Carbonatos

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KINETIC CONTROL OF MORPHOLOGY, COMPOSITION, AND MINERALOGY OF 
ABIOTIC SEDIMENTARY CARBONATES1 
R. KEVIN GIVEN AND BRUCE H. WILKINSON 
Department of Geological Sciences 
The University of Michigan 
Ann Arbor, Michigan 48109-1063 
ABSTRACT: Conventional thinking has long held that the abiotic precipitation of calcium carbonate occurs with a causal relationship 
between fluid Mg/Ca ratios and crystal morphology, crystal composition, and carbonate mineralogy, resulting in the formation of 
meteoric, equant, low-magnesium calcite and marine, acicular, high-magnesium calcite and aragonite. Problematically, calcites with 
varying amounts of incorporated magnesium occur either as equant or acicular crystals, and aragonite may coexist with calcite in 
either environment. 
Commonly, however, a systematic relation exists between crystal morphology, composition, mineralogy, and rates of reactant 
supply to growing crystal surfaces. For example, equant rather than acicular crystals of calcite form in modem, deep and/or cold 
marine, meteoriophreatic, and deep-burial settings where the degree of carbonate saturation and/or rates of fluid flow are low. In 
areas of higher saturation and/or fluid flow, such as in warm, shallow-marine and meteoric-vadose environments, acicular calcite 
may predominate. This relation is also seen in systems in which aragonite and calcite form in intimate association. Aragonite 
precipitation is favored when rates of reactant supply are high; calcite forms when rates are low. 
Such relations suggest that crystal morphology, composition, and mineralogy are controlled by the kinetics of surface nucleation 
and the amount of reactants, principally carbonate ions, at growth sites. Precipitating phases are the ones which can best accommodate 
such excess reactants; ambient MgKa ratios only indirectly control the nature of inorganically precipitated carbonate phases. 
INTRODUCTION 
The relations between various chemical and physical 
parameters, and the morphology, mineralogy, and com- 
position of abiotic calcium carbonates have been the sub- 
ject of intense interest among carbonate geologists since 
the pioneering work of Sorby (1 879). Furthermore, it has 
long been held that the composition and morphology of 
inorganically precipitated sedimentary carbonate crystals 
are, in large part, controlled by the Mg/Ca ratio of the 
precipitating solution. Classically, precipitation from so- 
lutions with low Mg/Ca ratios, such as meteoric or basinal 
connate fluids, has been thought to result in the formation 
of equant crystals of low-magnesium calcite (Fig. 1A). 
Conversely, precipitation from fluids with high Mg/Ca 
ratios, such as sea water or hypersaline brines, results in 
acicular forms of high-magnesium calcite and/or aragon- 
ite (Fig. IB, C). This apparent correlation between the 
composition of ambient fluids, the composition of car- 
bonate phases, their mineralogy, and their crystal habits 
has led many authors (e.g., Folk 1974; Lahann 1978) to 
suggest that the relative concentration of magnesium ions 
in ambient fluids and on growing crystal surfaces controls 
not only the composition but also the crystal habit of 
precipitated phases. 
Although most occurrences of inorganically precipitat- 
ed calcium carbonate in modem settings seem to corrob- 
orate this suggested control of composition and mor- 
phology by ambient Mg/Ca ratios, there are significant 
exceptions, both compositionally and morphologically, 
in marine and meteoric environments. These examples 
require that MgXa ratios have had little effect on crystal 
morphology, composition, and mineralogy. In the follow- 
ing, we examine the range of carbonate compositions and 
crystal morphologies known from both meteoric and ma- 
' Manuscript received 6 February 1984; revised 2 1 September 1984. 
rine environments and then develop a model which ex- 
plains both "normal" and "exceptional" carbonate mor- 
phologies, compositions, and minerdogies in various 
precipitational settings. 
Crystal Morphology 
Low-magnesium calcite, although the dominant ce- 
ment phase as equant crystals in meteoric and deep burial 
diagenetic environments, also may exhibit acicular mor- 
phologies. Such occurrences include whisker and needle 
cements (e.g., Fig. 2A, Longman 1980), beachrock ce- 
ments along freshwater lakes (e.g., Fig. 2B, Binkley et al. 
1980), travertine deposits at spring effluents (Braithwaite 
1979), and calcite in spelean deposits (e.g., Fig. 2C, Ken- 
dall and Broughton 1978). Such acicular, low-magnesium 
calcite crystals form almost exclusively in meteoric-va- 
dose settings, many from fluids with strikingly low Mg/ 
Ca ratios. 
Similarly, high-magnesium calcite, although a domi- 
nant cement phase as acicular crystals in shallow-marine 
environments, may also exhibit equant morphologies. 
Such forms have been reported as marine cement from 
Belize (James and Ginsburg 1979), Bermuda (Schroeder 
1972), Eniwetok (Warme and Schneidermann 1983), and 
the Bahamas (Pierson and Shinn 1983). A nearly identical 
spectrum of marine calcite cements, ranging from acicular 
(e.g., Walls et al. 1979; Krebs 1969) to equant (e.g., Long- 
man 1976; Delgado 1979) are also reported from the rock 
record. In total, the full spectrum of crystal habits exhib- 
ited by inorganic calcium carbonate can form in, and has 
been reported from, pore-water environments with rad- 
ically different ambient MgKa ratios. Furthermore, both 
high-magnesium and low-magnesium calcites exhibit a 
broad spectrum of morphologies ranging from equant to 
highly acicular (Figs. 3 and 4). 
