pressures favor the solubility of CO2 and for this reason carbonates precipitate more easily in shallow waters. Finally, the removal of CO2 from water causes calcium carbonate to precipitate. This can happen due to processes such as photosynthesis (promoting carbonate deposition in shallow waters, reached by sunlight), but also when water is agitated. For example, waves cause water to mix with air and CO2 to escape, causing carbonate precipitation. Organisms have learned how to take advantage of these processes and the availability of calcium carbonate at ambient conditions to produce shells and skeletal parts of calcite or aragonite.
As a result of all these processes combined, calcium carbonates sediments may form in several ways due to several biological and physical processes:
- Accumulation of the remains of organisms or active construction of carbonate structures (e.g. reefs) due to the activity of some organisms.
- Deposition of carbonate grains produced by inorganic and organic processes (allochems, see below).
- Deposition of carbonate grains deriving from erosion/reworking of pre-existing carbonate sediments and rocks (clastic limestones).
- Chemical precipitation due to organic/inorganic processes or evaporation.
This complexity results in a wide range of depositional textures in limestones.
Carbonate sedimentary rock
• skeletal grains (fossils)
• grain aggregates
• calcilutite, calcarenite, calcirudite…
• mudstone, wackestone, packstone, grainstone
• biomicrite, biosparite…
Recognition of limestone
Limestone can be recognized easily thanks to its effervescent reaction with hydrochloric acid (HCl). Calcite and aragonite, indeed, react with HCl diluted in water at 10% producing CO2, according to the reaction:
CaCO3 +2HCl⇌ CO2+ H2O+CaCl2
Dolomite also reacts with HCl but at such low dilution (10%), the reaction is very slow and does not produce the characteristic fizz (warning: dolomite will fizz with more concentrated HCl solutions, around 30% HCl). The reaction between limestones and HCl 10% etches away calcite and aragonite, leaving behind the eventual insoluble residuum, which may consist of dolomite, quartz (silica), or/and clay minerals. Quartz and dolomite can be recognized with a hand lens: quartz is transparent, whereas dolomite tends to form tiny rhomboedrons. The presence of clay minerals causes the water left after the reaction to become muddy.
Most limestones consist of very fine-grained crystalline material and hence tend to break along conchoidal fractures, which are commonly decorated by an irregular patina of carbonate material. The appearance of broken surfaces is similar to siliceous rocks like chert, but the hardness of limestone is much lower (4 on the Mohs scale, softer than metal) and siliceous rocks do not react with HCl. Limestones outcrops are often covered with karst features.
Components of limestones
Folk (1959, 1962) separated the components of limestone in two wide categories: allochemical and orthochemical components. Allochemical grains or allochems (from the Greek allos, ‘from outside’) are carbonate grains that were transported a short distance after their formation and deposited. They are the ‘carbonate counterparts’ of the grains of clastic sedimentary rocks. Orthochemical components or orthochems are the components formed in the site of deposition or that experienced very limited transport. Orthochems include microcrystalline carbonate matrix or carbonate mud, called micrite (< 4 µm diameter), and the crystalline calcium carbonate cement, sparry cement or spar, that crystallized in the pore spaces of limestones.
Skeletal grains are fossils, fragments, and remains of organisms, also known as bioclasts. The identification of the association of organisms in a carbonate rock is fundamental to understand its age and the environmental conditions where it formed.
Ooids are coated grains consisting of a nucleus and several concentric layers of calcium carbonate with an external nearly spherical shape and a maximum size of 2 mm. The nucleus of the ooid can be anything, from a grain of sand to the fragment of an organism. They form in high-energy environments subject to waves. When waves transport a grain in waters saturated with calcium carbonate, the sudden drop in pressure and mixing of water and air promotes the deposition of a layer of calcium carbonate. The process continues wave after wave producing several layers. When the grain is big enough (around 2 mm), waves are not able to carry it anymore in suspension and the process of coating that produced several layers stops. For this reason, ooids commonly show nearly the same size in lime sediments. Ooids form mostly in shallow waters < 5 m deep.
