Mineralogy, nucleation and growth of dolomite in the
laboratory and sedimentary environment: A review
JAY M. GREGG*, DAVID L. BISH † , STEPHEN E. KACZMAREK ‡ and
HANS G. MACHEL §
*Boone Pickens School of Geology, Oklahoma State University, Stillwater, OK 74078, USA
(E-mail: jay.gregg@okstate.edu)
†Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA
‡Department of Geological Sciences, Bridgewater State University, Bridgewater, MA 02325, USA
§Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3,
Canada
Associate Editor – Cathy Hollis
ABSTRACT
Dolomite [CaMg(CO
3
)
2
] forms in numerous geological settings, usually as a
diagenetic replacement of limestone, and is an important component of
petroleum reservoir rocks, rocks hosting base metal deposits and fresh water
aquifers. Dolomite is a rhombohedral carbonate with a structure consisting
of an ordered arrangement of alternating layers of Ca
2+
and Mg
2+
cations
interspersed with CO
2
3
anion layers normal to the c-axis. Dolomite has R
3
symmetry, lower than the (CaCO
3
) R
3c symmetry of calcite primarily due to
Ca–Mg ordering. High-magnesium calcite also has R
3c symmetry and differs
from dolomite in that Ca
2+
and Mg
2+
ions are not ordered. High-magnesium
calcite with near-dolomite stoichiometry (50 mol% MgCO
3
) has been
observed both in nature and in laboratory products and is referred to in the
literature as protodolomite or very high-magnesium calcite. Many dolomites
display some degree of cation disorder (Ca
2+
on Mg
2+
sites and vice versa),
which is detectable using transmission electron microscopy and X-ray dif-
fractometry. Laboratory syntheses at high temperature and pressure, as well
as studies of natural dolomites show that factors affecting dolomite ordering,
stoichiometry, nucleation and growth include temperature, alkalinity, pH,
concentration of Mg and Ca, Mg to Ca ratio, fluid to rock ratio, mineralogy of
the carbonate being replaced, and surface area available for nucleation. In
spite of numerous attempts, dolomite has not been synthesized in the labora-
tory under near-surface conditions. Examination of published X-ray diffrac-
tion data demonstrates that assertions of dolomite synthesis in the laboratory
under near-ambient conditions by microbial mediation are unsubstantiated.
These laboratory products show no evidence of cation ordering and appear
to be very high-magnesium calcite. Elevated-temperature and elevated-pres-
sure experiments demonstrate that dolomite nucleation and growth always
are preceded by very high-magnesium calcite formation. It remains to be
demonstrated whether microbial-mediated growth of very high-magnesium
calcite in nature provides a precursor to dolomite nucleation and growth
analogous to reaction paths in high-temperature experiments.
Keywords Crystallography, dolomite mineralogy, dolomitization, laboratory
synthesis, X-ray diffraction.
1 © 2015 The Authors. Sedimentology © 2015 International Association of Sedimentologists
Sedimentology (2015) doi: 10.1111/sed.12202