Journal of Sedimentary Research, 2007, v. 77, 424–432
Research Article
DOI: 10.2110/jsr.2007.035
A COMPARISON OF NANOMETER-SCALE GROWTH AND DISSOLUTION FEATURES ON NATURAL AND
SYNTHETIC DOLOMITE CRYSTALS: IMPLICATIONS FOR THE ORIGIN OF DOLOMITE
STEPHEN E. KACZMAREK AND DUNCAN F. SIBLEY
Michigan State University, Department of Geological Sciences, East Lansing, Michigan 48824, U.S.A.
e-mail: [email protected]
ABSTRACT: This paper describes experiments in which mechanisms of crystal growth are inferred from the surface
nanotopography of natural and synthetic dolomite using atomic force and scanning electron microscopy. Synthetic dolomite
was formed by replacement of calcite at 218˚ C in Mg–Ca–Cl solutions. The first products to form during dolomite synthesis
experiments, as detected with X-ray diffraction (XRD), are invariably poorly ordered (nonideal) dolomite. As the reaction
proceeds, the degree of cation order increases, yet the products remain relatively disordered. Upon examination, the surfaces of
nonideal dolomite are covered with mounds 10–200 nm wide and 1–20 nm high. Following depletion of the calcite reactant,
dolomite becomes relatively well ordered and stoichiometric (ideal). The surfaces of ideal dolomite are covered with broad, flat
layers separated by steps that measure tenths to tens of nanometers high. Comparison of natural and synthetic dolomite
surfaces after etching in 0.5% H2SO4 for 10–20 seconds indicates that high-temperature synthetic and low-temperature natural
nonideal dolomite surfaces are covered with mounds identical to the growth features found on unetched nonideal synthetic
dolomite. In contrast, synthetic and low-temperature ideal dolomite surfaces, when etched, are characterized by flat layers with
deep, euhedral etch pits. These results suggest that despite the wide range in formation conditions, natural and synthetic
dolomites form by the same growth mechanisms. Furthermore, the two different surface nanotopographies described here are
consistent with a model in which nonideal dolomite forms by precipitation of amorphous mounds that quickly crystallize to
nonideal dolomite while ideal dolomite later replaces nonideal dolomite and grows by spiral growth.
INTRODUCTION
Dolomite, CaMg(CO3)2, is a common mineral in ancient rocks and the
thermodynamically stable carbonate phase in modern seawater, yet it is
rare in modern marine environments. Why this is so has remained the
subject of scientific inquiry of over 200 years. There is very little
agreement concerning the details of dolomite formation except that most
natural dolomites form at Earth-surface temperatures and pressures
(Krauskopf and Bird 1995). Despite such consensus it has been extremely
difficult to synthesize dolomite abiotically at temperatures below 100uC,
even over many years (Land 1980, 1998). High-temperature laboratory
experiments have provided some insight into the nature of dolomite
formation (Katz and Matthews 1977; Baker and Kastner 1981; Sibley et
al. 1987; Sibley 1990; Malone et al. 1996; Kelleher and Redfern 2002), but
there exists no theoretical basis for extrapolating kinetic data over the 25–
250uC temperature range between sedimentary dolomites and temperatures commonly employed in the laboratory. As a result, the applicability
of high-temperature experiments to natural low-temperature systems
remains debated (Arvidson and Mackenzie 1999).
An understanding of dolomite formation has been further complicated
by the observation that many ancient dolomites are stoichiometric and
well ordered compared to their modern counterparts, which often contain
excess calcium, are poorly ordered, and display heterogeneous microstructures when observed with transmission electron microscopy (see
Reeder 1992, 2000 for reviews). Similar deviations from ideality have been
observed in early dolomite-like phases formed during high-temperature
synthesis experiments (Graf and Goldsmith 1956; Gaines 1974; Sibley
Copyright E 2007, SEPM (Society for Sedimentary Geology)
1990; Malone et al. 1996; Politi et al. 2004), the inference being that
nonideal dolomite represents a metastable precursor to ideal dolomite
(Nordeng and Sibley 1994).
Recently, a number of studies have documented the occurrence of
amorphous CaMg-carbonate precursors in highly supersaturated solutions with Mg:Ca ratios . 2 that later crystallize to Mg-calcite (Raz et al.
