Dolomite Perspectives on a Perplexing Mineral

A Metastable Lexicon 
Because it is a descriptive science based on 
observations made in the field, geology depends 
on a precisely tuned lexicon. When transferred 
from one analog to another, geological terminol-
ogy tends to evolve. Dolomite geology is rife with 
such terms. 
Dolomite is named in honor of Déodat Gratet 
de Dolomieu (1750–1801), a colorful and some-
what controversial geologist who described cal-
careous rock exposures in the southern Alps of 
northeastern Italy.
3
Dolomieu observed that these 
rocks looked like limestone but failed to effer-
vesce as limestone does when treated with weak 
1. Despite more than 200 years of research, the origin of 
dolomite remains the subject of considerable controversy, 
partly because critical chemical, biological and 
hydro logical conditions are poorly understood and partly 
because petrographic and geochemical data permit more 
than one interpretation.
For more on this controversy: Machel HG: “Concepts and 
Models of Dolomitization: A Critical Reappraisal,” in 
Braithwaite CJR, Rizzi G and Darke G (eds): 
The Geometry 
and Petrogenesis of Dolomite Hydrocarbon Reservoirs. 
London: Geological Society, Special Publication 235 
(2004): 7–63. 
2. Warren J: “Dolomite: Occurrence, Evolution and 
Economically Important Associations,” 
Earth Science 
Reviews 52, nos. 1–3 (November 2000): 1–81.
>
Solid-solution series. In its purest state, dolomite falls along the 
calcite-magnesite line in the solid-solution series of calcite, 
magnesite and siderite. Although the composition of dolomite is 
written as [CaMg(CO
3
)
2
], naturally occurring dolomite ranges from 
about Ca
1.16
Mg
0.84
(CO
3
)
2
to about Ca
0.96
Mg
1.04
(CO
3
)
2
.
MattV_ORAUT09_Fig_1_2
Calcite
CaCO
3
Dolomite
CaMg(CO
3
)
2
Ankerite
CaFe(CO
3
)
2
MgCO
3
Magnesite
FeCO
3
Siderite
3. de Dolomieu DG: “Sur un genre de pierres calcaires très 
peu effervescente avec les acides et phosphorescentes 
par la collision,” 
Journal de Physique 39 
(October 1791): 3–10. 
For an updated perspective: Zenger DH, Bourrouilh-Le 
Jan FG and Carozzi AV: “Dolomieu and the First 
Description of Dolomite,” in Purser BH, Tucker ME and 
Zenger DH (eds): 
Dolomites: A Volume in Honor of 
Dolomieu. Boston, Massachusetts, USA: Blackwell 
Scientific, International Association of Sedimentologists,
 
Special Publication 21 (1994): 21–28. 
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Oilfield Review
acid. So although the dolomite label was first 
applied to the rock, it also names the chief min-
eral constituent as well as the mountain range in 
which it was first described 
(above)
.
To distinguish between rock and mineral, the 
term dolostone was introduced in 1948.
4
This 
name refers to rock formed of the mineral
dolomite (more than 75%), along with other min-
erals.
5
Dolomicrite is formed when dolomite 
replaces very fine crystalline muds.
Other researchers felt it necessary to distin-
guish between different types of dolomite and 
introduced new terms to account for variations in 
magnesium and calcium content. The ideal com-
position of dolomite consists of equal amounts of 
Ca and Mg in alternating layers separated by CO
3
layers. The designation of high-calcium dolomite 
is given to the mineral when calcium carbonate 
[CaCO
3
] increases by 10% or more above its ideal 
composition; alternatively, it may also be called 
calcitic dolomite or lime dolomite. With a 
decrease in magnesium carbonate [MgCO
3
] con-
tent, such that CaCO
3
ranges from 50% to 90%, the 
rock may be called dolomitic limestone. Further 
reducing MgCO
3
to between 5% and 10% results in 
magnesian limestone, though some consider 
this term obsolete. With less than 5% MgCO
3
, the 
precursor rock is simply called limestone.
Protodolomite is a common metastable pre-
cursor of dolomite. Although it approximates 
dolomite in chemical composition, it is said to
be poorly ordered, or lacking the well-developed 
crystal lattices found in mature, ordered, 
stoi-
chiometric dolomite.
6
Like other terms in this 
discussion, some would strike it from the dolo-
mite vocabulary, while others find it useful.
The plural term dolomites may be used to col-
lectively describe different types of dolomite that 
vary in texture, composition or genesis.
7
When 
describing a carbonate that has been subjected 
to replacement, the adjective dolomitized may 
be used.
Dolomites can be divided into two major fami-
lies. Penecontemporaneous dolomites form soon 
after deposition of carbonate precursors as a 
result of geochemical conditions that prevail 
within the precursor’s environment of deposition. 
Most penecontemporaneous dolomites are of 
Holocene age and are restricted to certain
evaporitic lagoonal or lacustrine settings. 
Postdepositional dolomites form after carbonate 
sediment has been deposited and subsequently 
removed from the active zone of sedimentation. 
This may happen through progradation of the 
sedimentary surface, burial and subsidence, 
uplift and emergence, or eustatic sea-level fluc-
tuations. Almost all examples of massive, region-
ally extensive dolostones are postdepositional.
8
Important but sometimes confusing distinc-
tions have been drawn between various types of 
dolomite, based on how they form. A dolomite’s 
mode of origin is an important concept that can be 
tied to its general orientation and areal extent in 
the subsurface. Primary dolomite consists of