PHYSICAL PROPERTIES:
Chemistry: CaMg(CO3)2
Composition: Calcium Magnesium Carbonate
Class: Carbonates
Group: Dolomite
Crystal system: trigonal
Fracture:conchoidal
Hardness: 3.5-4
Specific gravity: 2.8
Luster: glassy to pearly
Streak: white
Cleavage: perfect in three directions
Color: Colorless, white, pinkish, or light
tints
darker colors even black when iron is present.
Transparency: transparent to translucent
Associated Minerals: barite, calcite,
fluorite, sulfide ore minerals, quartz and some times with gold.
COMPOSITION:
(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.
DISTINGUISHING CHARACTERISTICS:
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".
ENVIRONMENT:
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.
CRYSTAL DESCRIPTION:
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.
TESTS:
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)
LOCALITIES:
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.
USES:
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.
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