2000; Kelleher and Redfern 2002; Loste et al. 2003; Schmidt et al. 2005;
Kwak et al. 2005; DiMasi et al. 2006). Kelleher and Redfern (2002)
reported forming a hydrous calcium magnesium carbonate phase in
precipitation experiments that was compositionally and structurally
identical to recent dolomites from Abu Dhabi based on XRD and
infrared spectroscopy. They argued that a hydrous precursor phase could
circumvent kinetic barriers associated with dehydration of magnesium
ions in solution. Schmidt et al. (2005) showed that two amorphous phases
were produced in laboratory growth experiments, a hydrated CaMgcarbonate and an amorphous calcium carbonate. They found that the
hydrated CaMg-carbonate had similar characteristics as disordered
dolomite from the Coorong region in South Australia. In all cases,
amorphous precipitates are characterized by structural disorder and
rounded morphologies with a high degree of surface roughness.
Micrometer- to nanometer-scale surface topographies on synthetic and
mildly etched natural crystals have long been used to provide indirect
evidence of crystal growth mechanisms (see Sunagawa 1984 for review).
More recently, direct observation of crystal growth in a fluid cells using
atomic force microscopy has provided direct evidence that crystals grow by
spiral growth at low degrees of supersaturation and polynuclear growth at
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GROWTH AND DISSOLUTION FEATURES ON NATURAL AND SYNTHETIC DOLOMITES
425
TABLE 1.— Sample list and corresponding analytical data for natural dolomites.
Sample ID
MH-2
MH-6
MH-10
MH-11
MH-20
MH-52
SAL 7-5
SAL 7-11
7-9-99-3
SB 42
SC 10D
SB 35
RR 17
RR 19
PDC 1
L2 91
80-8-TS
80-8-17-1
Aruba Dol
8-9-84-5
8-9-84-3
13-4
16-8
S-1
8-23-86-7
DC37
Niagara
GWP 280-290
GWP287
FC 2C
7-22-02-3
Age
Formation
d(104) Position
Mole% CaCO3
Ordering Ratio
Etched Topography
Permian
Permian
Permian
Permian
Permian
Permian
Ordovician
Ordovician
Ordovician
Pliocene
Pliocene
Pliocene
Ordovician
Ordovician
Ordovician
Ordovician
Pliocene
Pliocene
Pliocene
Miocene
Miocene
Ordovician
Ordovician
Silurian
Silurian
Mississippian
Silurian
Holocene
Holocene
Mississippian
Pliocene
Hueco
Hueco
Hueco
Hueco
Hueco
Hueco
Saluda
Saluda
Saluda
Seroe Domi
Seroe Domi
Seroe Domi
Prarie DuChen
Prarie DuChen
Prarie DuChen
Prarie DuChen
Seroe Domi
Seroe Domi
Seroe Domi,
Terminal Carb. Complex, Spain
Terminal Carb. Complex, Spain
Galena
Galena
Lockport
Lockport
Burlington Keokuk
Niagara
Abu Dhabi Sabkha
Abu Dhabi Sabkha
Borden
Seroe Domi
31.00
31.03
31.02
31.02
31.03
31.03
30.84
30.87
30.85
30.91
30.87
30.87
31.00
31.00
31.04
31.03
30.85
30.85
30.84
31.00
31.00
31.06
31.03
31.03
31.04
30.92
31.04
30.89
31.02
30.87
30.88
50.00
50.00
50.00
50.00
50.00
50.00
53.73
52.75
53.23
51.50
52.75
52.75
50.00
50.00
50.00
50.00
53.37
53.37
53.43
50.00
50.00
50.00
50.00
50.00
50.00
51.33
50.00
52.00
50.00
52.75
52.33
1.13
1.08
1.08
1.07
1.11
1.22
0.82
0.67
0.54
0.60
0.57
0.73
0.96
1.00
1.33
0.97
0.67
0.48
0.46
0.44
0.62
1.14
1.04
1.07
1.04
0.68
1.00
0.56
0.67
1.17
0.56
pits
pits
pits
pits
pits
pits
mounds
mounds
mounds
mounds
mounds
mounds
pits
pits
pits
pits
mounds
mounds
mounds
mounds
mounds
pits
pits
pits
pits
mounds
pits
mounds
mounds
mounds
mounds
higher degrees of supersaturation (Gratz et al. 1993; Dove and Hochella
1993; Pina et al. 1998; Bosbach et al. 1998; Teng et al. 2000; Pina et al.
