Images
Formula: CaMg(CO3)2
Carbonate, dolomite group
Crystal System: Trigonal
Specific gravity: 2.84 to 2.86 measured, 2.876 calculated
Hardness: 3½ to 4
Streak: White
Colour: Colourless, white, gray, reddish-white, brownish-white, or pink; colourless in
transmitted light
Solubility: Readily soluble in hydrochloric acid
Common impurities: Fe,Mn,Co,Pb,Zn
Environments:
Carbonatites (essential)
Evaporite Deposits (rare primary dolomite)
Metamorphic environments
Hydrothermal environments
Dolomite overwhelmingly occurs in dolostone or in
marble formed from the
contact metamorphism of
dolostone. It is a mineral of the
prehnite-pumpellyite,
greenschist,
amphibolite and
granulite facies.
It also occurs in hypothermal (high temperature),
mesothermal (moderate temperature)
and epithermal (low temperature) veins, chiefly in lead and
zinc veins
in limestone, associated with
quartz,
fluorite,
calcite,
baryte,
magnesite,
sphalerite,
siderite,
galena,
pyrite and
chalcopyrite.
Few dolomites are primary in origin. The main exception
to this is the rare primary dolomite that forms in
evaporites as a relatively late product of seawater evaporation.
Sedimentary dolomite results from alteration of calcite and
aragonite.
In sedimentary
dolostone, dolomite is often associated with
calcite, aragonite,
gypsum, anhydrite
and halite.
Although uncommon, when dolomite occurs in altered
ultramafic igneous rocks, such as
serpentinite, it may be associated with
magnesite,
serpentine and talc.
Dolomite is an essential constituent of
dolostone.
It is a common but not esential constituent of
limestone and
skarn.
It also may be found in
marble and
serpentinite.
It is a mineral of the prehnite-pumpellyite,
greenschist,
amphibolite and
granulite facies
(MU).
Localities
The Two Mile and Three Mile deposits, Paddy's River, Paddys River District, Australian Capital Territory, Australia,
are skarn deposits at the contact between
granodiorite and volcanic rocks.
Dolomite is a primary carbonate associated with pink
calcite and in some cases with pyrite
cubes
(AJM 22.1.38).
At the Mount Kelly deposit, Gunpowder District, Queensland, Australia, the copper
ores overlie primary zone mineralisation consisting of
quartz-dolomite-sulphide veins hosted
in siltstone and schist.
Dolomite is common in the siltstone. The Paragenesis for the
primary zone is
dolomite followed by pyrite, then
chalcopyrite and sphalerite,
and lastly bornite
(AJM 22.1.25).
At the Mount Lyell mines, Queenstown district, West Coast municipality, Tasmania, Australia,
dolomite-ankerite crystals to 3 cm occur in veins in the sulphide ore, with
chalcopyrite, siderite and
quartz crystals
(AJM 21.2.24).
Dolomite from Mount Lyell - Image
At lots 10 and 11 of concession 1, Bathurst Township, Lanark County, Ontario, Canada (DeWitts corner), the deposit is
located in the Grenville Geological Province, which consists mostly of
marble, gneiss, and
quartzite.
Syenite-migmatite was
also reported in the area where the vein-dikes are located. Characteristic features of the vein-dikes include the
fact that perfectly formed euhedral crystals of different minerals can often be found floating in
calcite with no points of contact with the walls. Sometimes these crystals
have inclusions of calcite, irregular or rounded in shape. It has been argued
that at least some of the vein-dikes were formed as a result of melting of Grenville
marble.
Dolomite is rare and occurs as fine-grained white aggregates around grey
corundum pseudomorphs after
spinel in the grey calcite from the
central area of the vein-dike
(R&M 97.6.556-564).
Dolomite from Bathurst - Image
At the Wanshan mine, Wanshan Mercury Ore Field, Wanshan District, Tongren, Guizhou, China, dolomite occurs associated
with cinnabar
(AESS).
