Direct Evidence That Well-Ordered, Stoichiometric (Ideal) Dolomites are the Product of Recrystallization*
Stephen E. Kaczmarek1, and Duncan F. Sibley2
Search and Discovery Article #50693 (2012)**
Posted August 20, 2012
*Adapted from oral presentation at AAPG Annual Convention and Exhibition, Long Beach, California, April 22-25, 2012
**AAPG©2012 Serial rights given by author. For all other rights contact author directly.
1
2
Geological Sciences, Bridgewater State University, Bridgewater, MA ([email protected])
Geological Sciences, Michigan State University, East Lansing, MI
Abstract
Geochemical, structural, and mineral surface data from a broad assemblage of synthetic and natural dolomites suggest that
stoichiometric and well ordered (ideal) dolomites form through recrystallization of a nonstoichiometric and/or poorly ordered
(nonideal) dolomite precursor. Synthetic dolomite is formed by replacement of calcite in Mg-Ca-Cl fluids at high-temperature
(>200°C). The first synthetic dolomites to form are invariably poorly ordered with stoichiometries that strongly reflect the initial
Mg/Ca of the fluids. Near reaction completion, when calcite reactants are nearly consumed (>95% dolomite), nonideal dolomite is
rapidly replaced by a stoichiometric and relatively well-ordered, ideal dolomite.
Scanning electron (SEM) and atomic force microscope (AFM) observations of synthetic dolomite crystal surfaces reveal that nonideal
dolomites are covered with round growth mounds and ideal dolomite surfaces are characterized by flat growth layers separated by
elongate steps. When synthetic dolomite crystals are etched in dilute acid, the surfaces of nonideal dolomites remain covered with
rounded mounds, whereas ideal dolomites are characterized by flat layers with deep, euhedral etch pits. Comparable surface features
are also observed on the surfaces of natural dolomite crystals. Chemically etched nonideal dolomite crystal surfaces have mounds and
ideal dolomite crystal surfaces are characterized by flat surfaces with euhedral etch pits.
Mineral surface textures, integrated with dolomite stoichiometry and cation order data, are consistent with a stepwise growth model in
which nonideal dolomite first forms by precipitation of growth mounds, and ideal dolomite forms by a spiral growth mechanism only
through replacement of a nonideal dolomite precursor. Because layers and etch pits dominate the surfaces of etched ideal dolomites,
and ideal dolomites form by replacement of a nonideal dolomite precursor, flat layers with euhedral etch pits are interpreted as direct
physical evidence of recrystallization. An independent evaluation of recrystallization in natural dolomites is valuable because
geochemical data are commonly used to interpret the chemistry of dolomitizing fluids. If recrystallization can be established using an
independent test, chemical analyses can be more accurately interpreted as to whether they reflect the original dolomitizing fluid or
later diagenetic solutions.
Selected References
Kaczmarek, S.E., 2005, Crystal growth mechanisms in natural and synthetic dolomite; insight in dolomitization kinetics: Ph.D.
dissertation, Michigan State University, Dept. of Geological Sciences, 230 p.
Kaczmarek, S.E., and D.F. Sibley, 2011, On the evolution of dolomite stoichiometry and cation order during high temperature
synthesis experiments; an alternative model for the geochemical evolution of natural dolomites: Sedimentary Geology, v. 240/1-2, p.
30-40.
Kaczmarek, S.E., and D.F. Sibley, 2007, A comparison of nonometer-scale growth and dissolution features on natural and synthetic
dolomite crystals; implications for the origin of dolomite: JSR, v. 77/5, p. 424-432.
Kupecz, J.A., and L.S. Land, 1995, Progressive recrystallization and stabilization of early-stage dolomite; Lower Ordovician
Ellenburger Group, West Texas, in B. Purser, M. Tucker, and D. Zenger, (eds.), Dolomites; a volume in honour of Dolomieu: Special
Publication of the International Association of Sedimentologists, v. 21, p. 255-279.
Machel, H.G., 1997, Recystallization versus neomorphism, and the concept of “significant recrystallization” in dolomite research:
Sedimentary Geology, v. 113/3-4, p. 161-168.
Direct Evidence that Well Ordered,
Stoichiometric (Ideal) Dolomites are the
Product of Recrystallization
Stephen E. Kaczmarek1 and Duncan F. Sibley2
Bridgewater State University, Department of Geological Sciences
Michigan State University, Department of Geological Sciences
The Problem
“One of the most vexing problems in dolomite
research is to determine whether the textures
and compositions of replacive dolomites
represent the conditions of their formation, or
whether the present textures and geochemical
compositions represent merely the last of
possibly several episodes of recrystallization.”
Machel (1997, Sed. Geol. )
Dolomite Recrystallization
“Some studies of dolomite have suggested
that present-day geochemical signatures are
the product of recrystallization, not original
precipitation.”
-Land et al. 1975, Zenger 1981, Gregg & Sibley 1984,
Banner et al. 1988, Dorobek & Filby 1988, Moore et
al. 1988, Zenger & Dunham 1988, Montanez & Read
1992, and many others.
Kupecz & Land (1994, IAS Spec. Pub. 21)
Lack of Direct Physical Evidence
• Without direct physical evidence of
recrystallization, we have a circular argument…
“If the chemical signature of a dolomite appears
primary, we can use the chemical signature to
interpret the composition of the original
dolomitizing fluid.”