Pisoids are another type of coated grains that forms over a nucleus and are morphologically very similar to ooids. However, pisoids are bigger and can reach the size of several centimeters. They form due to a variety of processes, like chemical precipitation in karstic environment (cave pearls) or fluctuations of the water table in the ground, etc.
Oncoids are concentric structures up to several cm in size that form due to encrustation of algae caused by photosynthesis. Oncoids are different from ooids and pisoids because they are strongly irregular and can contain cavities (which is where the algal mat lived), eventually filled by spar or micrite. Many oncoids may roll or break when they are on the seafloor due to currents or waves, producing irregular balls. Their presence indicates formation of the carbonate sediment in the photic zone (shallow waters).
Peloids are fine-grained (0.1 – 0.5 mm) aggregates of carbonate without a clear internal structure with spherical to irregular shape. They originate due to various processes. Some are fecal pellets, others derive from the micritization of other coated grains. They form in low-energy shallow marine waters.
Grain aggregates form when different grains stick to one another, forming lumps and masses of several grains (grapestones, lumps, botroyds). In general, these aggregates form due to encrusting organisms that bind grains together in shallow waters with weak currents.
Limestone clasts: limestones may contain intraclasts and extraclasts. Intraclasts are clasts of carbonate material originated from the same sedimentary unit where they occur. We can recognize them by comparing their texture with that of the rock enclosing them. Several processes can produce them. For example, currents or tides may carry partially solidified lime mud and re-depose it in the same environment. Conversely, extraclasts derive from outside of the sedimentary basin where they deposited. Hence, they show different textures compared to those of the rock when they occur.
Micrite (matrix) is the fine-grained (< 4 µm) fraction of a carbonate rock consisting of consolidated carbonate mud. In hand samples, it is the undistinguishable dull and opaque material between grains (note: some rocks consist entirely of micrite). At the microscope, it is a dull brownish material barely able to polarize light. Micrite consists of aragonite and calcite and forms due to the physical and biological disintegration of other grains and due to chemical deposition. In carbonatic environments, grains produce micrite when they are abraded one against the other or when they dissolve in water. Many organisms actively contribute to the production of micrite and the disintegration of carbonate grains, for example by boring through shells and other carbonate material. Many algae contains very tiny needles of aragonite that disperse in the sediment when the algae dies. In deep marine environment, micrite consists of the remains of microscopic organisms with carbonate shells (nanoplankton). Micrite can deposit only in low-energy environments.
Spar (cement) results from the precipitation of carbonate crystals in the porosities of a carbonate rock. In hand samples and with the help of a hand lens, spar is recognizable because it is crystalline and can be transparent. Spar can also form due to recrystallization of original micrite. Therefore, it is necessary to use the microscope to distinguish primary spar (cement) from secondary spar (or microsparite).
Classification of limestones
Classification based on grain size
Grabau (1903, 1904) proposed a classification of limestones based on grain size, which parallels the classification of siliciclastic rocks. This classification considers only the size of the components of carbonate rocks, irrespective of their origin, defining three classes of limestones:
Calcirudite consisting of grains > 2 mm (lime-rubble) and representing the carbonate parallel of a conglomerate. Several authors use calcirudite irrespective of the rounding of grains, but some authors use lime-breccia for limestones with angular clasts.
Calcarenite is a limestone with sand-sized (0.0625 – 2 mm) carbonate grains (lime-sand).
Calcilutite is fine-grained limestone (< 0.0625 mm; lime-mud). Some authors even split calcilutites in calcisiltite (0.004 – 0.0625 mm) and micrite (< 0.004 mm).
Pros and cons:
– easy classification in the field
– does not require knowledge of the components of the rock
– useful for reworked and redeposited clastic limestones (e.g. calciturbidites)
– no information on the components of the carbonate rock
Dunham (1962) classifies carbonate rocks based on their texture and presence of mud/micrite. This classification requires the recognition of matrix-supported, grain-supported, and massive textures, defining five rock types.
