Description
what is Dolomite ? Dolomite is one of our major sources for the concrete so essential to modern society’s road and building infrastructure. Crystals of dolomite are common in hydrothermal vein deposits and in sedimentary rocks, where they fill pores in their host rock. By volume, however, most dolomite occurs in its massive form as dolostone or mixed dolostone/limestone sedimentary rocks. These dolostone rocks originally formed as limestone marine deposits on ancient shallow seafloors that were later altered to dolostone as magnesium-rich waters moved through them. Dolostones that formed from the alteration of limestone rock can retain much of the rock’s original depositional textures, such as fossils, bedding, and other sedimentary features, although sometimes all of this original fabric was lost as the rock recrystallized.
Description and Identifying Characteristics
Most often found as a massive, white to buff or gray, carbonate rock-forming mineral, dolomite is one of the three most abundant carbonate minerals, calcite and aragonite being the other two. Dolomite differs from calcite and aragonite in its crystal structure. In dolomite crystals, layers of carbonate ions alternate with layers of magnesium and calcium ions, rather than only having layers of calcium ions alternate with carbonate ions as in calcite and aragonite. Dolomite crystals usually form transparent to translucent rhombs that are colorless to light-colored, although crystals may be red to brown if iron impurities are present. Some dolomite crystals also exhibit crystal faces that form slightly curved surfaces, rather than flat planes.
Pure samples of dolomite and calcite may have a similar appearance and share many properties, so the easiest way to distinguish them is by their reaction with room temperature dilute acid. Calcite (and aragonite) will readily react with acid to form small bubbles (effervescence). Dolomite will only effervesce if the mineral is ground up into powder (or if the acid is heated). Unfortunately, natural massive samples often consist of a mixture of the two minerals, so it is sometimes difficult to distinguish whether dolomite is present in a mixed massive sample. Iron may also substitute for some of the magnesium in dolomite, so dolomite may grade into siderite, an iron carbonate (FeCO3), although dolomite is far more abundant than siderite.
what is Dolomite ?
In Our Earth: The Geologic Importance of Dolomite
Dolomite forms in hydrothermal veins or as a pore-filling mineral in carbonate rocks, and more rarely as an accessory component in igneous pegmatites or altered mafic igneous rocks. By far though, most dolomite occurs in altered sedimentary marine rocks called dolostones or in marbles formed from the metamorphism of dolostone. Because dolostones are composed primarily of the mineral dolomite, geologists once used the term ‘dolomite’ for both the mineral and the rock. The term is now only used for the mineral, since a dolostone may include other minerals besides dolomite.
Few dolostones are primary in origin. In other words, they did not originally form as dolostone, but instead formed from the alteration of limestone rock as magnesium-rich water moved through the limestone, altering its calcite and aragonite into dolomite. The main exception to this is primary dolomite that forms in evaporitic settings as a relatively late product of seawater evaporation. These primary dolomites are rare though. One of the more unusual primary occurrences of dolomite (where dolomite is precipitated directly from a fluid, rather than forming as an alteration of a pre-existing mineral) occurs in the kidneys of Dalmatian dogs! It appears this geologic peculiarity is unique to Dalmatians, as other dogs do not precipitate dolomite kidney.
Secondary sedimentary dolomites can be broadly separated into two informal groups. Many sedimentary dolomites occur
In Our Society: The Economic Importance of Dolomite
Dolomite shares a broadly similar chemistry with other carbonate minerals like calcite and aragonite, and consequently is used in much the same way. By volume, the most important uses of dolomite are in the production of concrete and as aggregate construction material. Significant amounts of dolomite are also used as dolostone and dolomitic marble building stones and in the manufacture of glass and ceramic glazes.
In industry, dolomite is an important source for magnesium and calcium metals, and is used as a flux for metallurgy. A flux is a material that melts easily and can be used to remove impurities from metal ores or to make the slag produced by metal ore smelting more fluid so it can be disposed of more easily.
Dolomite in the Upper Midwest:
Dolomite is abundant throughout most of the upper Midwest, being the primary mineral comprising most of the Paleozoic carbonate units that cover this region. Thousands of quarries in these rock units provided the bulk of the carbonate used locally for concrete in building and road construction. One of the more common, relatively inexpensive building stones used in the region is a porous dolostone known by the geologically incorrect label of ‘Winona Travertine’. A true travertine is a porous carbonate rock that is precipitated from groundwater at a spring or in a cave. In contrast, the ‘Winona Travertine’ is a dolostone that formed as an alteration of a marine limestone. Its pores are the void spaces left by the dissolution of calcite fossils and fragments. Trace fossils of burrowing organisms also extend throughout this dolostone, giving it a decorative pattern that increases its worth as building stone.
Although dolomite is not as soluble as calcite, the abundance of local dolomite also contributes to the ‘hard’ water problems common to Upper Midwest.
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