2004). Two-dimensional growth models assume that growth occurs by
generation and subsequent spread of surface nuclei (Nielsen 1964). In
contrast, spiral growth circumvents the energy barrier associated with
nucleation by taking advantage of screw dislocations, which provide
a continuous and self-perpetuating source of attachment sites (Burton et al.
1951; Pina et al. 2004). Kessels et al. (2000) used AFM to analyze the
surfaces of synthetic dolomite crystals from high-temperature growth
experiments. They showed that early in the reaction Ca-rich dolomites were
covered with rounded mounds, but late in the reaction, after dolomites
became stoichiometric, surfaces were covered with flat layers.
The work described here compares growth and dissolution nanotopography on natural and synthetic dolomite crystals with the aim of
addressing two hypotheses. (1) High-temperature synthetic dolomites and
low-temperature natural dolomites form by the same mechanism. If this
hypothesis is unsubstantiated, the applicability of high-temperature
experiments to natural low-temperature dolomite is tenuous at best.
(2) Precipitation of amorphous metastable CaMg-carbonate is the first
step in formation of dolomite. If their occurrence is common in nature,
metastable, amorphous precursors could provide an energetically easier
and more rapid mechanism by which to form dolomite. Therefore
amorphous precursors could be one missing link in a solution to the
‘‘dolomite problem,’’ which has eluded geologists for more than two
centuries.
METHODS
Preparation of Synthetic Dolomite
A series of eight experiments was conducted to dolomitize Iceland spar
calcite in Mg–Ca–Cl solutions at 218 6 2uC. Each experiment was
defined by the initial Mg2+:Ca2+ ratio in solution (0.43, 0.50, 0.66, 0.79,
1.0, 1.14, 1.27, 1.50), which was attained by mixing different proportions
of 1 M MgCl2 and CaCl2 solutions. Teflon-lined stainless steel reaction
vessels were loaded with 0.1 g calcite (ground and sieved to 40–60 mm size
fraction and cleaned ultrasonically in distilled water) and 15 ml solution.
Reaction vessels were then sealed and heated for a predetermined time. At
the end of each run individual reaction vessels were removed from the
oven and allowed to air-cool for approximately 30 minutes. The contents
of each vessel were then vacuum filtered, rinsed with distilled water, airdried, and stored in a vacuum desiccator until analysis.
Preparation of Natural Dolomite
Thirty-one natural dolomite rocks ranging in age from Ordovician to
Holocene were analyzed (Table 1). Whole-rock samples were broken into
pieces with a rock hammer, cleaned ultrasonically in distilled water, dried,
and stored in a vacuum desiccator.
X-Ray Diffraction (XRD)
Powder X-ray diffraction was used to characterize the phases present
and to determine composition and degree of cation order for reactionvessel contents and natural dolomites. XRD was carried out using CuKa
radiation. Diffraction data were collected between 25 and 45u 2h using
a step size of 0.004u with a 1 second count time. A small amount of CaF2
was added to powdered samples as an internal standard to calibrate the
position of the d(104) dolomite peak. The X-ray-determined d(104) peak
position was reproducible to within 0.006u 2h, or 6 0.33 mole% CaCO3.
The percent product, which provides a measure of reaction progress in
synthesis experiments, was determined by the peak-height ratio of the
d(104) dolomite peak to the d(104) dolomite plus the d(104) calcite peak
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S.E. KACZMAREK AND D.F. SIBLEY
FIG. 1.— Plot of induction period vs. initial solution Mg:Ca ratio.
(Royce et al. 1971). Product composition was calculated for synthetic and
natural samples according to the relative corrected position of the d(104)
dolomite peak (Lumsden and Chimahusky 1980). The degree of cation
order is reported as the ratio of the d015:d110 superlattice reflection peaks
following the methods of Goldsmith and Graf (1958).