Mudstone: a limestone with less than 10% grains (allochems) over micrite. Typical of low-energy environments.
Wackestone: a limestone with more than 10% surrounded by micrite (matrix-supported).
Packstone: a grain-supported limestone with interstices filled by micrite.
Grainstone: a grain-supported limestone that lacks micrite/mud. Interstices may be empty or filled predominantly by spar. Grainstones forms only in high-energy environments (beach, bar, etc.).
Boundstone: a collective term for all massive limestones built by reef-forming organisms (corals, stromatolites, bryozoans, etc.) and whose texture is the result of the biological activity of these organisms that built skeletal parts and bound together various components when they were alive (just think of the Great Barrier Reef).
Pros and cons:
– provides information on the depositional texture of the rock
– suited for rock descriptions of hand samples or field descriptions
– no information on the components of the carbonate rock
The classifications by Folk (1959, 1962), modified by Kendall (2005), consider the texture and components of carbonate rocks. The carbonate rock is given the root name -sparite if it contains over 50% spar (cement) over micrite and -micrite if it contains more than 50% micrite (mud) over sparite. The rock is then classified according to the most abundant allochem present, e.g. biosparite if it contains bioclasts and sparite > 50%, oomicrite if it contains ooids and micrite > 50%, and so on. According to this classification, a micrite is a rock lacking allochems (i.e. only carbonate mud), while a dismicrite contains micrite and sparse patches of spar. The boundstone of Dunham corresponds to a biolithite in the classification by Folk.
The classification can be further refined based on the proportion of allochems and the percentage of micrite and sparite. The diagram below shows an example for a biomicrite/biosparite.
Pros and cons:
-classification dense of textural and compositional information
-requires a microscope (not suited for field descriptions or hand samples)*
*with some effort it can be used also in the field
Classification of Embry & Klovan
Dunham’s and Folk’s classification do not classify boundstones (or biolithites). The classification of Embry & Klovan (1971) expanded the classification by Dunham to include other five categories divided in allochtonous rocks, with > 10% grains of > 2 mm grain size that were not bound together at deposition, and autochthonous rocks, formed by components bound together at deposition.
Floatstone is a matrix-supported limestone with grains > 2 mm. It is the coarse-grained equivalent of the wackestone of Dunham.
Rudstone is a grain-supported limestone with grains > 2 mm. It is the coarse-grained equivalent of the packstone and grainstone of Dunham.
Bafflestone is a boundstone consisting of organism that trap carbonate sediment acting as barriers.
Bindstone is a type of boundstone produced by encrusting organism that trap sediments in layers (e.g. stromatolite).
Framestone consists of a network or framework of structures built by organisms where sediments is trapped in cavities (e.g. coral reef).
Classification of crystalline carbonates
Carbonates are some of the minerals that more easily recrystallize during burial and diagenesis of the sediment, sometimes very shortly after deposition. Recrystallization destroys original sedimentary textures, including spar, micrite, and allochems, developing crystalline limestones, whose texture testifies this process. Here I show the classification of these rocks by Friedman (1965), who considers the texture and relative size of crystals. Other classifications consider also the grain size of crystals.
Which classification should I use?
Apart from some classification schemes that can be used for specific rock types, such as Embry & Klovan for boundstones and Friedman and similar classifications for crystalline limestones, the answer is: it depends. Classifying limestones based on grains size (Grabau) is useful for deposits that are produced only by physical processes like talus breccias or calciturbidites. In this case, carbonate grains are, indeed, arranged by currents and other processes and recognizing allochems and fossils is useful only to obtain information on the source area. On the other, Dunham and Folk are very useful for marine carbonates, whose texture is the result of both physical and biological processes, whose recognition can help us understand the sedimentary environment where they form. Each classification has its pros and cons. Dunham’s classification is very straightforward to use in the field, while Folk’s, even if more precise, often requires the petrographic recognition of all components of a limestone.
Examples of limestone
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