Limestone is a sedimentary rock that is composed of at least 50% calcium carbonate (CaCO3) in the form of calcite, its main origin is biochemical-organic in a shallow marine environment, but it can also be formed by precipitation. chemistry. in evaporitic continental environments.
The main components of the limestone They are calcite (more than 50%), magnesium carbonate and aragonite.
Characteristics and properties of limestone
Chemical and mineralogical composition
mineralogically a limestone it is dominated by calcite (more than 50%), aragonite, and magnesium carbonate, however, there may also be minor amounts of quartz, clay minerals, feldspars, siderite, and some sulfide such as pyrite.
Calcium carbonate can normally be formed by the accumulation of carapaces and shells of living organisms or by direct chemical precipitation from aqueous solutions.
| % dolomite (ore) | % calcite | rock type |
| 0 – 10 | 90 – 100 | Limestone |
| 10 – 50 | 50 – 90 | Magnesian limestone or dolomitic limestone |
| 50 – 90 | 10 – 50 | calcareous dolomite |
| 90 – 100 | 0 -10 | dolomite (dolomitic rock) |
Regarding the chemical composition of limestone, the ratio of calcium (calcite) and magnesium (dolomite) that is in the rock plays a very important role.
In this regard, it should be mentioned that pure limestone theoretically would have more than 95% calcite in its composition and possibly 4% magnesium (dolomite).
While if the rock is between 90% calcite and 10% dolomite, it would be called magnesian limestone.
If calcite is already only between 50% to 90% of the composition of the rock and dolomite already appears between 10% and 50%, then it is called dolomitic limestone.
Finally, if dolomite begins to dominate in the rock and exceeds 50% of the total volume, it begins to be called dolomite.
Limestone texture
The limestone They are very varied due to the different origin and formation that they may have, however, based on the Dunham 1962 classification, the following textures can be defined.
- matrix supported
- grain supported
- crystal clear
In this case the grains are usually fossils, shells or shells of microorganisms, and also oolites, peloids and intraclasts.
While the matrix is generally micrite or calcareous mud. Regarding the crystalline texture, it is crystallized calcareous cement, where no original texture is recognized. Also the crystalline texture may have formed because the rock originated from direct precipitation of calcite.
Structural components
Skeletal grains (bioclasts/fossils) are the dominant constituents of various types of limestone.
The types of skeletal grain present depend on environmental factors during sedimentation (eg, water temperature, depth, and salinity), as well as the state of invertebrate evolution and diversity at that time.
The main groups of organisms that contribute skeletal material are molluscs (bivalves and gastropods), brachiopods, corals, echinoderms (especially crinoids), bryozoans, calcareous algae, stromatoporoids, and foraminifera.
Other groups of minor or local importance are sponges, crustaceans (especially ostracods), annelids (serpulids) and cricoconarids (tentaculites).
Carbonate skeletons (bioclasts/fossils) have different parent mineralogies, and the preservation of bioclasts in the limestone depends on this.
Oolites or ooids are spherical to subspherical grains, usually in the 0.2–0.5 mm size range, but not uncommonly reaching several millimeters in diameter. Structures larger than 2 mm are called pisoids or pisolites.
Ooids consist of concentric shells around a nucleus, usually a carbonate particle or quartz grain. Most modern marine ooids are composed of aragonite, but the ancient ones were generally calcite.
Peloids are elongated to subspherical grains of micrite (lime mud or calcareous mud) generally less than 1 mm in length. They are of fecal origin or altered bioclasts.
Intraclasts are fragments of reworked carbonate sediment.
Many are flakes up to several centimeters long, derived from desiccation of tidal carbonate mud or penetrating erosion, especially by storms.
The micrite is the matrix of many limestone bioclastics and the main component of the limestone fine grained It consists of carbonate particles mostly less than 4 μm in diameter.
Much of modern carbonate mud, the precursor to micrite, is biogenic in origin, forming through the disintegration of carbonate skeletons or grains such as calcareous algae.
Esparit (sparitic calcite) is a transparent, sometimes white coarse cement, equivalently precipitated in the pore space between grains and in larger cavity structures. It is primarily a cement, although it may be a freshwater precipitate near the surface.
