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.

Allochems
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.

Orthochems
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).