Dolomite – A Useful Mineral


Dolomite – A Useful Mineral

Dolomite is also known as dolostone and dolomite rock.  It is a sedimentary rock which primarily consists of the mineral dolomite. It is found in sedimentary basins worldwide. Dolomite rock is similar to limestone rock. Both dolomite and limestone rocks share the same colour ranges of white-to-gray and white-to-light brown (although other colours such as red, green, and black are also possible). Both the rocks have approximately the same hardness, and they are both soluble in dilute hydrochloric (HCl) acid.

The original mineral name ‘dolomie’ was given by NT Saussare, in 1792, in honor of the French geologist Deodat Guy de Dolomieu (1750–1801). Dolomite, the rock, contains a large proportion of dolomite the mineral. Ideal dolomite has a crystal lattice consisting of alternating layers of Ca and Mg, separated by layers of CO3 and is typically represented by a stoichiometric chemical composition of CaMg(CO3)2, where calcium and magnesium are present in equal proportions.

Dolomite originates in the same sedimentary environments as limestone i.e. in warm, shallow, marine environments where calcium carbonate (CaCO3) mud accumulates in the form of shell debris, fecal material, coral fragments, and carbonate precipitates. Dolomite is thought to form when the calcite in carbonate mud or limestone is modified by magnesium-rich groundwater. The available magnesium facilitates the conversion of calcite into dolomite. This chemical change is known as dolomitization. Dolomitization can completely alter a limestone into a dolomite, or it can partially alter the rock to form a dolomitic limestone.

Dolomite is a complex mineral. It is relatively a soft mineral which can be easily crushed to a soft powder. The mineral is an anhydrous carbonate mineral consisting of a double carbonate of calcium (Ca) and magnesium (Mg). It is chemically represented by CaMg(CO3)2 or CaCO3.MgCO3. It theoretically contains 54.35 % of CaCO3 and 45.65 % of MgCO3 or 30.41 % of CaO, 21.86 % of MgO, and 47.73 % of CO2 and has a molecular ratio of 1:1.

However, in nature, dolomite is not available in the above exact proportion. The generally accepted custom today is to say that a true dolomite carries over 40 percent MgCO3. Hence, in commercial language, the rock containing 40 % to 45% of MgCO3 is usually called dolomite. Between 40 % and 45 % of MgCO3, the excess CaCO3 is generally carried in solid solution or as minute calcite inclusions in the dolomite. In limestones with much less Mg, however, there often are still recognizable dolomite crystals.

The dolomitization process results in a slight volume reduction when limestone is converted into dolomite. This can produce a porosity zone in the strata where dolomitization has occurred. These pore spaces can be traps for subsurface fluids like oil and natural gas. This is why dolomite is often a reservoir rock which is sought during the exploration for oil and natural gas. Dolomite can also serve as a host rock for lead, zinc, and copper deposits.

Dolomite deposits are normally associated with limestones. This fact, together with the presence of variable amounts of impurities such as silica (SiO2), sulphur (S), iron oxides, and alumina (Al2O3), has an important bearing on the suitability of dolomite for specific applications.

Dolomite is grouped under flux and construction minerals, It has a number of applications. One of its large applications by volume is in construction where it may substitute for limestone. It is an important raw material for iron and steel, ferro-alloys, glass, fertilizer, and many more industries. Dolomite chips are also used in flooring tiles. For some of the applications, dolomite is specifically valued for its Mg content, and chemical composition is all important. In some other applications, its chemical properties (or degree of whiteness) are important. The MgO content is generally expressed after calcination (removal of CO2). However, glass industry normally uses pre-calcined MgO content for control purposes and there is a theoretical maximum of around 21.8 % of MgO. Dolomite when used in construction is a low value commodity, and hence does not normally lend to long transport distances. However, this is not so in case of high quality industrial dolomite.

For many of the  applications in the iron and steel industry there are strict limits on the chemistry of the dolomite used, which mainly needs to be low in SiO2 (frequently less than 1 % or even less than 0.55 %) in addition to low in sulphur (less than 0.1 %) and phosphorus (less than 0.02 %). In glass making industry, the iron content of dolomite is considered as serious impurity since it affects the manufacture of colourless glass.

