Metal carbonates generally decompose on heating, liberating carbon dioxide from the long term carbon cycle to the short term carbon cycle and leaving behind an oxide of the metal. This process is called calcination, after calx, the Latin name of quicklime or calcium oxide, CaO, which is obtained by roasting limestone in a lime kiln.
Although the carbonate salts of most metals are insoluble in water, the same is not true of the bicarbonate salts. In solution this equilibrium between carbonate, bicarbonate, carbon dioxide and carbonic acid changes constantly to the changing temperature and pressure conditions. In the case of metal ions with insoluble carbonates, such as CaCO3, formation of insoluble compounds results. This is an explanation for the buildup of scale inside pipes caused by hard water.
It is generally thought that the presence of carbonates in rock is strong evidence for the presence of liquid water. Recent observations of the planetary nebula NGC 6302 show evidence for carbonates in space, where aqueous alteration similar to that on Earth is unlikely. Other minerals have been proposed which would fit the observations.
Classification Two classification schemes are in common use by those who work on carbonate rocks. Although you will use only the Folk classification in lab, you should also become familiar with the Dunham classification since it is widely used as well. Folk Classification- The Folk classification, which we will use in lab, is shown below. The classification divides carbonates into two groups. Allochemical rocks are those that contain grains brought in from elsewhere (i.e. similar to detrital grains in clastic rocks). Orthochemical rocks are those in which the carbonate crystallized in place. Allochemical rocks have grains that may consist of fossiliferous material, ooids, peloids, or intraclasts. These are embedded in a matrix consisting of microcrystalline carbonate (calcite or dolomite), called micrite, or larger visible crystals of carbonate, called sparite. Sparite is clear granular carbonate that has formed through recrystallization of micrite, or by crystallization within previously existing void spaces during diagenesis.
Textures Textures of carbonate rocks are extremely variable. Textures can vary from those similar to clastic sediments, showing characteristic grain sizes, sorting, and rounding, to those produced by chemical precipitation. In carbonates the matrix can range from fine grained carbonate mud to crystalline calcite or dolomite. But carbonates can also show textures derived from the growth of living organisms. Many limestones (carbonate rocks in general) show characteristics similar to those of clastic sediments, like sandstones. Sandstones are composed of sand grains, a mud or clay matrix, and a crystalline cement produced during diagenesis. Similarly carbonate rocks are composed of allochemical grains (grains produced by precipitation somewhere else and transported, usually short distances, to the depositional site), mud matrix, consisting of fine-grained carbonate minerals, and a crystalline cement of calcite (or dolomite) precipitated during diagenesis. From the figure shown here, one can see that the average sandstone and mudrock have similar proportions of analogous constituents to average sparry (crystalline) limestones and micritic (fine grained crystalline limestones. This suggests a similarity of processes involved in the formation of clastic sediments and carbonate rocks. Grains in Carbonate Rocks - The grains that occur in carbonate rocks are called allochemical particles or allochems. They are grains often precipitated by organisms that formed elsewhere and became included in the carbonate sediment. Because calcite and aragonite, the main biochemical precipitates, are soft and soluble in water, the distance of transport is usually not very far. Unlike clastic sediments, the degree of rounding and sorting of the grains may not be a reflection of the energy of the transporting medium, but may be biologically determined. For example some organisms produce particles that already have a rounded shape. If many of the same size organisms die at the same place, then the grains may be well sorted. Grains found in carbonate rocks are as follows:
Carbonates on large Solar System bodies like Earth and Mars (the latter represented by the meteorite ALH84001) form through the weathering of silicates in a watery (CO3)2- solution. The presence of carbonates in interplanetary dust particles and asteroids (again, represented by meteorites) is not completely understood, but has been attributed to aqueous alteration on a large parent body, which was subsequently shattered into smaller pieces. Despite efforts, the presence of carbonates outside the Solar System has hitherto not been established. Here we report the discovery of the carbonates calcite and dolomite in the dust shells of evolved stars, where the conditions are too primitive for the formation of large parent bodies with liquid water. These carbonates, therefore, are not formed by aqueous alteration, but perhaps through processes on the surfaces of dust or ice grains or gas phase condensation. The presence of carbonates which did not form by aqueous alteration suggests that some of the carbonates found in Solar System bodies no longer provide direct evidence that liquid water was present on large parent bodies early in the history of the Solar System.