JOURNAL OF SEDIMENTARY PETROLOGY, VOL. 55, NO. 1, JANUARY, 1985, P. 010941 19 
Copyright O 1985, The Society of Economic Paleontologists and Mineralogists 0022-4472/85/0055-0109/$03.00 
1 l0 R. KEV IN G IVEN AND BRUCE H. WILK INSON 
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F1G. 1.--SEM images of typical carbonate cements. A) Equant, low- 
magnesium calcite; Miocene Camp Davis Vormation, Wyoming. Such 
habits and compositions are typical of meteoric and deeper-burial ce- 
ment crystals. B) Elongate Holocene high-magnesium calcite marine 
cement crystal; central Texas shelf. Growth has been most rapid in the 
c-axis direction• C) Acicular Holocene aragonite marine cement; central 
Texas shelf. 
FiG. 2. -- Acicular, low-magnesium calcite cements. A) Thin section 
of whisker calcites in a vadose pore; Pleistocene Key Largo Limestone, 
Florida. B) SEM image ofacicular, low-magnesium calcite from modern 
beach rock along a temperate-region marl lake; Ore Lake, southeastern 
Michigan. This is a composite trigonal prism made up of smaller trigonal 
prisms with rhombic terminations. Elongation is parallel to the c-axis. 
C) Negative print of a thin section (crossed polars) of low-magnesium 
calcite from a cave in the Cretaceous Edwards Formation, Comal Coun- 
ty, Texas. These, and most other cave carbonate crystals, are acicular 
in form. 
ABIOTIC SEDIMENTAR Y CARBONATES 111 
FIG. 3.--SEM images of morphologies in Holocene low-magnesium and high-magnesium calcites. A) Slightly elongate, meteoric, low-magnesium 
calcite cement from Ore Lake, Michigan. B) Slightly elongate, marine, high-magnesium calcite from the central Texas shelf. Note that, at least 
at this level of resolution, this cement and that in Figure 3A are almost indistinguishable. 
Crystal Composition 
As with morphology, the specific mineralogy and com- 
position of calcium carbonate phases may bear little re- 
lationship to the Mg/Ca ratio of the precipitating fluid. 
There is little doubt that in meteoric environments with 
low Mg/Ca ratios, the common carbonateis low-mag- 
nesium calcite (Fig. 1A). In marine settings with higher 
Mg/Ca ratios, high-magnesium calcite is typically the 
dominant phase (Fig. 1B), commonly coexisting with ara- 
gonite (Fig. IC), whereas in hypersaline lacustrine and 
supratidal settings with extremely high Mg/Ca ratios, ara- 
gonite may predominate (Muller et al. 1972). While these 
occurrences superficially implicate ambient Mg/Ca ratios 
as controlling compositions (e.g., Muller et al. 1972), sig- 
nificant exceptions exist. Low-magnesium calcite precip- 
itation from normal marine water is reported from Tongue 
of the Ocean (Schlager and James 1978), near England 
(Al-Hashimi 1977), and offTasmania (Rap 1981). In ad- 
dition, inorganically precipitated aragonite is reported 
from many meteoric spelean deposits (e.g., Curl 1962; 
Siegel 1965; Konishi and Sakai 1972). This spectrum of 
carbonate compositions from a variety of cementation 
environments requires that, except for the obvious fact 
that high-magnesium calcite will not precipitate from 
fluids greatly depleted in magnesium, crystal mineralogy 
and composition are not simply controlled by ambient 
Mg/Ca ratios. 
MORPHOLOGY AND GROWTH RATE 
If, as suggested by these occurrences, ambient, fluid 
Mg/Ca ratios have little effect on the nature of the car- 
bonate phase precipitated, what then controls crystal 
morphology, composition, and mineralogy? When the 
range of calcium carbonate crystals forming in all natural 
environments is considered, it appears that a strong cor- 
relation exists between the rate at which crystals grow and 
their morphology. 
In a general sense, the growth rate of carbonate phases 
is a direct function of the local degree of saturation (e.g., 
Berner and Morse 1974). In many meteoric environ- 
ments, the ions available for cementation are derived 
from the dissolution of metastable marine carbonate 
phases; hence, the concentration of Ca ++ and CO3 = ions 
in pore fluids is limited by the solubility of marine ara- 
gonite and high-magnesium calcite. As these metastable 
phases are only slightly more soluble than low-magne- 
sium calcite, the degree of supersaturation in meteoric 
waters is generally low, cement crystal growth rates are 
retarded, and the great majority of carbonate cements are 
equant (e.g., Fig. IA). Acicular crystals, on the other hand, 
are only found in vadose pores (e.g., Fig. 2) where pre- 
cipitation is driven by rapid degassing of CO2 and/or 
evaporation, leading to extremely high degrees of super- 
saturation. 
In marine environments, the bulk of carbonate cemen- 
tation takes place in relatively warm, shallow waters where 
precipitation is driven by decreases in the solubility of 
CO2 through heating and/or agitation. Shallow sea water, 
therefore, can be significantly supersaturated with respect 
to carbonate phases, and crystal growth rates may be 
correspondingly high. Accordingly, nearly all sustained 
precipitation of inorganic high-magnesium calcite or ara- 
gonite results in acicular crystal habits (e.g., Fig. 1 B, C). 
Equant low-magnesium calcite only forms in marine set- 
tings in cool-temperature climates or in deep, cold waters 
where dissolved COe (and, by implication, acidity) is high, 
the degree of supersaturation is low, and crystal-growth 
rates are slow. 