The long association with the iron and steel industry has made the dolomite mineral as mineral of considerable economic importance.

Dolomite has CAS-number 83897-84-1, EINECS- number 281-192-5 and REACH Registration number 01-2119474891-28. Important characteristics and properties of dolomite are given in Tab 1.

Tab 1 Important characteristics and properties of dolomite
Sl.No. Item Characteristics/properties
1 Category Carbonate minerals
2 Chemical formula CaMg(CO3)2
3 Chemical composition Calcium – 21.73 %, Magnesium – 13.18 %, Carbon – 13.03 % and Oxygen – 52.06 %
4 Molecular weight 184.4 grams/mol
5 Rock Type Igneous, Sedimentary, Metamorphic
6 Strunz classification 5.AB.10
7 Colour Colourless, white, gray, peach, pink, yellow, and orange. Rarely yellow, green, red, and black.
8 Specific gravity 2.8–2.9
9 Bulk density of dolomite chips Around 0.9 ton/cum
10 Melting point More than 2600 deg C
11 Mohs Hardness 3.5 to 4
12 Streak White
13 Luster Vitreous to pearly
14 Fracture Conchoidal
15 Tenacity Brittle
16 Transparency or diaphaneity Transparent to translucent
17 Crystal system Hexagonal
18 Crystal forms and aggregates Common in groups of small rhombohedral crystals, often with curved, saddle-like faces. Also prismatic, (although usually slightly curved), grainy, botryoidal, coxcomb, and massive. Uncommon in large rhombohedrons or rhombohedral aggregates.
19 Twinning Common as simple contact twins
20 Solubility Effervesces in hydrochloric acid. Slowly dissolves in nitric acid and hydrochloric acids.
21 Other characteristics Occasionally fluorescent bluish-white or pink in shortwave ultraviolet light.

 

Calcined dolomite has a density of around 3.5 tons/cum at 20 deg C, bulk density of around 0.85 tons/cum, and pH value in the range of 12-13. Its solubility in water is around 9 %. Melting point of calcined dolomite is around 2800 deg C. It is not combustible or flammable. However, it reacts vigorously with water, and may release heat sufficient to ignite combustible materials in specific instances. It is not considered to be an explosion hazard.

Calcium magnesium oxide reacts exothermally with water to form calcium hydroxide (CaO.MgO + H2O = Ca(OH)2 + MgO + 276kcal/kg CaO). MgO also reacts exothermally with water but the reaction is slow.

The mineral dolomite crystallizes in the trigonal-rhombohedral system. It forms white, tan, gray, or pink crystals. Dolomite is a double carbonate, having an alternating structural arrangement of calcium ions and magnesium ions. Dolomite occurs in a solid solution series with ankerite [CaFe(CO3)2]. When small amounts of iron (Fe) are present, the dolomite has a yellowish to brownish colour. Dolomite and ankerite are iso-structural. Kutnohorite [CaMn(CO3)2] also occurs in solid solution with dolomite. When small amounts of manganese (Mn) are present (upto 3 % of MnO), the dolomite is generally coloured in shades of pink. Kutnohorite and dolomite are iso-structural.

Under practical conditions, dolomite comprises a group of minerals with similar but not identical Mg/Ca ratios, and with differing ideal chemical and lattice compositions. Dolomite mineral shows variation not only in chemical composition but also in the atomic arrangements. Very few, if any, sedimentary dolomites are truly stoichiometric [CaMg(CO3)2]. In practice, dolomite mineral is better represented as Ca(1-x)Mg(1-y)(CO3)2 . Documented compositions range from Ca(1.16) Mg(0.94)(CO3)2 to Ca(0.96)Mg(1.04)(CO3)2, encompassing the spectrum from calcium (Ca) to magnesium (Mg) dolomites.

Thus, the term dolomite describes a mineral series that encompasses a range of chemical variation and lattice structures. It encapsulates an array of natural Ca-Mg carbonates with chemical compositions close to ideal dolomite, but with weak or diffuse X-ray reflections that indicate varying degrees of cation disorder. There are several varieties of dolomite mineral, each with different thermochemical properties depending on the degree of lattice ordering and non-stoichiometry.

