They are the salts of carbonic acid or esters with the group ROC (= O) -O-R ‘.
Composition of carbonates
Carbonate minerals contain the anionic complex (CO 3) 2, which has a triangular coordination, that is, with a carbon atom in the center and an oxygen atom in each corner of an equilateral triangle.
These anionic groups are tightly bound and do not share oxygen atoms with each other.
Triangular carbonate groups are the basic building units of all carbonate minerals and are largely responsible for the particular properties of the class.
Carbonates are frequently identified using the Acid Effervescence Test.
The reaction that results in the characteristic effervescent effect, 2H CO 2/3 → H 2 O + CO 2, makes use of the fact that the carbon-oxygen bonds of the CO 3 groups are not as strong as the corresponding carbon.
Division of common anhydrous carbonates
Common anhydrous carbonates are divided into three groups that differ in type of structure:
The copper carbonates azurite and malachite are the only notable hydrated varieties
The members of the calcite group given in the table share a common type of structure.
It can be considered as a derivative of the NaCl structure in which CO 3 groups replace chlorine ions and calcium cations replace sodium cations.
As a result of the triangular shape of the CO 3 groups, the structure is rhombohedral rather than isometric as in NaCl.
The CO 3 groups are in planes perpendicular to the triple caxis, and the calcium ions occupy alternate planes and are attached to six oxygen atoms of the CO 3 groups.
Members of the calcite group exhibit perfect rhombohedral cleavage.
The CaCO 3 composition most commonly occurs in two different polymorphs: rhombohedral calcite with calcium surrounded by six closest oxygen atoms and orthorhombic aragonite with calcium surrounded by nine closest oxygen atoms.
When CO 3 groups combine with large divalent cations (generally with ionic radii greater than 1.0 Å), orthorhombic structures are produced.
This is known as the aragonite type of structure.
Members of this group include those with large cations: BaCO 3, SrCO 3, and PbCO 3. Each cation is surrounded by nine nearest oxygen atoms.
The aragonite group shows a more limited solid solution than the calcite group.
The type of cation present in aragonite minerals is largely responsible for the differences in physical properties between members of the group.
Specific gravity, for example, is roughly proportional to the atomic weight of metal ions.
Dolomite [CaMg (CO 3) 2], kutnahorite [CaMn (CO 3) 2] and ankerite [CaFe (CO 3) 2] are three isostructural members of the dolomite group.
The dolomite structure can be considered as a calcite-like structure in which magnesium and calcium cations occupy the metal sites in alternate layers.
Calcium (Ca 2+) and magnesium (Mg 2+) ions differ in size by 33 percent, and this results in the ordering of the cations with the two cations occupying separate and specific levels in the structure.
Dolomite has a calcium to magnesium ratio of approximately 1: 1, which gives it an intermediate composition between CaCO 3 and MgCO 3.
Other relatively common carbonate minerals serve as metallic minerals: siderite, for iron; rhodochrosite, for manganese; strontianite, for strontium; smithsonite, for zinc; conerita, for barium; and cerussite, for lead.
Carbonate minerals other than simple carbonates include hydrated carbonates, bicarbonates, and compound carbonates that contain other anions in addition to carbonate.
The first two groups include nahcolite, trona, natron, and shortite; they typically occur in sedimentary evaporite deposits and as low-temperature hydrothermal alteration products.
Members of the third group generally contain rare earth elements and almost always result from hydrothermal disturbances at low temperatures.
Examples of these carbonate minerals are:
Carbonates and health
There are several carbonates applied in the field of health and food.
Some of the most used are:
- Calcium carbonate.
- Sodium carbonate.
- Magnesium carbonate.
- Potassium carbonate.
Calcium carbonate is a dietary supplement that is used when the amount of calcium taken in the diet is not enough.
The body needs calcium for healthy bones, muscles, nervous system, and heart.
Calcium carbonate is also used as an antacid to relieve heartburn, acid indigestion, and an upset stomach. It is available with or without a prescription.
Sodium carbonate reduces stomach acid. It is used as an antacid to treat heartburn, indigestion, and an upset stomach.
Sodium carbonate is a very fast acting antacid. It should be used only for temporary relief.
If you need to treat long-term heartburn problems (such as peptic ulcer disease, GERD), talk to your doctor about other medications.
Although these carbonates can intervene satisfactorily in some health conditions, it is essential to talk with your doctor before using them, since like all medicines, they can have unwanted side effects that can seriously affect your health.