Dolomite from Wanshan - Image
At the Nakhlak Mine, Anarak District, Nain County, Isfahan Province, Iran, epigenetic (formed later than the
surrounding or underlying rock formation) vein deposits and metasomatic replacement bodies are hosted by a chalky
Upper Cretaceous (100.5 to 66 million years ago) limestone. The
limestone underwent
dolomitisation prior to sulphide mineralisation. The principal
primary ore mineral is
galena, associated with minor or trace amounts of
sphalerite, tetrahedrite
-tennantite, pyrite and
chalcopyrite as inclusions. The main
secondary ore mineral is
cerussite, sometimes associated with minor amounts of
anglesite, plattnerite,
wulfenite, minium,
mimetite, covellite,
chalcanthite, malachite and
goethite. Many trace elements are present in the
primary galena, but
most notably it is rich in silver and
antimony and poor in bismuth.
Crystals of dolomite to 1 mm show saddle shapes and curved edges, and are typically associated with
galena mineralisation. Intergrowth textures show it to be cogenetic with
galena
(Minrec 54.3.383-408).
At Tsumeb, Namibia, dolomite pseudomorphs after
aragonite and after calcite have been found
(KL p174, 175).
Dolomite from Tsumeb - Image
At Berg Aukas, Grootfontein, Otjozondjupa Region, Namibia, dolomite is the major component of the
dolostone country rock, but it also occurs as well formed crystals of
secondary origin up to a few mm in length and sometimes associated with
descloizite
R&M 96.2.130-131).
Dolomite from Berg Aukas - Image
At the Kloof Mine, Eastern Sector, Far West Rand, West Rand District, Gauteng, South Africa,
secondary dolomite has been found in this
gold mine as small pink to white saddle-shaped crystals, associated with transparent,
colourless quartz. Cubic pyrite to 0.5 mm
is found with some of these specimens
(R&M 96.4.324).
Dolomite from the Kloof Mine - Image
At the Blackdene Mine, Ireshopeburn, Stanhope, County Durham, England, UK, dolomite occurs on
limestone with siderite
(SY p149).
Dolomite from the Blackdene Mine - Image
At Frizington, Arlecdon & Frizington, Copeland, Cumbria, England, UK, dolomite occurs as crystals with a slight
iridescence due to coatings of microscopic tarnished pyrite crystals
(SY p149).
Dolomite from Frizington - Image
The Sunnyside Deposit, Whitwell, Bolsover District, Derbyshire, England, UK, is hosted by late Permian (256 to 248
million years ago) dolostone that lies above a thick sequence of
Carboniferous (354 to 290 million years ago) Coal Measures sediments.
Cavities in the dolostone are commonly lined with rounded opaque
white dolomite rhombs up to about 1 mm on edge. Although it is present in cavities in the
wall-rock, there is no evidence that dolomite was deposited in the epigenetic
baryte - galena mineralisation at
the Sunnyside Deposit
(JRS 24.37-59).
At Croft quarry, Croft, Blaby, Leicestershire, England, UK, dolomite has been recorded in veins in
breccia and
quartz-diorite. The only
associated mineral is calcite
(JRS 20.14).
At Lane's Hill quarry, Stoney Stanton, Blaby, Leicestershire, England, UK, a specimen has been found that contained three generations
of Fe-rich dolomite with later calcite and
well-crystallised, microcrystals of chlorite. This site had a large vein of Fe-rich
dolomite that contained large quartz crystals, and
showed a complex paragenetic sequence commencing with ferroan dolomite -
pyrite, then ferroan dolomite -
quartz, and finally ferroan dolomite
(JRS 20.15).
At Enderby Warren Quarry, Enderby, Blaby, Leicestershire, England, UK, dolomite occurs in
quartz-diorite and
tonalite associated with palygorskite
and calcite
(R&M 20.13).
At the Cloud Hill quarry, Breedon on the Hill, Leicestershire, England, UK, dolomite crystals have been found on
goethite, with druses of microcrystalline
quartz
(RES p207).
At the Suever Stone Company quarry, Delphos, Van Wert county, Ohio, USA, dolomite occurs occasionally as white or pink crystals
associated with fluorite, which it precedes in the paragenesis. The
fluorite can be two stages, brown, then clear and colourless
(R&M 95.6.505).