While establishing a correlation between rates of pre- 
cipitation and crystal habits does not in itself demonstrate 
that growth rates control morphology, theoretical and 
mineralogical considerations suggest that precipitation 
112 R, KEV IN G IVEN AND BRUCE H. WILK INSON 
FtG. 4.--SEM images of nearly identical morphologies in low-magnesium and high-magnesium calcite cements. Each are Holocene composite 
crystals as trigonal prisms consisting of aggregates of smaller-trigonal prism crystallites. A) Low-magnesium calcite ( 1.5 mole% MgCO3) from Ore 
Lake, Michigan. B) High-magnesium calcite (17 mole% MgCO3) from coastal southwestern Louisiana. 
rates should influence crystal habits. Because natural car- 
bonate crystals exhibit a broad spectrum of lengths and 
widths, it follows that there must be separate and distinct 
controls on growth rates along different crystallographic 
axes. Moreover, in order for an acicular crystal to form, 
local fluid chemistry must influence growth parallel to the 
c-axis differently than growth in any other direction. As 
noted by several authors (e.g., Folk 1974; Lahann 1978), 
this difference is directly related to the mineralogic struc- 
ture of carbonate phases. 
In a general sense, the growth of carbonate crystals is 
controlled by the incorporation of reactants into the grow- 
ing crystal lattice. During precipitation from an aqueous 
fluid, the rate-limiting step commonly is the dehydration 
of reactants. As calcium ions are more strongly hydrated 
than carbonate ions, growth rates should be limited by 
the dehydration of, and by extension, the concentration 
of, Ca+* ions. This is indeed the case for growth in any 
direction not parallel to the c-axis; growth in a c-axis 
direction, however, may not be limited by the dehydra- 
tion of Ca *+ ions. Planes perpendicular to the c-axis will 
always consist of layers of carbonate or calcium. When 
pH values are below 8, or when pH values are higher but 
there are substantially more calcium ions than carbonate 
ions in the fluid, as is the case in sea water, these surfaces 
are positively charged (Somasundaran and Agar 1967; 
Lahann 1978), indicating that they are composed of ad- 
sorbed Ca ++ ions. Apparently, once a carbonate ion at- 
taches to the growing crystal surface, it immediately at- 
tracts a calcium ion. The incorporation of the calcium 
ion is, in effect, instantaneous; this rapid adsorption in- 
dicates that the dehydration of Ca ++ in no way slows its 
incorporation into the growing crystal lattice. As would 
be predicted from studies of growth on uniformly charged 
crystal surfaces (Doremus 1958), therefore, the rate of 
precipitation in the c-axis direction is controlled by the 
availability of CO3 = ions (Doremus 1958; Lahann 1978), 
the less concentrated of the two reactants. 
It is important to realize that the actual situation is 
much more complex. Elongation along the c-axis, for ex- 
ample, is not literally the result of repeated wholesale 
addition of basal planes. Rather, c-axial growth probably 
occurs through the stacking of some (hkil) planar ele- 
ment, probably (1011) (Fig. 1B). The effect of positively 
charged planes on c-axial growth rates occurs on a fine 
scale, probably on the order of only a few unit cells. Note 
also that the net growth rate of calcium carbonate parallel 
and perpendicular to the c-axis will be a function of the 
concentration of both calcium and carbonate, as has been 
reported by numerous authors (Berner and Morse 1974; 
Mucci and Morse 1983; and many others). 
The control of c-axis growth by the rate of supply of 
CO3 = ions readily explains the range of carbonate mor- 
phologies seen in nature. As mentioned earlier, the degree 
of saturation in most meteoric waters is limited by the 
solubilities of metastable carbonate phases. Moreover, to 
reach saturation with respect to low-magnesium calcite, 
CO2-charged waters must have dissolved significant 
amounts of calcium carbonate. Most of the CO3 = ions 
derived in this manner combine with H ÷ generated from 
the dissociation of dissolved CO2. When this fluid attains 
calcite saturation, it has substantially more calcium ions 
than carbonate ions. An equivalent situation exists in 
temperate and deep marine environments, where Ca ++ 
ion concentrations are maintained at levels much higher 
than CO3 = ion concentrations. In both cases, growth sites 
on precipitating crystals are "starved" for carbonate ions, 
c-axis growth is retarded, and precipitation results in 
equant crystals. Elongatecrystal habits, on the other hand, 
are found only in meteoric-vadose and shallow-marine 
environments as low-magnesium and high-magnesium 
calcite, respectively. In these settings, precipitation is 
driven by rapid degassing of CO2 and the subsequent 
generation of CO3 = ions from the dissociation of riCO3-; 
the system is carbonate "rich," c-axis growth is enhanced, 
and elongate crystals result. 
ABIO 77C SEDL~ENTAR Y CARBONATES 113 
COMPOSIT ION AND GROWTH RATE 
Considerations of carbonate composit ions and min- 
eralogy fall into two main categories: 1) the control of 
magnesium content in calcite phases, and 2) the precip- 
itation of aragonite versus calcite. Although both aspects 8 
may be discussed separately, they are in fact closely re- r-~ 
lated in that the former is dependent on growth rate and (.~ 
the Mg/Ca ratio of the precipitating fluid, while the latter 
is entirely controlled by growth rate. ~ 6 
Calcite Composition 
When considering precipitation, most workers rou- 
tinely employ distribution coefficients to apport ion minor 
and trace elements between solid and fluid phases. At a 
given temperature and pressure, a specific coefficient is 
mult ipl ied by the molar ratio of a specific minor element 
to the major element in the fluid to give a corresponding 
ratio in the solid phase. A fundamental question with 
respect to calcite composit ions is to what degree are mag- 
nesium contents controlled by an invariant distribution 
coefficient (e.g., Mucci and Morse 1983) and to what de- 
gree are they controlled by kinetically mediated coeffi- 
cients which are dependent on rates of crystal growth (e.g., 
Berner 1978; Lahann and Siebert 1982). In other words, 
with respect to magnesium concentrations in calcite, is 
there a constant distribution coefficient that is unrelated 
to crystal-growth rates, implying a unique Mg/Ca ratio 
in the precipitated solid for a particular Mg/Ca ratio in 
the liquid? 