Atomic Force Microscopy (AFM) and Scanning Electron
Microscopy (SEM)
Samples used in AFM and SEM analyses were selected for relatively
flat crystal faces. Because only the dominant rhombohedral form was
observed, synthetic dolomite crystal surfaces are assumed to correspond
to the d104 growth face. Natural dolomite crystal surfaces are assumed to
correspond to the d104 cleavage plane. Natural and synthetic dolomite
crystals used in the etching experiments were submerged in 0.5% H2SO4
solutions for 10–20 seconds at room temperature. Etched samples were
then flushed with distilled water and dried in a vacuum desiccator.
AFM analysis was conducted on a Digital Instruments Nanoscope III
using constant force mode. Images were acquired under atmospheric
conditions using silicon nitride cantilevers (NP-20) with an average tip
radius of 20 nm. According to the manufacturer (Veeco Probes), 1 in 10
NP-20 tips have a radius of 5 nm. This is important because a smaller tip
enhances the ability to resolve smaller surface features (Eggleston 1994).
To limit artifacts, the force between each sample and the cantilever was
minimized. According to our force calibration procedures the applied
force was 5–20 nN. Although artifacts can form as a result of tip-surface
interaction in this force range, no change in surface nanotopography was
observed during extended scans (e.g., 20 minutes). Artifacts can also arise
during imaging because of irregularly shaped cantilevers, as well as
irregular cantilever movement across a crystal surface. In an effort to
detect and minimize these types of artifacts, samples were periodically
rotated and cantilevers replaced. All images reported here were
reproduced on more than one crystal from each sample using multiple
cantilever tips.
Following AFM analyses, samples were prepared with a 2 nm
electrically conductive osmium coating for observation with a JEOL
6300 field emission SEM at 10 kV. The mineralogy of individual crystals
was verified with energy dispersive spectroscopy (EDS).
FIG. 2.—SEM micrograph of dolomite (d) forming on the edges and corners of
a dissolving calcite crystal (c).
RESULTS
High-temperature synthesis experiments are characterized by a relatively long induction period in which no products are detected by XRD after
the onset of experimental conditions. The length of the induction period,
as shown in Figure 1, is a function of the initial Mg:Ca ratio in solution.
Products are detected sooner in solutions with high Mg:Ca ratios and
later in solutions with low Mg:Ca ratios. In all experiments, the first
products detected by XRD are invariably poorly ordered crystalline
dolomite. SEM observations indicate that the first dolomite rhombohedra form preferentially on edges and corners, and in crystallographic
continuity with the dissolving calcite substrate (Fig. 2). Synthetic
dolomite compositions range from stoichiometric to Ca-rich (, 61
mol% CaCO3) with magnesium contents dictated by the initial Mg:Ca
ratio in solution (R2 5 0.97). Figure 3 illustrates a marked increase in
mole percent MgCO3 with increasing Mg:Ca ratio in solution for runs
ended prior to calcite depletion. The overall dolomitization reaction rate
is proportional to the initial Mg:Ca in solution, and in all cases, proceeds
very rapidly once products are detected. Figure 4 plots percent product
vs. time for the Mg:Ca 5 1.27 experiment. Provided that not all calcite
reactants have been depleted, the slope of the reaction curve is used as
a measure of the reaction rate. After depletion of calcite reactants all
dolomites become stoichiometric.
Although major reflections indicate that products are crystalline,
dolomite from runs ended before 50% replacement lack superstructure
reflection peaks, thus indicating poor ordering. Cation order in dolomite
from runs with greater than 50% product relatively rapidly increases with
time. In some experiments cation order increased linearly with time, but in
others it increased less rapidly after calcite reactants were depleted (Fig. 5).
Despite differences in solution chemistry and product composition,
synthetic dolomites always become well ordered and stoichiometric after
calcite reactants are exhausted. Comparison of data from these experiments indicates that synthetic dolomite produced in experiments stopped
prior to calcite depletion have ordering ratios between 0.0 and 0.45,
whereas dolomites from experiments stopped after calcite depletion have
ordering ratios between 0.50 and 0.88. Because of differences in cation
order, the terms nonideal and ideal are used herein to describe poorly
ordered and relatively well ordered synthetic dolomite, respectively.
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GROWTH AND DISSOLUTION FEATURES ON NATURAL AND SYNTHETIC DOLOMITES
427
FIG. 5.— Plot of cation order vs. time for the Mg:Ca 5 1.5 experiment.