High Temperature Dolomite Synthesis
Replacement Reaction
Mg2+ + 2CaCO3
CaMg(CO3)2 + Ca2+
Dissolution-Reprecipitation Reaction
2 μm
50 μm
Kaczmarek & Sibley (2007, J. Sed. Res .)
I - - - - - - - - Increasing Reaction Progress - - - - - - - - -
Kaczmarek & Sibley (2011, Sed. Geol. )
Nonideal & Ideal Dolomite
• Stoichiometry & Cation Order
• Composition – Ingredients
• Arrangement – Recipe
• Nonideal
• Ideal
Smyth (U. Colorado)
X-Ray Diffraction
XRD Pattern for Ideal Dolomite
Ideal Ordering
12000
31.00
8000
intensity
Intensity
16000
CaF2
Standard
4000
0
20
25
30
35
40
500
450
400
350
300
250
200
150
100
50
0
33
2-theta
34
35
36
37
38
37
38
2-theta
XRD Pattern for Nonideal Dolomite
Nonideal Ordering
4000
250
intensity
Intensity
300
30.90
3000
CaF2
Standard
2000
1000
200
150
100
50
0
20
25
30
2-theta
35
40
0
33
34
35
36
2-theta
Surface Topography
Nonideal Synthetic Dolomite
Ideal Synthetic Dolomite
Kaczmarek & Sibley (2007, J. Sed. Res .)
AFM – Atomic Force Microscope
photodiode
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Surface Nanotopography
Nonideal Synthetic Dolomite
Kaczmarek & Sibley (2007, J. Sed. Res .)
Nonideal Synthetic Dolomite
Kaczmarek (2005)
Ideal Synthetic Dolomite
Kaczmarek & Sibley (2007, J. Sed. Res .)
Ideal Synthetic Dolomite
Kaczmarek (2005)
Increasing Cation Order
Reaction Progress
Amorphous Growth:
Nonideal Dolomite
Mounds
CaCO3
Substrate
Amorphous Mounds Crystallize
Nonideal Dolomite
Spiral Growth:
Ideal Dolomite
Layers
Nonideal Dolomite
Dissolves
Substrate
Dissolves
CaCO3 Depletion
Recrystallization
The difficulty with
evaluating this
model in natural
dolomites is that
natural samples
rarely preserve
pristine growth
surfaces.
Ideal Dolomite
Kaczmarek (2005)
Chemical Etching Experiments
• Attacks chemical and structural imperfections
incorporated into crystal during growth
Growth mounds have interfacial
free energy (like surface tension
between soap bubbles)
Dislocations produce
strain in the crystal lattice
(like a loaded spring)
Chemically Etched Synthetic Dolomite
Nonideal Synthetic Dolomite
• Mounds
Ideal Synthetic Dolomite
Chemically Etched Natural Dolomite
Nonideal Natural Dolomite
Ideal Natural Dolomite
Secti on Analysi s
n"
.
~
N
L
71.244 nM
9.259
R"S
Ie
DC
RaU oJ 1.368
RMilX
Rz
0
Rz Cnt 2
~-
,
N
,
,
,
,
,
0
200
400
GOO
800
n"
Surface distance 79.028
Horiz distanceCLJ 71. 244
SpectruM
,
DC
05081426.001
5.777
5.777
n"
n"
n"
n"
n"
n"
Uert distance
30.469
Angle
23.155 de.
Surface distance
Horiz distance
Uert distance
Angle
Spectral perioel
Hin
n"
Spectral freq
Spectral RHS aMp
11 Formations - Ordovician to Cenozoic
Ideal Dolomite
Amorphous Growth
350 nm
Spiral Dislocation
Spiral
Growth
Nonideal Dolomite
Kaczmarek
350 nm
Conclusions
• Synthetic dolomite grows by same
mechanisms as natural dolomite
• Stepwise Replacement Growth Model
– Nonideal Dolomite rapidly replaces calcite
• Mound Growth
– Ideal Dolomite more slowly replaces nonideal
dolomite
• Layers/Spiral Growth
Conclusions
• Etch pits are evidence for layer/spiral
growth in ideal dolomite
• Layer/spiral growth occurs only during
replacement of nonideal dolomite
precursor
• Etch pits are direct physical evidence of
recrystallization in ideal dolomite
Direct Evidence that Well Ordered,
Stoichiometric (Ideal) Dolomites are the
Product of Recrystallization
Stephen E. Kaczmarek1 and Duncan F. Sibley2
Bridgewater State University, Department of Geological Sciences
Michigan State University, Department of Geological Sciences
Solution Chemistry & Stoichiometry
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0.3 0 .4 0.5 0.6 0.7 0.8 0.9
1
1.1 1.2 1.3 1.4 1.5 1.6
Inital Solution Mg:Ca (mol/mol)
Kaczmarek & Sibley (2011, Sed. Geol. )
Crystal Growth Models
Spiral Growth
Birth and Spread
Polynuclear Growth
Amorphous Growth
Ideal Synthetic Dolomite
Kaczmarek & Sibley (2007, J. Sed. Res .)
Significant Recrystallization
'Significant recrystallization" is a modification via
recrystallization of the original texture, ordering,
chemical composition, or magnetic properties that is
larger than the original range during dolomite formation.. Significant recrystallization is recognized, if
recrystallization modified at least one of the above
properties to a range larger than the original
Machel (1997, Sed. Geol. )