Mucci and Morse (1983) concluded that the distribu- 
tion coefficient for magnesium in calcite was independent 
of growth rate. However, aspects of their study suggest 
that this conclusion may be invalid when applied to nat- 
ural sedimentary calcites. First of all, their experimental 
procedure allowed only the Ca ++ ion concentration to 
vary; the COt- ion concentration (pCO2 and pH) was kept 
relatively constant during calcite precipitation. In a sim- 
ilar manner, Katz (1973) also concluded that the mag- 
nesium content of calcite may be independent of fluid 
Ca + + ion concentrations. As discussed previously, growth 
on any face not perpendicular to the c-axis is l imited by 
the rate of dehydration of Ca ++ ions. As the dehydration 
of the smaller Mg +* ion requires much higher free energy 
(L ippmann 1960; Christ and Hostetler 1970), it seems 
reasonable that the incorporation of magnesium on a side 
face is solely a function of the Mg/Ca concentrations in 
the fluid, and that on these faces magnesium incorpora- 
tion may well be controlled by a constant distribution 
coefficient. However, on c-axis faces where the charged 
surface facilitates cation dehydration, growth is con- 
trolled by the local availabil ity of carbonate ions. When 
crystal growth is predominantly in a c-axis direction, 
magnesium incorporation depends on rates of CO3- ion 
supply, indicating that magnesium partit ioning is kinet- 
ically mediated and that magnesium contents in calcite 
are directly related to crystal-growth rates. 
Second, the conclusion of Mucci and Morse (1983), 
that the magnesium content of calcite is dominant ly in- 
I..- 4 
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FIG. 5.--Relationships between ambient Mg/Ca ratios in precipitating 
fluids (vertical axis), a variable distribution or partitioning coefficient 
determined by the availability of CO3 ions at growing crystal surfaces 
(horizontal axis), and the composition of precipitated calcites (shown 
as the population of lines defining equal Mg/Ca ratios in the solid phase). 
Note that precipitated phases will contain increasingly elevated mag- 
nesium concentrations either as ambient fluid Mg/Ca ratios increase or 
as growth rates and kinetic distribution coet~cients increase. The di- 
vision between low-magnesium and high-magnesium calcite was (ar- 
bitrarily) placed at a calcite Mg/Ca molar ratio of 0.1, a value corre- 
sponding to a composition of Cao~bMN,.~+CO3. Note that the precipitation 
of magnesium-enriched calcites, depending on growth rates, may or 
may not relate to the Mg/Ca ratio of precipitating fluids. 
fluenced by the Mg/Ca ratio of the water from which it 
precipitates, is inconsistent with known marine cement 
composit ions. Using their constant distr ibution coeffi- 
cient of 0.0123, marine calcite cement should contain 
about 6 mole% MgCO 3. As we have seen, however, at 
roughly equivalent temperatures this same marine water 
will precipitate acicular high-magnesium calcite contain- 
ing up to 17 mole% MgCO3 in shallow tropical settings 
when ambient CO3- ion concentrations rise through the 
degassing of CO2. Similarly, calcite with less than 4 mole% 
MgCO 3 will precipitate from marine water with greater 
amounts of dissolved CO2, such as in temperate regions 
or at greater depths. Both the Mg/Ca ratio and the con- 
centration of Ca ++ ions are roughly the same in all of 
these settings; only the COl- ion concentration varies. 
Growth rate in the c-axis direction must largely control 
magnesium concentrations in natural marine calcites. 
Further support for kinetic control o f magnesium con- 
tents in calcite is shown by extrapolation from high-tern- 
1 14 R, KEVIN GIVEN AND BRUCE H. WILKINSON 
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O01D DIAMETER (mm) 
Fio. 6. - Relationships between exterior conical mineralogy and grain 
diameter for ooids from Batfin Bay, Texas. Note that with decreasing 
size, the proportion of cortical high-magnesium calcite (HMC) relative 
to aragonite increases. 
perature experimental work which suggests that the ther- 
modynamically stable calcite (in the presence of a dolomite 
phase) probably contains less than 1 mole% MgCO3. This 
would be the calcite phase precipitated at infinitely slow 
growth rates; at any higher growth rates, some extraneous 
magnesium is included. The control of magnesian calcite 
composition by growth rates (as well as ambient Mg/Ca 
ratios) has been suggested by numerous authors (Berner 
1975, 1978; Thorstenson and Plummer 1977, 1978). This 
concept is also in accord with recent experiments which 
describe reverse (magnesium expulsion) reactions occur- 
ring on the growing surfaces of magnesian calcites 
(Schoonmaker et al. 1982). The slower the carbonate pre- 
cipitates, the more magnesium is expelled from surface 
layers and the lower the magnesium concentration. The 
final magnesium content of natural calcites is a function 
of two variables: the Mg/Ca ratio of ambient fluids and 
the rate of crystal growth as determined by fluid Ca *+ and 
CO3- ion concentrations (Fig. 5). 