At the Mufulira Mine, Mufulira, Mufulira District, Copperbelt Province, Zambia, most of the hanging wall strata
above the A orebody consists of alternating layers of porous
dolostone, siltstone
and quartzite. Small, delicately pink, saddle-shaped crystals of
dolomite occur commonly in cavities throughout the mine in association with
secondary copper
minerals and sometimes with transparent gypsum. Most dolomite crystals
are small, rarely exceeding 1 cm in size
(MinRec 55.4.465-466).
Alteration
ankerite, dolomite and quartz to
augite and CO2
Ca(Mg,Fe)(CO3)2 + 2SiO2 → Ca(Mg,Fe)Si2O6 + 2CO2
(DHZ 2A p384)
ankerite-dolomite and quartz to
diopside-hedenbergite
and CO2
Ca(Fe,Mg)(CO3)2 + 2SiO2 = Ca(Fe,Mg)Si2O6 + 2CO2
(DHZ 2A p274)
aragonite or calcite and Mg2+
(from Mg-rich fluid) to dolomite and Ca2+
2CaCO3 + Mg2+ ⇌ CaMg(CO3)2 + Ca2+
diopside and dolomite to
forsterite,
calcite and CO2
CaMgSi2O6 + 3CaMg(CO3)2 →
2Mg2SiO4 + 4CaCO3 + 2CO2
This is a high-grade metamorphic change occurring at temperature in excess of 600oC
(MOM, DHZ 5B p213).
diopside, dolomite, CO2 and H2O to
tremolite and calcite
4CaMgSi2O6 + CaMg(CO3)2 + CO2 + H2O =
Ca2Mg5Si8O22(OH)2 + 3CaCO3
Diopside is produced by the metamorphism of siliceous dolostone, and
if water is introduced at a later stage
tremolite may be produced from the above reaction, or by the reaction of diopside with CO2 and
H2O
(DHZ 2A p276).
diopside, dolomite and
H2O ⇌ hydroxylclinohumite,
calcite and CO2
2CaMgSi2O6 + 7CaMg(CO3)2 + H2O ⇌
Mg9(SiO4)4(OH)2 + 9CaCO3 + 5CO2
In the nodular dolomites, clinohumite
associated with
calcite occurs in a narrow zone in the central parts of the
nodules due to the above reaction
(DHZ 1A p264).
dolomite and
chert to talc and
calcite
3CaMg(CO3)2 + 4SiO2 + H2O →
Mg3Si4O10(OH)2 + 3CaCO3 + 3CO2
Metamorphism of siliceous carbonate rocks causes the formation of hydrous phases such as talc and tremolite.
(DHZ 5B p127)
This is a very low-grade metamorphic reaction occurring at temperature between about 150oC and
250oC
(MOM, DHZ 5B p213).
dolomite and coesite to diopside,
diamond and oxygen
MgCa(CO3)2 + 2SiO2 → CaMgSi2O6 + 2C + 2O2
The coexistence of diamond and carbonate minerals in mantle
eclogite is explained by the above reaction.
dolomite, K-feldspar and H2O to
phlogopite, calcite and
CO2
3CaMg(CO3)2 + KAlSi3O8 + H2O =
KMg3AlSi3O10(OH)2 + 3CaCO3 + 3CO2
In the presence of Al and K the metamorphism of dolomite leads to the formation of
phlogopite according
to the above equation
(DHZ 5B p213).
dolomite and muscovite to
phlogopite,
calcite, CO2 and Al2O3
3CaMg(CO3)2 + KAl2(AlSi3O10)(OH)2
→ KMg3(AlSi3O10)(OH)2 + 3CaCO3 + 3CO2
+ Al2O3
The excess alumina may be used to form spinel
(DHZ 3 p51).
dolomite and
quartz to
diopside and CO2
CaMg(CO3)2 + 2SiO2 → CaMgSi2O6 + 2CO2
In siliceous dolostone dolomite and quartz may react to form either
diopside
or forsterite, with diopside forming
at a lower temperature
than forsterite
(DHZ 2A p270) .
