The Mineral Dolomite
Composition: Calcium Magnesium Carbonate
Crystal System: trigonal
Specific Gravity: 2.8
Luster: glassy to pearly
Color: Colorless, white, pinkish, light tints, darker colors even black when iron is present.
Cleavage: perfect in three directions
Transparency: transparent to translucent
Associated Minerals: barite, calcite, fluorite, sulfide ore minerals, quartz and some times with gold.
(Calcium Magnesium Carbonate) The ideal composition of dolomite is 30.4% CaO, 2 1.7% MgO, 47.9% C02. The calcium and magnesium ratio is basically 1:1. A large number of other elements may substitute for the calcium and magnesium and varieties containing manganese, iron, cobalt, lead and zinc are known. When a large amount of iron is present, the mineral ankerite forms and when excess manganese is present the mineral kutnahorite forms instead of dolomite. All these minerals have the same internal structure, but differ chemically from each other. This is known as an isomorphous series of minerals. The chemical structure may vary slightly in either direction and still be called dolomite, even when some iron has also intruded. 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. The slow effervescence in cold acid distinguishes it from calcite which has rapid effervescence and magnesite that effervesces only in hot acid. The intermediate specific gravity will help when a pure piece can be obtained. Most often not fluorescent but some locations do. Glen Brown says as a collector of fl. minerals I know that there are a number of locations where the dolomite is fluorescent; I've found it myself in multiple locations in Utah. I have in my collection a truly amazing piece from Sweden that fluoresces very bright carmen red. You can also read about some different localities of dolomite that fluoresces in Manny Robbins latest book "Fluorescence Gems and Minerals under Fluorescent Light".
Dolomite is a common sedimentary rock-forming mineral that can be found in massive beds known as dolomites. These beds may be several hundred feet thick. Dolomite also forms as sediments in ore veins such as limestone. Limestone where dolomite is present in more than a small amount are called dolomitic limestone. Dolomite is only rarely found in higher temperature metamorphic surroundings.
Dolomite crystals are rarer and as a rule smaller than calcite. Dolomite differs slightly in its crystal form from the other rhombohedral carbonates and does not occur in scalenohedrons. Dolomite usually forms early in a mineral series, and directly coats the wall rock, underlying minerals like calcite, sphalerite, galena, fluorite, celestite, or gypsum, which come later in the depositional sequence. Dolomite can also be prismatic, massive, granular and rock forming.
Effervesces in week hydrochloric acid or vinegar like calcite. Except calcite effervesces easily when acid is applied to it and dolomite 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)
The pearly clusters are particularly common in association with galena, sphalerite, and calcite in low temperature veins from sites in Midwestern quarries of the USA. Especially in the Missouri, Kansas and Oklahoma lead district and areas like our specimens from the Meridian Quarry Black Rock, Lawerence County, Arkansas. Nice dolomite crystals are found in limestone pockets in the quarries at Rochester, New York. Large fine crystals have been found in Switzerland, in pegmatitic seams in North Carolina, and in veins in Colorado. Other locals include Ontario, Canada; Pamplona, Spain and Mexico.
Dolomite is used for manufacturing certain types of refractory bricks used in steel making. The dolomite is heated to a high temperature to drive off the carbonate as carbon dioxide and the remaining material, a mixture of calcium and magnesium oxides, is blended with carbon and other materials and pressed into blocks for the furnaces. The magnesium and calcium oxides have very high melting points and are an excellent, inexpensive refractory material. Dolomite is also used as a source of magnesium oxide for making magnesium metal and for chemical uses, such as the common laxative milk-of-magnesia. Dolomitic limestone's and dolomites are mined along with limestone and used for crushed stone and aggregates for manufacture of pavement, concrete for construction and as fill material. Dolomite is also used in some cements, as a source of magnesium. Of course Dolomite is also used as mineral specimens.
FACTS & HISTORY:
Dolomite was named for the French mineralogist Deodat de Dolomieu.
There are disputes 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. It is thought that these dolomite beds were formed in warm ocean like environments which created a process similar to metamorphism called diagenesis.