Cyanobacteria have affected major geochemical cycles (carbon, nitrogen, and oxygen) on Earth for billions of years. In particular, they have played a major role in the formation of calcium carbonates (i.e., calcification), which has been considered to be an extracellular process. We identified a cyanobacterium in modern microbialites in Lake Alchichica (Mexico) that forms intracellular amorphous calcium-magnesium-strontium-barium carbonate inclusions about 270 nanometers in average diameter, revealing an unexplored pathway for calcification. Phylogenetic analyses place this cyanobacterium within the deeply divergent order Gloeobacterales. The chemical composition and structure of the intracellular precipitates suggest some level of cellular control on the biomineralization process. This discovery expands the diversity of organisms capable of forming amorphous calcium carbonates.
In plastics, Imerys engineered calcium carbonates improve the quality of breathable films for diapers, hygiene, medical and roofing applications. They are excellent reinforcement agents maximizing mechanical properties in PVC building profiles and flooring.
In paper production, Imerys calcium carbonates are cost-effective filler substitutes for wood fiber where they improve brightness and paper optics. As paper and paperboard coatings, they enhance whiteness and opacity as well as the printability of the finished paper.
In industry, Imerys limes are valued raw materials and process enablers for the steel industry, sugar industry, water treatment and the building industry. In oil and gas production, ground calcium carbonates reduce filtration loss in drilling fluid formulations.
Analysis of biogenic carbonates by EA-IRMS versus GasBench-IRMSThe SIF does not regularly analyze biogenic carbonates by EA-IRMS due to poor combustion of the inoganic matrix resulting in poor data precision. Instead, please consider the analysis of biogenic apatite and carbonates by GasBench-IRMS.
Removing carbonates from calcareous soils and sediments before organic 13C analysisInorganic C in the form of carbonates can interfere with the measurement of organic 13C in soils. In low carbonate materials (less than 3% carbonate), you can remove inorganic C by acid fumigation. Weigh soil samples into silver capsules (tin is corroded when exposed to acid) and arrange samples in a 96-well tray. Add a small amount of water to each open capsule to wet the soil. Place the whole 96-well tray in a desiccator containing a beaker of concentrated (12M) HCl. Carbonates are released as CO2 in 6 to 8 hours. Dry the samples at 60 C and carefully crimp-seal the capsules. The capsules become brittle after drying, resulting in leaks; be careful not to lose material when crimping. We recommend placing the whole capsule into a new tin (Sn) capsule and crimp it closed. The additional tin capsule is an important combustion catalyst, so it is advantageous to use tin capsules for re-encapsulating leaking samples.
For more information on acid fumigation to remove carbonates, please refer to: Harris, D., Horwath, W.R., and van Kessel, C., 2001. Acid fumigation of soils to remove carbonates prior to total organic carbon or carbon-13 isotopic analysis. Soil Science Society of America Journal 65: 1853-1856.
The GPM Carbonates module enables geoscientists to simulate the growth and diagenesis of carbonates across a range of depositional settings such as carbonate platforms and reefs. Users have access to the parameters that control growth and diagenetic alteration, including erosion and redistribution of carbonate material, compaction, tectonics, and wave action. Carbonate growth can be modeled as dependent on several factors, such as water depth, wave energy, and suspended sediment. All these factors can be combined additively or multiplicatively. Reworked carbonates can also be included and differentiated in the model. Additionally, users have access to processes for modeling sediment diffusion and sediment accumulation along with several output properties for quality checking of the results.
Salts or ions of the theoretical carbonic acid, containing the radical CO2(3-). Carbonates are readily decomposed by acids. The carbonates of the alkali metals are water-soluble; all others are insoluble. (From Grant & Hackh's Chemical Dictionary, 5th ed)