FIG. 3.— Plot of dolomite stoichiometry vs. solution Mg:Ca for experiments
ended prior to calcite reactant depletion (R2 5 0.97, N 5 69).
The natural dolomites studied here have compositions that range from
stoichiometric to Ca-rich (, 54 mol% CaCO3) and ordering ratios that
range from 0.44 to 1.22 (Table 1). In general, XRD reflection peaks for
ancient dolomites are stronger than those observed for synthetic
dolomites with comparable magnesium contents.
Examination of crystal surfaces using SEM and AFM shows that ideal
and nonideal dolomites have different surface topographies. SEM
observations show that nonideal synthetic dolomites have hummocky
surfaces with irregular growth fronts (Fig. 6A), whereas ideal synthetic
dolomites are generally flatter, with layers separated by straight-sided
FIG. 4.— Plot of percent product vs. time for the Mg:Ca 5 1.27 experiment.
steps (Fig. 6B). Despite these differences, only AFM provides the
resolution necessary to clearly distinguish the nanotopography of
nonideal and ideal dolomites.
AFM observations of synthetic dolomite growth surfaces also reveal
two principal features, described herein as mounds and layers. Mounds
are rounded positive-relief features (Fig. 7) 10–200 nm in diameter and 1–
20 nm high. Growth mounds cover the surfaces of all nonideal dolomites
prior to calcite depletion despite highly variable solution Mg:Ca ratios
and stoichiometries. Following calcite reactant depletion, ideal dolomite
crystal surfaces are characterized by flat, laterally extensive layers with
steps that measure a few tenths (, 0.2 nm) to tens of nanometers high.
Layers have very flat surfaces and do not have mounds on top of them.
Some layers have straight-sided steps (Fig. 8A), whereas others are
curved steps (Fig. 8B). In either case, steps generally run parallel to one
another. In some cases, layers emerge from otherwise flat crystal faces
(Fig. 8C), but no classic multilayer, helical growth spirals (hillocks) were
observed. Following reactant depletion, all ideal synthetic dolomites
observed with AFM had layers regardless of the solution Mg:Ca ratio.
Chemically etched synthetic dolomite crystals are characterized by two
distinct surface nanotopographies: mounds, and layers with euhedral etch
pits. Mounds characterize etched nonideal synthetic dolomites, whereas
euhedral pits characterize the surfaces of etched ideal synthetic dolomites.
Mounds on etched crystals are indistinguishable from the dolomite
growth mounds described above. Euhedral etch pits have pointed
bottoms and occur on relatively flat layers. Etch pits measure 30–
200 nm in diameter and 1–30 nm deep (Fig. 9).
Chemically etched natural dolomite crystal surfaces have the same
features as chemically etched synthetic dolomites. Mounds were observed
on all chemically etched natural nonstoichiometric dolomites as well as
stoichiometric dolomites with cation ordering ratios below 0.67, whereas
layers with euhedral pits were observed on the surfaces of stoichiometric,
relatively well ordered ($ 1.0) dolomite (Table 1). Individual mounds are
rounded topographical highs (Fig. 10) that measure 20–300 nm in
diameter and 1–20 nm high. Mound-covered nonideal dolomites do not
have etch pits or flat layers with steps. Holocene dolomites from Abu
Dhabi have mounds on both etched and unetched surfaces. Prior to
chemical etching the mounds were less pronounced, but they became
more pronounced after being exposed to etching solutions for a few
seconds. Chemically etched ideal natural dolomites exhibit euhedral etch
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FIG. 7.—AFM image of a nonideal synthetic dolomite from the Mg:Ca 5 1.0
experiment showing mound nanotopography.
FIG. 6.— SEM micrographs of dolomite growth surfaces comparing synthetic
dolomites from the Mg:Ca 5 1.0 experiment. A) Nonideal dolomite showing
hummocky growth features. B) Ideal dolomite showing relatively flat growth
surfaces separated by straight-sided steps.
pits with flat inter-pit layers separated by steps that run parallel to one
another (Fig. 11). Euhedral etch pits are typically rhombic and measure
20–500 nm wide and 5–90 nm deep with pointed bottoms (Fig. 12).