Carbonate Mineralogy 
Numerous authors have suggested that the precipita- 
tion of aragonite as opposed to calcite is in some way 
controlled (e.g., Muller et al. 1972; Folk 1974) or at least 
favored (e.g., DeGroot and Duyvis 1966; Berner 1975) 
by elevated fluid Mg/Ca ratios. The presence of both high- 
magnesium calcite and aragonite cements in tropical ma- 
rine environments and the occurrence of both low-mag- 
nesium calcite and aragonite as speleothems in meteoric 
settings, however, requires that some factor other than 
Mg/Ca ratio exerts a direct control on aragonite versus 
calcite precipitation. Like magnesium concentrations in 
calcite, there is an excellent correlation between calcite 
and aragonite precipitation andthe expected rates of car- 
bonate crystal growth in specific environments. Marine 
aragonites are generally found where conditions are fa- 
vorable for the rapid precipitation of a carbonate phase, 
FIG. 7.--Thin section ofa bimineralic aragonite-calcite ooid from the 
Pennsylvanian Plattsburg Limestone, southwestern Kansas. The inner 
(dark) cortex consists of elongate, radially oriented nannograins, an 
original fabric ofcalcitic cortical laminae. The outer (ligh0 cortex con- 
sists ofequant, blocky, neomorphic spar which has presumably replaced 
originally aragonite cortical laminae. 
such as in larger reefal pores of shallow tropical systems 
where wave agitation and current flow rates are maxi- 
mized. Meteoric aragonite is found only in vadose settings 
(with acicular calcites) where precipitation under condi- 
tions of elevated pH and/or higher temperature is driven 
by rapid degassing and evaporation (Curl 1962). As yet, 
aragonite is unreported in meteoric-phreatic settings, and 
is rare in cool, temperate marine settings. 
In order to gain further insight into the influence of 
degrees of carbonate saturation on the precipitation of 
calcite versus aragonite, it is informative to comment on 
specific instances where both calcite and aragonite have 
been precipitated, where a transition from one phase to 
the other during precipitation has occurred, and where 
this change is accompanied by concomitant changes in 
the fewest number of variables. Bimineralic ooids and 
marine cements serve as such examples. 
Bimineralic Ooids.-Ooids, as inorganically precipitated 
carbonate grains, are occasionally bimineralic, exhibiting 
cortices of either low-magnesium calcite-aragonite (e.g., 
Popp and Wilkinson 1983) or high-magnesium calcite- 
aragonite (e.g., Land et al. 1979). Bimineralic high-mag- 
nesium calcite-aragonite ooids from Baffm Bay, Texas, 
serve as an excellent case in point. Here, Land et al. (1979) 
report that along a 10-km-shore segment where ooids are 
forming, grains of identical diameter are increasingly ara- 
gonitic in higher energy areas and contain greater pro- 
portions of cortical high-magnesium calcite where wave 
energy is lowest. As fluid Mg/Ca ratios and Ca ++ ion 
concentrations are constant in this relatively small area, 
such variation in mineralogy strongly suggests that in 
higher energy settings, increased CO2 degassing gives rise 
to higher CO3 = ion concentrations and, hence, to en- 
hanced aragonite precipitation. 
Local variations in CO2 degassing, however, do not 
ABIOT IC SED1MENTAR Y CARBONATES 1 15 
FIG. 8.--Slab of the massive reef facies of the Permian Capitan For- 
mation, Texas. Most of this surface transects early (former) aragonite 
marine cement which initially lined voids (A). The light isopachous 
cement (C) then precipitated topotactically as marine magnesian calcite 
on aragnnite during continued pore filling. The remaining pore space 
was then filled with equant calcite spar(s). 
® 
\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 
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FIG. 9.--Relations between fluid shear and marine carbonate min- 
eralogy, suggested by bimineralic marine ooids and cements. Top-phase 
transition from small calcite ooids to larger aragonite ooids with in- 
creasing fluid shear (as depicted by arrow size) at growing crystal sur- 
faces. Bottom-phase transition from aragonite to calcite with decreasing 
fluid shear during occlusion of pore throats by marine cement. 
explain why individual ooids are bimineralic. Because 
Land et al. (1979) related wave energy to variation in 
mineralogy for a constant grain size, and because varia- 
tion in current velocity must surely relate to variation in 
ooid sizes, we decided to evaluate variation in exterior 
cortical mineralogy as a function of diameter among ooids 
at an individual site. One of the several sites at which 
Land et al. (1979) reported microprobe data on Mg and 
Sr concentrations in cortical high-magnesium calcite and 
aragonite was sampled for analysis. Ooids were washed 
and dry-sieved into 12 size fractions, 9 of which consisted 
exclusively of coated grains. Dissolution experiments were 
then conducted on individual size fractions in order to 
remove the outer few microns of cortical carbonate. The 
volume dissolved was determined by atomic absorption 
spectroscopic analysis of total calcium; the proportion of 
high-magnesium calcite to aragonite was determined by 
analyses of total Mg ++ and Sr ++. Within Baffin Bay ooids, 
a direct correlation exists between external cortical min- 
eralogy and grain diameter (Fig. 6). Smaller ooids are 
more caleitie, while larger ooids are more aragonitic. 