dolomite and
quartz to
forsterite,
calcite and CO2
2CaMg(CO3)2 + SiO2 → Mg2SiO4 + 2CaCO3
+ 2CO2
In siliceous dolostone dolomite and quartz may react to form
either diopside
or forsterite, with diopside forming
at a lower temperature
than forsterite
(DHZ 2A p270, 1A p264).
dolomite and quartz to diopside and
CO2
CaMg(CO3)2 + 2SiO2 → CaMgSi2O6 + 2CO2
This reaction is the result of metamorphism of siliceous, Mg-rich
limestone or dolostone
(MOM p482).
dolomite, quartz and H2O to
tremolite, calcite and CO2
5CaMg(CO3)2 + 8SiO2 + H2O →
Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 7CO2
This is a metamorphic reaction in dolomitic limestone
(MOM p496).
dolomite and tremolite to
forsterite, calcite, CO2
and H2O
Ca2Mg5Si8O22(OH)2 + 11CaMg(CO3)2
→ 8Mg2SiO4 + 13CaCO3 + 9CO2 + H2O
(DHZ 5B p213)
dolomite, tremolite and
forsterite to diopside,
enstatite
and H2O
Ca2Mg5Si8O22(OH)2 + Mg2SiO4 ⇌
2CaMgSi2O6 + H2O
At a pressure of 4 kbar the equilibrium temperature is about 840oC
(granulite facies)
(JVW p97).
forsterite, dolomite and
H2O to
calcite, hydroxylclinohumite
and CO2
A forsterite-clinohumite
assemblage in the silica-rich
dolomite in the aureole of the Alta
granodiorite in Utah, USA, is
probably due to the reaction:
4Mg2SiO4 + CaMg(CO3)2 + H2O →
Mg9(SiO4)4(OH)2 +CaCO3 + CO2
A forsterite-clinohumite
assemblage in the silica-rich
dolomite in the aureole of the Alta
granodiorite in Utah, USA, is
probably due to the above reaction
(DHZ 1A p264).
kaolinite,
dolomite,
quartz and H2O to chlorite,
calcite and CO2
Al2Si2O5(OH)4 + 5CaMg(CO3)2 + SiO2
+ 2H2O ⇌ Mg5Al(AlSi3O10)(OH)8 + 5CaCO3 +
5CO2
Chlorite often forms in this way from reactions between clay minerals such as kaolinite and carbonates such as
dolomite
(KB p377).
talc, calcite and CO2 to
dolomite,
quartz and H2O
Mg3Si4O10(OH)2 + 3CaCO3 + 3CO2 ⇌
3CaMg(CO3)2 + 4SiO2 + H2O
(JVW p144)
talc and calcite to
tremolite
dolomite, CO2 and H2O
2Mg3Si4O10(OH)2 + 3CaCO3 +4SiO2 →
Ca2Mg5Si8O22(OH)2 +
CaMg(CO3)2 + CO2 +H2O
This is a low-grade metamorphic change, occurring at temperature between about 250oC and 450oC
(MOM).
tremolite and CO2 to dolomite,
talc and SiO2
☐Ca2Mg5Si2O22(OH)2 + CO2
⇌ 2CaMg(CO3)2 + Mg3Si4O10(OH)2
+ 4SiO2
In the greenschist facies
tremolite may be converted to
talc according to the above reaction
(DHZ 3 p127).
tremolite and calcite to
diopside,
dolomite, CO2 and
H2O
Ca2Mg5Si8O22(OH)2 + 3CaCO3 ⇌
4CaMgSi2O6 + CaMg(CO3)2 + CO2 + H2O
The forward reaction is a diopside-forming metamorphic reaction
(DHZ 2A p249).
tremolite and dolomite to
forsterite, calcite, CO2
and H2O
Ca2Mg5Si8O22(OH)2 + 11CaMg(CO3)2 →
8Mg2SiO4 + 13CaCO3 + 9CO2 + H2O
(DHZ 1A p264)
tremolite, dolomite and
H2O ⇆
hydroxylclinohumite, calcite
and CO2
Ca2Mg5Si8O22(OH)2 + 13CaMg(CO3)2
+ H2O ⇆ 2Mg9(SiO4)4(OH)2 + 15CaCO3
+ 11CO2
(DHZ 1A p264)
Back to Minerals