DISCUSSION
Mounds observed on the surfaces of natural and synthetic dolomites
indicate that natural and synthetic dolomites form in a similar manner,
but the mechanism is unclear. Mounds are not characteristic of any of the
three classic crystal growth models—mononuclear, polynuclear, and
spiral growth (Nielsen 1964: Ohara and Reid 1973; Sunagawa 1984).
None of these models predict rounded topographic highs that do not have
steps or crystal faces. Polynuclear growth features, often referred to as
islands, have been observed on the surfaces of calcite, gypsum, and barite.
Islands tend to be flat topped and straight edged with layers one to several
monolayers thick (Hillner et al. 1992; Dove and Hochella 1993; Bosbach
and Rammensee 1994; Bosbach et al. 1998; Pina et al. 1998; Teng et al.
2000; Pina et al. 2004). It is possible that dolomite mounds have steps that
are below the resolution of the AFM. If steps of any height are separated
by only a few nanometers, then the AFM tip may ride over them with
little or no deflection. The best indication of the size and spacing of steps
that can be imaged on mounds is the size and spacing of steps that we
have imaged in etch pits. We have clearly imaged steps that are less than
0.2 nm in height and 4 nm apart. We are also certain that it is possible to
image straight edges that are at least six nanometers long because this is
the shortest straight edge we have imaged on an etch pit. If steps are
present on mounds, then they must be less than 0.2 nm high and 4 nm
apart. Mounds also differ from islands in that islands are flat topped
whereas mounds have rounded tops. Growth hillocks centered on spiral
dislocations with steps too small to resolve could appear rounded, like the
mounds we observe. In this case, pits should form at the apices of
dislocations when chemically etched, but etching of mounds only reveals
more mounds. These comparisons indicate that mounds are most likely
smooth and rounded topographic highs, which are different from
crystalline growth features formed by mononuclear, polynuclear, or
spiral growth.
Because dolomite growth mounds are rounded and lack crystal faces,
they are similar in shape to the rounded morphologies of amorphous
calcium carbonate (ACC) (Reddy and Nancollas 1976; Raz et al. 2000;
Kelleher and Redfern 2002; Loste et al. 2003; Schmidt et al. 2005). Reddy
and Nancollas (1976) added Mg2+ to [Ca2+]:[CO322] 5 1 solutions and
observed the formation of amorphous precipitates after 3 minutes. After
20 days combinations of aragonite spherulites and irregularly shaped
calcite were formed. Raz et al. (2000) documented the formation of
magnesium calcites via a partially stabilized amorphous phase that
contained up to 21 mol% magnesium. Loste et al. (2003) demonstrated
that ACC was the first phase to form during precipitation from highly
supersaturated solutions containing magnesium and that ACC later
crystallized into Mg-calcite. They also showed that the magnesium
content of the ACC increased with the Mg:Ca ratio in solution, which is
consistent with our findings (see Fig. 3). Stolarski and Mazur (2005)
etched biogenic calcium carbonate believed to have formed via
amorphous calcium carbonate precursors and found mounds similar to
those observed in this study. DiMasi et al. (2006) found that during
biomimetic mineralization of CaCO3, the formation of amorphous
calcium carbonate films was partially controlled by magnesium.
The mound-shaped nanotopography on the surfaces of nonideal
dolomites is consistent with an amorphous CaMg-carbonate precursor,
but crystallization to nonideal dolomite must take place soon after
deposition because nonideal dolomite was detected by XRD throughout
the reaction. This is not unreasonable at the high temperatures employed
in our experiments. At room temperature amorphous CaMg-carbonates
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GROWTH AND DISSOLUTION FEATURES ON NATURAL AND SYNTHETIC DOLOMITES
429
FIG. 8.—AFM images of ideal synthetic dolomite growth surfaces with layers. A) Dolomite growth layers from the Mg:Ca 5 1.0 experiment showing a straight-sided
step that measures 0.6 nm high. B) Stacked layers with curved steps on a dolomite growth surface from the Mg:Ca 5 1.5 experiment. Step height between arrows is
0.2 nm. C) Dolomite surface from the Mg:Ca 5 0.66 experiment showing emerging growth layers.
have been shown to crystallize to calcite and/or aragonite in minutes to
days (Reddy and Nancollas 1976; Tracy et al. 1998, Loste et al. 2003), and
according to Schmidt et al. (2005) the time for crystallization of
amorphous CaMg-carbonate decreased with increasing temperature.
