Dolomite

minerals

forsterite

clinohumite

tremolite

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Formula: CaMg(CO3)2
Carbonate
Specific gravity: 2.84 to 2.86
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
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

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).

At Tsumeb, Namibia, dolomite pseudomorphs after aragonite and after calcite have been found (KL p174, 175).

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).

At the Blackdene Mine, Ireshopeburn, Stanhope, County Durham, England, UK, dolomite occurs on limestone with siderite (SY p149).

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).

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).

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)

Common impurities: Fe,Mn,Co,Pb,Zn

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