Identical relationships are also seen in ancient, origi- 
nally bimineralic, ooids. Numerous grains from the Or- 
dovician West Spring Creek Formation of Oklahoma 
(Sandberg 1983a) and grains from the Pennsylvanian 
Plattsburg Limestone of Kansas (Kettenbrink and Manger 
1971) exhibit inner cortical layers preserving fine radial 
fabric; outer layers now consist of blocky neomorphic 
spar only preserving ghosts of original concentric banding 
(e.g., Fig. 7). In both cases, inner laminae were calcitic; 
outer laminae were precipitated as aragonite. These three 
examples, two ancient and one recent, indicate that ooid 
compositions may be bimineralic and that cortical min- 
eralogy may be a function of ooid size. Because in all 
three cases water chemistry was probably invariant during 
cortex accretion, this relation suggests that some physical 
factor related to ooid size may determine the mineralogy 
of precipitated carbonate phases. 
Bimineralic Marine Cements.--Like some marine ooids, 
high-magnesium calcite and aragonite may occur together 
as marine cements, both in ancient and in recent settings. 
Like modern ooids in Baffin Bay, recent marine cements 
may also show a spatial variation in mineralogy, with 
aragonite predominating in higher energy settings such as 
along windward atolls, while high-magnesium calcite in- 
creases in importance in more sheltered settings. Also, 
like bimineralic ooids, in some instances a mineralogic 
transition from one phase to the other has taken place 
during the filling of individual pores. Excellent examples 
occur in the massive reef facies of the upper Capitan 
Formation in the Permian Reef Complex of West Texas. 
Volumetrically, these units contain up to 80% marine 
cement which was precipitated either as botryoids ofacic- 
ular aragonite or as isopachous crusts ofacicular magne- 
sian calcite (Mazzullo 1980). Without exception, these 
two phases exhibit a distinct sequence from aragonite, 
precipitated when voids were large, to magnesium calcite, 
precipitated after partial filling of pore spaces and con- 
stfiction of pore throats (Fig. 8). Isotopic study of these 
two cement phases (Given and Lohmann 1983) dem- 
onstrates that both were precipitated from normal marine 
water, and that both temperature and the composition of 
marine fluids were invariant during aragonite and magne- 
sian calcite precipitation. 
Nearly identical aragonite to calcite transitions during 
void filling by marine cement are reported from several 
other ancient sequences. These include marine cements 
from the Proterozoic of Canada (Grotzinger and Read 
1984), the Cambrian of Labrador (James and Kobluk 
1978), the Pennsylvanian of Utah and Colorado (Roy- 
lance 1983), and the Pleistocene of Japan (Sandberg 
1983b). Such aragonite to calcite transitions in normal 
marine settings suggest that some physical factor related 
to the size of the pore being filled may determine the 
mineralogy of precipitated carbonate phases. 
Carbonate Thresholds.-The one physical factor which is 
1 16 R. KE V IN G IVEN AND BRUCE H. WILK INSON 
t-- 
Z 
Ld 
O0 
< 
I 
o 
A Fuchtbauer ~ Hordie ll976) 
• Thorstenson & Plurnmer (1977)o5 .~b 15 ~o as 
CALCITE Mg/Ca 
FIG. 10.--Relation between ambient fluid molar Mg/Ca ratios and 
the occurrence of calcite and aragonite precipitates. Note that since the 
"equilibrium" condition of Thorstenson and Plummer (1977) is, in 
essence, a nongrowth situation, very slow, controlled-growth experi- 
ments (e.g., Morse et al. 1979) more closely approximate equilibrium 
than do rapid-growth experiments (e.g., Glover and Sippel 1967). Vari- 
able distribution or partitioning coefficients (plotted as the x-axis in Fig. 
5) here would comprise a population of lines radiating from the lower 
left of the figure; note the constant coefficient of 0.0123 suggested by 
Mucci and Morse (1983) and a coefficient of 0.04 which is an approx- 
imate linear limit of calcite magnesium contents. This limit represents 
a growth rate, and by implication a rate of carbonate ion supply, that 
corresponds to the calcite-aragonite threshold. The low-magnesium- 
high-magnesium calcite boundary was, as in Figure 5, taken at a calcite 
composition of 9 mole% MgCO3. SW = sea water. Modified from La- 
harm and Siebert (1982). 
common to both ooid and cement mineralogic transitions 
is the amount of fluid shear at growing crystal surfaces; 
shear is higher during aragonite precipitation and lower 
during calcite precipitation. As ooids grow, an ever-in- 
creasing amount of shear is required for agitation (e.g., 
Heller et al. 1980). Conversely, as pore throats constrict, 
total flow through pores and boundary shear across ce- 
ment growth sites must decrease (Fig. 9). I f the compo- 
sition of fluids is invafiant, as is the case in normal marine 
systems, the only effect fluid shear can have on crystal 
growth is on the rate of supply of reactants, carbonate 
ions in particular. 
Such a scenario implies that a discrete rate of supply 
of carbonate ions separates calcite and aragonite precip- 
itation. Higher CO3 = availability gives rise to aragonite; 
lower availability gives rise to calcite. Data on the oc- 
currences of these two polymorphs in natural meteoric 
and marine systems give some hint as to the nature of 
this threshold. As discussed previously, at any given fluid 
Mg/Ca ratio, the MgCO3 content of inorganically precip- 
itated calcite is dependent on CO3- ion supply rates. In 
shallow, tropical marine environments, calcite cements 
may contain up to 17 mole% MgCO3, and almost in- 
variably these calcites occur with aragonite. This suggests 
that during precipitation, the flux of CO3- ions to growing 
calcite crystal surfaces approximates that which defines 
the calcite-aragonite threshold; increases in the avail- 
ability of CO3- ions gives rise to aragonite rather than 
calcite precipitation. Inorganic calcites with magnesium 
contents greater than about 17 mole% do not form from 
marine fluids because any higher rates of CO3 = supply 
give rise to aragonite precipitation. Low-magnesium cal- 
cite and aragonite phases in spelean deposits record the 
same threshold in meteoric fluids with low Mg/Ca ratios. 