The ability to detect growth mounds in ancient nonideal dolomites with
chemical etching indicates that mounds maintain their growth structure
and remain as separate physical entities over a long time. If adjacent
growth mounds with slightly different composition or cation order meet
during growth, lattice mismatch is likely to result. In this case, the
interfacial free energy between mounds could prevent them from
coalescing in a way that anneals the boundary. If such interfaces exist,
crystals grown by mounds would have a pervasive internal defect
structure. This is consistent with the modulated microstructures observed
with TEM in ancient Ca-rich dolomites (Barber 1977; Frisia 1994; Reeder
2000; Schubel et al. 2000). Modulations, which measure nanometers to
tens of nanometers in wavelength, are attributed to chemical and/or
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FIG. 9.— AFM image of an etched ideal synthetic dolomite from the
Mg:Ca 5 0.66 experiment showing euhedral etch pits on flat layers.
FIG. 11.—AFM image of an etched natural ideal dolomite surface (Sample S-1)
showing euhedral etch pits with flat inter-pit layers.
structural domains caused by fluctuations in stoichiometry and ordering
created during growth (Reeder 2000). Without defective growthinterfaces, mounds would coalesce into defect-free layers. As a result,
these interfaces would be undetectable with chemical etching because the
energetic drive for preferential dissolution between individual growthmounds would not exist.
Layers and euhedral etch pits on the surfaces of ideal dolomite indicate
either spiral or polynuclear growth. In spiral growth, the most common
growth mechanism found in natural crystals (Sunagawa 1977, 1981),
material is added to step and kink sites along layers created by spiral
dislocations. The most convincing evidence for spiral growth is the
presence of multilayer spiral hillocks (Hillner et al. 1992; Gratz et al.
1993; Bosbach and Rammensee 1994; Bosbach et al. 1998; Pina et al.
1998; Sunagawa 2005). Some layers observed on ideal synthetic dolomite
merge into the crystal surface (see Fig. 10C), thus producing a geometry
that is consistent with a spiral dislocation outcrop (Maiwa et al. 1998;
Pina et al. 1998; Teng et al. 2000). However, no multi-layer helical growth
spirals were observed. Kessels et al. (2000) suggested that stacked layers
on synthetic dolomite form by birth and spread. Birth-and-spread growth
is a variety of polynuclear growth characterized by attachment of growth
FIG. 10.— AFM image of an etched natural nonideal dolomite (Sample SB 35)
with a mound-covered surface.
FIG. 12.— AFM image of an etched natural ideal dolomite surface (Sample
MH-2) showing crystallographically oriented rhombic etch pits.
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GROWTH AND DISSOLUTION FEATURES ON NATURAL AND SYNTHETIC DOLOMITES
units (ions or molecules) along the periphery of individual surface nuclei.
These nuclei can advance laterally until they intersect and fuse with other
nuclei to form layers. Therefore, the birth-and-spread model could also
explain some of the geometries observed on synthetic dolomite surfaces,
such as stacked layers.
Euhedral etch pits that are hundreds of monolayers deep are indicative
of dissolution at extended line defects, such as screw dislocations at the
center of growth spirals (Brantley et al. 1986; Sangwal 1987; Lin and Shen
1993). Extended line defects are the site of preferential dissolution because
defects have excess strain energy (Lasaga 1990). Point defects, such as
precipitates and inclusions, may also be sites of etch pit formation
because they too are zones of elevated reactivity. Point defects, however,
promote only shallow, flat-bottomed etch pits, because once a pit
nucleates and the point defect is removed, the drive for dissolution in
a direction normal to the crystal surface is eliminated. Gratz et al. (1991)
observed the formation of shallow and deep pits in quartz crystals during
chemical etching experiments. Hillner et al. (1992) and Liang et al. (1996)
made similar observations in calcite using AFM. These authors argued
that shallow, flat-bottom pits corresponded to point defects, whereas
deep, pointed pits formed at extended line defects. Based on his
observations using in situ AFM techniques, Teng (2004) reported that
the value of undersaturation at which deep etch pits began to form on
freshly cleaved calcite crystals was in good agreement with the value
predicted by dislocation theory for pit formation at line dislocations.
