What is Dolomite?

• Chemistry: CaMg(CO₃)₂, Calcium Magnesium Carbonate
• Class: Carbonates
• Group: Dolomite
• Uses: in some cement, as a source of magnesium and as mineral specimens.
• Specimens
Dolomite, which is named for the French mineralogist Deodat de Dolomieu, is a common sedimentary rock-forming mineral that can be found in massive beds several hundred feet thick. They are found all over the world and are quite common in sedimentary rock sequences. These rocks are called appropriately enough dolomite or dolomitic limestone. Disputes have arisen as to how these dolomite beds formed and the debate has been called the "Dolomite Problem". Dolomite at present time does not form on the surface of the earth; yet massive layers of dolomite can be found in ancient rocks. That is quite a problem for sedimentologists who see sandstones, shales and lime stones formed today almost before their eyes. Why no dolomite? Well there are no good simple answers, but it appears that dolomite rock is one of the few sedimentary rocks that undergoes a significant mineralogical change after it is deposited. They are originally deposited as calcite/aragonite rich lime stones, but during a process call diagenesis the calcite and/or aragonite is altered to dolomite. The process is not metamorphism, but something just short of that. Magnesium rich ground waters that have a significant amount of salinity are probably crucial and warm, tropical near ocean environments are probably the best source of dolomite formation.
Dolomite in addition to the sedimentary beds is also found in metamorphic marbles, hydrothermal veins and replacement deposits. Except in its pink, curved crystal habit dolomite is hard to distinguish from its second cousin, calcite. But calcite is far more common and effervesces easily when acid is applied to it. But this is not the case with dolomite which only weakly bubbles with acid and only when the acid is warm or the dolomite is powdered. Dolomite is also slightly harder, denser and never forms scalenohedrons (calcite's most typical habit).
Dolomite differs from calcite, CaCO₃, in the addition of magnesium ions to make the formula, CaMg(CO₃)₂. The magnesium ions are not the same size as calcium and the two ions seem incompatible in the same layer. In calcite the structure is composed of alternating layers of carbonate ions, CO₃, and calcium ions. In dolomite, the magnesiums occupy one layer by themselves followed by a carbonate layer which is followed by an exclusively calcite layer and so forth. Why the alternating layers? It is probably the significant size difference between calcium and magnesium and it is more stable to group the differing sized ions into same sized layers. Other carbonate minerals that have this alternating layered structure belong to the Dolomite Group. Dolomite is the principle member of the Dolomite Group of minerals which includes ankerite, the only other somewhat common member.
Dolomite forms rhombohedrons as its typical crystal habit. But for some reason, possibly twinning, some crystals curve into saddle-shaped crystals. These crystals represent a unique crystal habit that is well known as classical dolomite. Not all crystals of dolomite are curved and some impressive specimens show well formed, sharp rhombohedrons. The luster of dolomite is unique as well and is probably the best illustration of a pearly luster. The pearl-like effect is best seen on the curved crystals as a sheen of light can sweep across the curved surface. Dolomite can be several different colors, but colorless and white are very common. However it is dolomite's pink color that sets another unique characteristic for dolomite. Crystals of dolomite are well known for their typical beautiful pink color, pearly luster and unusual crystal habit and it is these clusters that make very attractive specimens.
Dolomite is after calcite the second most important and abundant of the carbonate minerals. Chemically and structurally it may be regarded as calcite with half the calcium ions replaced by magnesium. Iron or manganese may substitute for magnesium in dolomite, forming isostructural series with ankerite and Kutnahorite.
The crystal structure, hexagonal-rhombohedral, is similar to that of calcite, with alternate layers of calcium ions totally replaced by magnesium. This ordered arrangement of cations slightly impairs the overall symmetry of the structure but is essential to the stability of the mineral. Hardness is 4.5-5, specific gravity 2.85, luster vitreous to pearly, color ranges from colorless to white with green, brown, or pink tints, and cleavage is perfect in three directions.
Like calcite, dolomite occurs in virtually all geologic settings: in igneous rocks as carbonatite, in metamorphic rocks as marble, and in hydrothermal deposits. Also like calcite, the most abundant occurrences are in sedimentary rocks; rock composed primarily of dolomite is sometimes referred to as dolostone.
There is uncertainty as to the cause of its formation, as vast deposits are present in ancient rock, but it is very rarely found being produced in modern environments. This is referred to as the "Dolomite Problem". Dolomite accounts for about 10% of all sedimentary rock, including much that would have been produced near the surface of the Earth. However, experiments have only been able to synthesize dolomite under the high temperatures and pressures present in deeper layers.