Data on magnesian calcites from experimental systems 
further constrain the nature of the calcite-aragonite 
threshold. Much of this work has recently been reviewed 
by Lahann and Siebert (1982), who point out that at a 
given fluid Mg/Ca ratio, precipitated calcites exhibit a 
range of Mg/Ca ratios reflecting the influence of precip- 
itation kinetics on the distribution of Mg ÷÷ between so- 
lution and crystal. More specifically, these data demon- 
strate that, as suggested earlier, both rates of crystal growth 
and fluid Mg/Ca ratios determine calcite magnesium con- 
tents. Whereas fluids with low Mg/Ca ratios will only give 
rise to low-magnesium calcites, fluids with elevated Mg/ 
Ca ratios may give rise to calcites with highly variable 
magnesium contents (Fig. 10). 
As pointed out by Lahann and Siebert (1982), an even 
more striking apsect of these data is that there is a unique 
upper limit to the amount of magnesium that is incor- 
porated into calcites at any specific fluid Mg/Ca ratio. 
This apparent limit corresponds to a magnesium distri- 
bution coefficient of 0.04 (Fig. 10). In addition, given a 
marine water Mg/Ca ratio of 5.1, this distribution coef- 
ficient corresponds to a calcite with a composition of 
Cao 83Mgo 17CO3, approximately the most magnesium-en- 
riched calcites found in marine waters. On the basis of 
the agreement between maximum magnesium contents 
in these experimental and natural calcites, it is likely that 
the limit of magnesium incorporation defined by the ex- 
perimental data from Lahann and Siebert (1982) is, in 
fact, the calcite-aragonite threshold suggested by data on 
carbonate phases in natural systems. Significantly, this 
threshold is not related to fluid Mg/Ca ratios; rather, be- 
cause it corresponds to a magnesium distribution coef- 
ficient of 0.04 (Fig. 10), the calcite-aragonite threshold is 
apparently related only to a rate of crystal growth and, 
by implication, to a finite rate of CO3 = ion supply to 
growing calcium carbonate crystals. 
SUMMARY 
In the foregoing, we have attempted to demonstrate 
that the relations between ambient fluid chemistry, car- 
ABIOT IC SEDIMENTAR Y CARBONATES 11 7 
bonate crystal habit, composition, and mineralogy are 
best described by kinetically mediated processes. To view 
these only in a context of ambient Mg/Ca activities is to 
perceive them oversimplistically and probably inaccu- 
rately. Rate of crystal growth, in most cases determined 
by rate of carbonate ion supply to growing crystal surfaces, 
controls the nature of abiotic sedimentary carbonate 
phases in nearly all meteoric and marine settings (Fig. 
I l), and possibly in burial diagenetic environments as 
well. 
With regard to modern environments, the availability 
of carbonate ions to growing crystals relates to a number 
of environmental variables including temperature, fluid 
flow, the presence of gas in pores during cementation, and 
other factors not identified herein. Although the appli- 
cability of this model will only be determined as addi- 
tional data on the nature of carbonate sediments become 
available, within this framework, several interesting as- 
pects of carbonate phases become less puzzling. For ex- 
ample, it has long been recognized that the dominant 
mineralogy of modern marine ooids is aragonite; magne- 
sian calcite cortices are only known from 3 or 4 settings 
and, to our knowledge, all but one of these are relict grains 
on various continental shelves. The only place where cal- 
cite ooids are known to be forming in a setting that ap- 
proaches normal marine conditions is Baftin Bay, Texas, 
which in fact is a coastal estuary. In contrast, it has also 
long been recognized that although aragonite marine ce- 
ments are common, magnesian calcite makes up the vast 
majority of recent carbonate cementsin shallow, tropical 
settings. Given that both ooids and cements precipitate 
from normal marine water, why is their dominant min- 
eralogy so different? 
We suggest that this difference reflects the availability 
of carbonate ions during precipitation. Ooids generally 
form in warm, shallow, high-energy environments where 
CO2 degassing gives rise to elevated carbonate ion con- 
centrations and where fluid shear brings these ions into 
frequent contact with cortical crystal surfaces. Marine 
cements, on the other hand, are precipitated in inter- and 
intragranular pores, where their parent marine fluids 
probably experience less agitation and lower flow veloc- 
ities than do coeval ooids. These factors, in conjunction 
with the potential for CO2 liberation to pore fluids during 
organic decay, suggest that both the concentration of car- 
bonate ions and the frequency with which they come into 
contact with cement crystals is lower than that for ooids. 
As a result,ooids are predominantly aragonite while ma- 
rine cements are predominantly magnesian calcite. 
In even a broader context, the role of carbonate ion 
concentrations at growing crystal surfaces indeed may 
have had a profound influence on the bulk compositions 
of carbonate sediments in the geologic past. Several work- 
ers (e.g., MacKenzie and Pigott 1981; Sandberg 1983a, b) 
have recently suggested that the relative dominance of 
aragonite versus calcite has fluctuated throughout the 
Phanerozoic, with significant aragonite precipitation only 
taking place during times of continental emergence. In 
addition, it is now generally accepted that over this time 
the principal factor controlling the original mineralogy of 
g W/~,:,sN r - - - ~-W~ -~- c 
< ~ ~ ° - 3 ' c - \ { EAT..EE _\ 
• - - "~. - :x~ . . . . 
