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.
Dolomite Physical Characteristics
Hardness |
3.5-4 |
Specific gravity |
2.86 (average) |
Cleavage |
|
Color |
Often pink or pinkish and can be colorless, white, yellow, gray or even brown or
black when iron present in the crystal. |
Density |
|
Diaphaniety |
|
Fracture |
Conchoidal |
Crystal Habits |
Include saddle shaped rhombohedral twins and simple rhombs some with slightly
curved faces, also prismatic, massive, granular and rock forming. Never found in
scalenohedrons. |
Luminescence |
|
Luster |
Pearly to vitreous to dull |
Streak |
White |
Synonym |
|
Transparency |
Crystals are transparent to translucent |
Crystal System |
Trigonal; bar 3 |
Cleavage |
Perfect in three directions forming rhombohedrons. |
Other Characteristics |
Unlike calcite, effervesces weakly with warm acid or when first powdered with
cold HCl. |
Associated Minerals |
Include calcite, sulfide ore minerals, fluorite, barite, quartz and occasionally
with gold |
Notable Occurrences |
Many localities throughout the world, but well known from sites in Midwestern
quarries of the USA; Ontario, Canada; Switzerland; Pamplona, Spain and in Mexico
|
Best Field |
Typical pink color, crystal habit, hardness, slow reaction to acid, density and
luster |
Dolomite Chemical Analysis
Chemical Analysis |
% |
SiO2 |
------ |
Al2O3 |
0.04 |
Fe2O3 |
0.024 |
TiO2 |
N.D |
CaO |
32.218 |
MgO |
20.179 |
Na2O |
------ |
K2O |
------ |
Insoluble |
0.094 |
Na2CO3 |
------ |
Loss |
47.33 |
Total |
99.885 |
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.
Minerals That Belong to the Dolomite Group
• Ankerite Ca(Fe, Mg, Mn)(CO3)2
• Benstonite (Ba, Sr)6(Ca, Mn)6Mg(CO3)13
• Dolomite CaMg(CO3)2
• Huntite CaMg3(CO3)4
• Kutnohorite Ca(Mn, Mg, Fe)(CO3)2
• Minrecordite CaZn(CO3)2
• Norsethite BaMg(CO3)2
The borate minerals nordenskoldine and
tusionite are isostructural with the
Dolomite Group minerals.
Specimen: |
Cluster of curved, pink
Dolomtite crystals ("Pearl
Spar") |
Locality: |
Picher, Ottawa Co., Oklahoma |
Magnification: |
Specimen Grade |
Collection: |
Hershel Friedman |
Photograph: |
Hershel Friedman |
Specimen: |
Curved white Dolomite crystals
with Chalcopyrite |
Locality: |
Picher, Ottawa Co., Oklahoma |
Magnification: |
+2 |
Specimen Grade: |
B |
Collection: |
Hershel Friedman |
Photograph: |
Hershel Friedman |
Noteworthy Localities
There are many localities that produced
fine Dolomite specimens. Most locations
are in regions which contain an
abundance of this mineral throughout the
region. These regions are.
Dolomite Industrial Applications
Dolomite is chiefly used as refractory,
ramming, and fettling material in steel
melting shop, and as fluxing material in
blast furnace operation in secondary
steel and ferromanganese manufacture. To
a lesser extent it is used in the glass
industry especially in sheet-glass
manufacture. It also finds use in the
manufacture of mineral wool.
In England, dolomite has become a useful
source for the production of magnesite
by reacting calcined dolomite with
sea-water. The UK is meeting nearly 50%
of her magnesite requirements by this
method. Dolomite is also a good source
of magnesium metal. The magnesium metal
is extracted from dolomite by the
well-known fero-silicon process.
Dolomite decomposes completely above
900ºC. The product resulting from this
relatively law-temperature calcination
is highly porous and reactive and is
known as 'calcinated dolomite'. Dolomite
is sometimes used both in the raw and
calcined form as refractory material for
hearth maintenance and for banking door
in open hearth furnaces.
For most refractory uses, it is
desirable to subject the dolomite to a
heat treatment at a high temperature of
the order of 1700ºC, to shrink the
material thoroughly and render it less
reactive. Dead burnt (D.B.) dolomite is
sthe term generally used for the
refractory made by firing dolomite, with
or without additives, at high
temperature to produce dense,
well-shrunk particles.
In basic converters the bricks employed
are generally of D.B. dolomite and
sometimes also of D.B. magnesite.
Dolomite bricks are kept in the outer
lining because it has lower thermal
conductivity than magnesite.
Terms of Dolomite Use
Dolomite is quarried for building and
ornamental stone, road stone, and the
production of refractory brick. It is
the principal ore of magnesium metal and
the source of the magnesium used by the
chemical industry.
1. Dolomite is used to make magnesia,
which has important medicinal
applications. Dolomite specimens from
Iran are very popular among mineral
collectors and dealers.
2. Dolomite Rock is used as an
ornamental and structural stone, and for
extracting certain metals from their
ores. It is useful in the chemical
industry in the preparation of magnesium
salts.
3. Dolomite occurs in a different
crystal class than the Calcite Group.
This can be noted by the fact that
Dolomite generally forms more elongated
crystals than the Calcite Group. In
addition, Dolomite never occurs in
scalenohedrons, whereas minerals of the
Calcite Group do.
Dolomite Industrial Use
A major source of magnesium,
particularly for agricultural and
pharmaceutical applications.
Dolomite General Use
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 cement, as
a source of magnesium. Of course
Dolomite is also used as mineral
specimens.
Dolomite specimens from the Iran are
very popular among mineral collectors
and dealers. The clear transparent
specimens from Iran are rare and
unusual, and are in high demand by
Collectors.
Dolomite Rock is used as an ornamental
and structural stone, and for extracting
certain metals from their ores. It is
useful in the chemical industry in the
preparation of magnesium salts.
Recommended Filled of Application
Kind of powder |
Talc |
Mica |
Kaolin |
Red Iron oxide |
Fluorine |
Dolomite |
Calcite |
Bentonite |
Barite |
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