Lendvay et al. (1971) found that formation of etch pits in undersaturated
conditions corresponded to sites of spiral-growth hillocks in experimentally grown crystals. Therefore, flat growth layers with steps observed in
conjunction with deep, euhedral etch pits on both natural and synthetic
ideal dolomites are consistent with the spiral-growth model.
Mounds always form first during dolomitization of calcite, whereas
layers form only after calcite reactants have been depleted. Therefore,
over the conditions tested, dolomite surface nanotopography is independent of the Mg:Ca ratio in solution. Because the overall rate of the
reaction is proportional to the Mg:Ca ratio in solution, surface
nanotopography is also independent of the overall rate of dolomitization.
Although we do not fully understand the mechanism for changing surface
topography, it appears to be a function of carbonate ion concentration
because the carbonate ion concentration probably decreases significantly
when the calcite reactant is used up. Based on the experimental design, the
carbonate ion concentration for any Mg:Ca ratio is likely to remain
relatively constant as long as calcite is present because prior to reactant
depletion calcite solubility dictates the concentration of carbonate ions in
solution. As dolomite begins to form and sequesters carbonate ions, more
calcite dissolves. Following calcite depletion, however, metastable,
nonideal dolomite is the only source of carbonate reactants for the
growth of ideal dolomite. Although it is not possible to measure the
carbonate activity in the reaction vessels, it must be lower after calcite
depletion because nonideal dolomite is less soluble than calcite (Carpenter
1980; Chai et al. 1995). Therefore, mounds form early in the reaction
when the carbonate ion concentration is relatively high and layers form
after calcite depletion when carbonate ion concentration is relatively low.
This is consistent with the fact that all low-temperature precipitation of
synthetic amorphous calcium carbonate has been accomplished in
solutions with high carbonate ion concentrations (Reddy and Nancollas
1976; Tracy et al. 1998, Kelleher and Redfern 2002; Loste et al. 2003).
CONCLUSIONS
AFM and SEM observations of natural dolomites reveal two distinct
etch features: amorphous mounds, and layers with euhedral pits. Mounds
are observed on the surfaces of Ca-rich and/or disordered (nonideal)
dolomite, whereas layers with pits are characteristic of stoichiometric and
relatively well-ordered (ideal) dolomite. Mounds and layers with etch pits
431
also characterize the surfaces of high-temperature synthetic nonideal and
ideal dolomites, respectively. The observations presented here indicate
that despite differences in formation conditions, high-temperature
synthetic dolomites and ancient dolomites have the same surface
nanotopography, thus indicating that they form by similar mechanisms.
Therefore, the results of this study are further evidence that hightemperature synthetic dolomitization reactions may be accurate analogs
for low-temperature natural dolomitization.
In all cases, the formation of well-ordered, stoichiometric synthetic
dolomite proceeds through a poorly ordered, metastable phase characterized by rounded growth mounds. The presence of mounds on the
surfaces of nonideal dolomites is inconsistent with standard models of
crystal growth but is indicative of an amorphous precursor. The two
different surface nanotopographies described here are consistent with
a model in which nonideal dolomite forms by precipitation of amorphous
mounds that quickly crystallize to nonideal dolomite while ideal dolomite
replaces nonideal dolomite and grows by spiral growth. This new model
for dolomite crystal growth will have to be tested with actively forming
natural dolomites before we can be confident that it is accurate. If
nonideal dolomites form via an amorphous precursor, interpretations of
the origin of many dolomites will change. For example, Schmidt et al.
(2005) postulated that amorphous precursors to dolomite might be
important in understanding the oxygen isotope composition of dolomite.
Also, if nonideal dolomites form via amorphous precursors, then
nonideal dolomites probably form in solutions characterized by high
degrees of supersaturation with respect to dolomite and high carbonate
ion concentrations.
ACKNOWLEDGMENTS
The authors would like to thank Kitty Milliken, Leslie Melim, Patricia
Dove, John Southard, and an anonymous reviewer for helpful editorial
comments during the review process. We would also like to further
acknowledge the thoughtful comments of Patricia Dove and thank her for
introducing us to the idea of amorphous carbonate precipitation.
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Received 5 December 2005; accepted 2 November 2006.