RATE OF COa SUPPLY 
FIG. 1 I.--Diagrammatic summary of the relations between rates of 
CO3 = ion supply (horizontal axis), fluid Mg/Ca ratios (vertical axis), 
crystal morphologies, crystal compositions, and crystal mineralogies for 
abiotic sedimentary carbonates. Three compositional fields as low-mag- 
nesium calcite (LMC), high-magnesium calcite (HMC), and aragonite 
(ARG), are superimposed on two morphological fields as equant (EQ) 
and acicular (AC). Although unquantified here, the marine water line 
(Mg/Ca = 5.1) and the meteoric water line (Mg/Ca = 0.3), the horizontal 
and vertical axes, and the LMC-HMC phase boundary are from Figure 
5. The division between LMC and HMC was taken at 9 mole% MgCO3 
(Mg/Ca in calcite = 0.1); the calcite-aragonite threshold was taken at a 
magnesium distribution coefficient of 0.04 (Fig. 10) from Figure 5. 
Placement of the equant-acicular morphological boundary was on the 
basis of many natural occurrences of carbonate crystals; typical examples 
are shown as circled numbers; (I) EQ MARINE LMC as deep and/or 
temperate marine cement; (2) EQ MARINE HMC as reef cement; (3) 
AC MARINE HMC as tropical reef and shelf cement and rare relict 
ooids; (4) AC BIMINERALIC MARINE HCM and ARG as rcefal 
cement and Baflin Bay ooids; (5) AC MARINE ARG as tropical reef 
and shelf cement and most marine ooids; (6) EQ METEORIC LMC as 
phreatic and vadose cement; (7) AC METEORIC LMC as vadose spe- 
lean, travertine, and beach-rock carbonates; (8) AC BIMINERALIC 
METEROIC LMC and ARG as vadose spelean deposits. Many precip- 
itational situations such as caliche crusts, concretion-forming sediments, 
deeper burial settings, etc., are, primarily through a lack of data, not 
plotted. Rate ofCO3 ~ supply as employed here is admittedly qualitative 
in that the local presence of precipitation inhibitors such as organic 
complexes, orthophosphate, and sulfate has not been incorporated. 
sedimentary carbonates has been atmospheric CO2 con- 
centrations (e.g., Berner et al. 1983). Although studies of 
cyclic variation in hydrospheric-atmospheric chemistry 
are still in their infancy, the role of carbonate ion con- 
centrations in predicting both mineralogy and composi- 
tion is in accord with documented changes in marine 
carbonates and also affords a framework in which to as- 
sess the significance of these variations throughout geo- 
logic time. 
Undoubtedly, this synthesis will also prove to be overly 
simplistic as additional data become available on the more 
common, as well as the more unusual, carbonate-forming 
settings, and as the relative roles of other variables which 
influence growth rates, such as organic complexes, ortho- 
phosphates, and sulfates, become better understood. 
However, the identification of kinetic factors which affect 
any thermodynamically driven process such as carbonate 
precipitation is critical to our interpretations of carbonate 
sediments. Adequate understanding of the processes re- 
sponsible for the abundance of various carbonate phases 
118 R. KEV1N GIVEN AND BRUCE H. WILK INSON 
in modem and ancient meteoric, marine, and burial set- 
tings, and of the processes which give rise to the many 
other truisms we generally accept for, and perhaps too 
often misapply to, sedimentary carbonates, depends on 
these relations. Rates of carbonate ion supply, rates of 
growth parallel to c- and a-axes, rates of magnesium in- 
corporation into growing crystal faces, and the relations 
between these processes and the precipitation of various 
calcium carbonate polymorphs are important variables 
in sedimentary carbonate systems. 
ACKNOWLEDGMENTS 
Many individuals at The University of Michigan, at 
other academic institutions, and with Corporate America 
have directly or indirectly contributed to the thoughts 
and speculations presented herein. We particularly ac- 
knowledge and thank R. L. Folk; his impact on one of us 
as a teacher and on both of us as a scientist has been 
immeasurable. Numerous graduate students at Michigan 
have suffered through long hours of discussion of abiotic 
carbonates and offered telling criticisms of several tan- 
gents not included here. To Joyce M. Budai, Luis A. 
Gonzalcz, and Karen Rose Cercone, we offer particular 
thanks. Our colleagues at Michigan, Kyger C Lohmann, 
Robert M. Owen, and James Lee Wilson are also grate- 
fully acknowledged for sharing their knowledge and ex- 
pertise throughout this study. Gale D. Martin carried out 
all AAS analysis on Baffin Bay ooids; Robert M. Owen 
aided with the linear algebraic manipulations which re- 
duced AAS data to proportional mineralogies. Lynton S. 
Land provided transportation and guidance during sam- 
ple collection in Baftin Bay. Christopher P. Weiss pro- 
vided the SEM photographs of the Texas shelf cements. 
John M. Kocurko kindly provided the photograph of the 
coastal Louisiana cement. Early drafts of the manuscript 
were reviewed by Karen Rose Cercone, Henry S. Chafetz, 
Philip W. Choquette, David E. Eby, Eric J. Essene, Robert 
L. Folk, Kyger C Lohmann, Donald R. Peacor, William 
B. Simmons, Randolph P. Steinen, and William G. Zem- 
polich. Special thanks go to Marie I. Schatz for typing 
the original draft and numerous revisions. Research on 
Phanerozoic carbonates at The University of Michigan 
is supported by the National Science Foundation, NSF 
Grants OCE-81-17699 and EAR-82-05727. 
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