Fibrous calcite is also a cement, coating grains and fossils and filling cavities. It is generally of marine origin and is common in reef rocks.
Thin sheet limestone
In a thin slice or under a microscope it will be possible to clearly observe the components of the limestone, for example the characteristic calcareous organisms.
For example, in the image you can see gastropods filled with calcite, which indicates that the limestone has formed in a shallow marine environment, in this case the rock is called Wackestone (see the classification of limestones below).
Origin and formation
The limestone It can be related to two formation environments: the shallow marine (it is the most common and extensive) and the continental environment.
Regarding the marine environment, the limestone They are formed by the accumulation of the shells of microorganisms on the ocean floor and shells.
Microorganisms such as foraminifera take calcium ions (Ca2+) dissolved in seawater to form their shells.
When these marine organisms die, these shells fall to the bottom of the ocean and accumulate for thousands of years.
Thus, those that are already buried hundreds of meters undergo diagenesis and form the different limestone bioclastic or also fossiliferous limestones.
Another example is the coquina, this is formed by the accumulation of shells that are made up of calcium carbonate. These shells are joined with some type of cement and form a coquina.
Regarding the limestone Of continental origin, these are formed mainly by direct chemical precipitation of calcium ions from aqueous solutions to form calcite.
An example is the sequence of evaporites where the calcite is accompanied by gypsum, anhydrite, silvite, halite, etc.
The appropriate environment to form these rocks is coastal lakes, where seawater remains stagnant and calcite precipitates through evaporation and accumulates to form the rock.
These types of rocks are much less extensive than marine ones.
Also, there is another type of limestone that form in caves, are known as travertine (stalactites and stalagmites).
When the water that enters the caves through the fissures in the rock is enriched in calcium ions, when the water evaporates they tend to accumulate from the ceiling of the cave as drops downwards and form stalactites.
If these calcium-enriched drops fall to the ground and the water evaporates, they begin to accumulate upwards and form stalagmites.
Limestone classification
The most widely used descriptive classification to define the type of limestone found in field work is that of Dunham (1962).
For the classification it is necessary to base it on the texture, structural characteristics, the relationships of the primary structural components: grains (intraclasts, granules, pelloids, wrapped grains, bioclasts and skeleton), carbonate mud and calcite cement.
In addition, it is also based on the presence or absence of calcareous mud, the relative proportion of grains and calcareous mud, skeletal signs of organogenic junction on its development, lithification in situ and the position of growth. The system is simple and easy to apply, in the field description with a geologist’s magnifying glass.
There are 6 basic types:
- The limestone crystalline, which does not present a recognizable sedimentary texture, rather it has crystalline carbonate.
- Mudstone containing carbonate mud and less than 10% grains between 0.03 and 2 mm.
- Wackestone, where there are grains between 0.03 and 2mm in a calcareous mud matrix (supported matrix) that does not exceed 50% of the rock volume.
- Packstone containing grains, which touch each other, in the manner of supported grains, but there is still a calcareous mud matrix between the grains.
- Grainstone, does not contain calcareous mud, but only grains that touch each other, and calcite cement between the grains.
- Boundstone, containing primary skeletal components (fossils) united with sedimentation, lithified in their habitat in the growth position or the individual components related to organisms, with sedimentation and formation of biostromas, bioherms or stromatolites.
How to identify limestone?
With the twenty magnification magnifying glass, try to see if you have fossil components in the rock, or if the texture is clearly crystalline and also take into account the following characteristics.
Outcrop
When calcareous rock is associated with a shallow marine environment, it generally occurs over large areas. That is to say, the outcrop of limestone marine environment is usually very extensive.
However, you can also find it interbedded with calcareous sandstones, shales and siltstones in turbidite sequences. They usually occupy strata that vary from a few centimeters and even exceed 2 meters in thickness, and when only calcareous sequences appear, they usually exceed 10 meters in thickness.
The strata are massive and subhorizontal, and when they are affected by tectonism they do not fold, rather they fracture and due to dissolution it is common to observe veinlets of calcite.
Color
The limestone Marinas are usually dark grey, due to the high percentage of calcareous mud that they usually have. However, they can range from light gray to dark gray in color, depending on their composition. They are usually whitish when generated by chemical precipitation.