The Present Scenario About Dolomite

Dolomite at present time, does not form on the surface of the earth; yet massive layers of dolomite can be found in ancient rocks. That is quite a problem for sedimentologists who see sandstones, shales and limestones formed today almost before their eyes.
DOLOMITE is a double carbonate of calium and magnesium, CaCO₃, MgCO₃. The mineral was first identified by Count Dolomien in 1791 and named after its discoverer. It is of sedimentary origin and is supposed to have been formed due to chemical action of sea-water containing high percentage of magnesia, on limestone.

Theoretically, Dolomite Contains

CaCO3	54.35%
MgCO3	45.65%

In Other Words, Dolomite Contains

CaO	30.4%
MgO	21.7%
CO2	47.9%

In nature, considerable variations in the composition of dolomite relating to lime and magnesia percentages are found. When the percentage of CaCO₃ increases by 10% or more over the theoretical composition, the mineral is termed 'calcitic dolomite', 'high-calcium dolomite' or 'lime-dolomite'. With the decrease in percentage of MgCO₃, it is called 'dolomitic limestone'. With the variations of MgCO₃ between 5 to 10%, it is called 'magnesian limestone', and upto 5% MgCO₃ or less it is taken to be limestone for all purposes in trade and commercial parlance.
Dolomite usually contains impurities, chiefly silica, alumina and iron oxide. For commercial purposes, the percentage of combined impurities should not go beyond 7% above which, it becomes unsuitable for industrial use. It is then used only for road ballasts, building stones, flooring chips etc.

Hardness	3.5-4
Associated	include calcite sulfide ore minerals fluorite barite quartz and occasionally with gold
Minerals Chemical/Typical composition	white
Color	often pink or pinkish and can be colorless, white, yellow, gray or even brown or black when iron is present in the crystal
Characteristics	Unlike calcite, effervesces weakly with warm acid or when first powdered with cold HCl
Luster	pearly to vitreous to dull
Field Indicators	typical pink color, crystal habit, hardness, slow reaction to acid, density and luster

Dolomite Physical Characteristics

Dolomite Chemical Analysis

Dolomite Habits

Crystalline - Coarse - Occurs as well-formed coarse sized crystals. Massive - Uniformly indistinguishable crystals forming large masses., Blocky - Rhombohedral - Crystal shape resemb les rhomohedrons.
Associated Minerals include albite, anatase, calcite, chlorite group, fluorapatite, fluorite, galena, gmelinite, marcasite, molybdenite, pyrite, quartz, rutile, siderite and sphalerite
Crystal habits include saddle shaped rhombohedral twins and simple rhombs some with slightly curved faces, also prismatic, massive, granular and rock forming. Streak is white.

The Dolomite Group of Minerals

The Dolomite Group is composed of minerals with an unusual trigonal bar 3 symmetry. The general formula of this group is AB(CO₃)₂, where A can be either calcium, barium and/or strontium and the B can be either iron, magnesium, zinc and/or manganese.
The structure of the Dolomite Group is taken from the Calcite Group structure. The Calcite Group structure is layered with alternating carbonate layers and metal ion layers. The structure of the Dolomite Group minerals is layered in such a way that the A metal ions occupy one layer which is followed by a carbonate layer which is followed by the B metal ion layer followed by another carbonate (CO₃) layer, etc. The layering looks like this: |A|CO₃|B|CO₃|A|CO₃|B|CO₃|A|... This ordered layering of different or nonequivalent ions causes a loss of the two fold rotational axes and mirror planes that are present in the Calcite Group structure. Dolomite's symmetry class is bar 3 whereas the Calcite Group's symmetry class is bar 3 2/m. The loss of symmetry allows only simple crystal forms to be used by the Dolomite Group minerals, mostly rhombohedrons.
Dolomite is a very common mineral and ankerite is much more scarce. The other members are considered rare to very rare. The rarity of the members of this group can be tied to the closeness in radius of the A and B ions. In dolomite the A and B ions are calcium and magnesium which have the largest ionic radius differential of the group (approximately 33%). If the A and B ions are close in radius, then they tend to not segregate as easily into the separate A and B layers, which is required to form this structure and therefore these minerals.

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