Structure of dolomite crystal and bonding of water molecules with the crystal of dolomite are shown in Fig 1.

Fig 1 Structure of dolomite crystal and bonding of water molecules with the crystal of dolomite

When dolomite is heated, the carbonates present in the dolomite are decomposed and CO2 is released. The calcined dolomite has a composition of CaO.MgO. The calcination reaction takes place in two steps. In the first step CO2 is start releasing from the MgCO3 component of the dolomite and in the second step CO2 is start releasing from the CaCO3 component of the dolomite. The decomposition temperature is dependent on the partial pressure of the CO2 present in the process atmosphere. The decomposition of MgCO3 starts at around 550 deg C while the decomposition of CaCO3 starts at around 810 deg C. Since during calcination, CO2 is removed from the raw dolomite, the calcined dolomite (i) is porous (ii) has higher surface area, (iii) has high reactivity, and (iv) is hygroscopic. The calcined dolomite is alkaline in nature.

Uses of dolomite

Dolomite has several uses. Since very early days, it is being used as ornament stone. For some uses, dolomite is specifically valued for its MgO content and chemical composition is all important.  It is being used in raw form, calcined form or in sintered form. It is being used as construction material, in agriculture and in industry. In industry it is being used (i) as a fluxing material, (ii) as a refractory material, and (iii) as a filler material. The major industries where dolomite is being used are iron and steel industry, ferro alloy industry, plastic industry, ceramic industry, glass industry, fertilizer industry, and soap and detergent industry etc.

As construction material, dolomite is crushed and sized for use as a road base material, an aggregate in concrete and asphalt, railroad ballast, rip-rap (armour shorelines, streambeds, bridge abutments, pilings and other shoreline structures against scour and water erosion), or fill. Dolomite is chosen as a construction material due to its increased hardness and density. Asphalt and concrete applications prefer dolomite as filler because of its higher strength and hardness.

In iron and steel industry, the main uses of dolomite are (i) as a fluxing material (ii) for protection of refractory lining, and (iii) as a refractory raw material. Dolomite in iron and steel industry is normally used in three forms. These are (i) raw dolomite which is also the natural form of dolomite, (ii) calcined dolomite, and (iii) sintered dolomite. When dolomite is used as a fluxing material then it is used as either raw dolomite or calcined dolomite. When dolomite is used for the protection of refractories, it is used in calcined form and when dolomite is being used as a refractory raw material, it is used in the form of sintered dolomite.

In the chemical industry, dolomite is used as a source of MgO. In agriculture, dolomite is used as a soil conditioner and as a feed additive for livestock.

Dolomite has been used as a minor source of magnesium, but today most magnesium is produced from other sources. Calcined dolomite is also used as a catalyst for destruction of tar in the gasification of biomass at high temperature.

Dolomite is relatively soft and can easily be crushed to soft powder. Dolomite powder is used in agriculture to reduce soil acidity and also to adjust magnesium deficiency. Magnesium is an important plant nutrient. Dolomite powder is used as filler in various products like detergent, paints, rubbers, agriculture products, animal feed,  putty, and other adhesives and sealants,.  It is used in the manufacture of plastics as well as ceramic tiles. Dolomite is finely ground to precise size specifications. Pure white (high brightness) filler grades are preferred but these are rare.

Another important use of dolomite is in glassmaking industry. Most commercial glasses consist essentially of silica together with soda (Na2O) and lime (CaO), the lime being partly replaced by magnesia for the same purpose. Lime is introduced into the glass melt as limestone and magnesia by adding dolomite. However, in flat gas industry, most lime is introduced with the dolomite and only a small amount of limestone is used to balance the CaO/MgO ratio. Lime and magnesia improve the durability of glass but magnesia also inhibits the devitrification process, which is particularly important for the manufacture of flat glass.

Dolomite is also used in container glass. A critical property of the glassmaking grade of dolomite is the iron content, since it is considered a serious impurity in the manufacture of colourless glasses. Very low content of Fe2O3 is needed for the manufacture of colourless glass containers and, sometimes, low iron flat glasses. In contrast to silica sand, mineral processing cannot effectively lower the iron content of dolomite.