Dolomite

The mineral called dolomite is a common sedimentary mineral found on the earth. Dolomite is named after the French mineralogist Deodat de Dolomieu, this rock forming sedimentary mineral occurs in massive beds several hundred feet thick in the earth. Dolomite rocks are frequently encountered in sedimentary rock sequences and are found all over the world in deposits. Appropriately enough, these rocks are called dolomite or dolomitic limestone in the common terminology as dolomite forms the largest component among all the various minerals. Ancient rock strata contain all the present day supply of dolomite in massive layers in the older rocks and the mineral does not form on the surface of the earth in the present day. Dolomite represents a different type of problem for present day sedimentologists who are used to seeing all kinds of sandstones, shales and limestones forming today in natural processes almost in front of their eyes. The question that can be asked is why dolomite does not form naturally these days? The reason seems to be that dolomite rock is one of the few sedimentary rocks that mature only following significant mineralogical change that occurs much later after it’s deposited in an area. Dolomite rocks are originally deposited as limestones rich in calcite or aragonite, they then undergo a transformation process called diagenesis during which the calcite or the aragonite is slowly transformed into dolomite -completing the transformation process. This chemical transformation process is not to be confused with metamorphism; it can be considered to be a process just short of metamorphic transformation of rocks. Dolomite formation probably depends on the presence of magnesium rich ground waters possessing a significant amount of salinity, possibly in warm or tropical areas and near oceanic environments - such conditions might be considered as the best areas for the formation of dolomite.

Deposits of the mineral dolomite also occur in metamorphic marbles, in many hydrothermal veins and replacement deposits besides its common occurrence in sedimentary deposits of all types. Calcite and dolomite are hard to distinguish, the exception being that dolomite has a pink and curved crystal habit. The other notable difference between dolomite and calcite is the much more common occurrence of calcite and its ease of effervescing on the application of acid. Dolomite is quite different as it an only bubble weakly on the application of acid and that too occurs only under warm acid or if the dolomite is powdered down. Compared to calcite the mineral dolomite is a little harder, denser and will never form the structures called scalenohedrons - the most typical habit displayed by the mineral calcite.

The chemical formula of calcite is CaCO3, dolomite differs from this in the addition of magnesium ions and has the chemical formula, and CaMg(CO3) 2-magnesium ions thus primarily make the difference between dolomite and calcite. The size of the calcium ions and the magnesium ions are not the same and the two ions seem incompatible when existing in the same layer in crystals. The chemical structure of calcite is a composition of alternating layers of ions of carbonate, CO3 and the ions of calcium. Magnesium ions present in dolomite occupy one layer and are then followed by a layer of carbonate layer, this is then followed by a layer made only of calcite and this alternation occurs all along the mineral. The question to ask is why these alternating layers are present in the first place? The layers are probably formed due to the very significant size difference between ions of calcium and magnesium; these layers are also more stable in arranging groups of differing sized ions into similarly sized layers along the mineral. All carbonate based minerals which also posses this structure of alternating layers can be considered to belong to the dolomite group of minerals. The mineral dolomite is the principal mineral of the dolomite group of minerals; another mineral ankerite is the only other mineral which can be considered somewhat common in occurrence in dolomite bearing rocks.

The typical crystal habit formed in dolomite are rhombohedrons. However, some of the crystals do curve saddle shaped crystals for some unknown reason, possibly due to a chemical property known as twinning. The term classical dolomite is applied to these unique crystal habits. Some of the crystals of dolomite are not necessarily curved and certain very impressive crystalline specimens of dolomite do display very well formed and sharp rhombohedron structures. Considered to be one of the best examples of a pearly luster, the luster of crystalline dolomite is considered unique. In dolomite, the pearl like visual effect is best observed along the curved crystals when a bean of light sweeps along and across the curved surface of the crystal. Colorless and white dolomites are the most commonly occurring forms of the crystal, though the mineral can be found in several different colors or shades. The most unique characteristic for dolomite is its pink color in some rock layers. Dolomite crystals are famous for their typical appearance in a beautiful pink color, with a lustrous pearly surface and formed in an unusual crystal habit, such clusters of the mineral dolomite in rock layers make for very attractive specimens and are visually appealing.

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