Geology Reference
In-Depth Information
trolled by physicochemical constraints and triggered
by biological factors such as organic matter (OM)
present in carbonate cements, skeletal and non-skel-
etal grains, and carbonate skeletons. OM occurs within
grains and on grain surfaces and has been found in pores
between submicroscopic subunits of cement crystals
(Barnes et al. 1990a, 1990b for reviews). Current re-
search is focussed on the location of OM within the
carbonate host, the nature of OM associated with car-
bonates of different origins, and the influence of OM
on the growth of inorganic and organic carbonate crys-
tals (e.g. Ramseyer et al. 1997). Evidence for the lead-
ing role of OM in the formation of organic and inor-
ganic carbonates has been revealed by experimental
work showing that specific organic substances induce
calcite and Mg-calcite precipitation, while others cause
aragonite to precipitate.
Aragonitic skeletal grains are susceptible to disso-
lution causing secondary porosity. The skeleton of to-
tally aragonite fossils is replaced by calcite crystals that
are commonly one or more orders of magnitude larger
than the aragonite crystals they replace. Aragonitic
grains can suffer two main fates: They are either dis-
solved to form complete or partial molds that are sub-
sequently filled with cement, or they may be trans-
formed to Low-Mg calcite by the simultaneous vol-
ume-per-volume dissolution of aragonite and precipi-
tation of calcite along intervening solution films. Skel-
etal aragonite dissolution is widespread in shallow-
marine carbonates and is commonly regarded largely
as a near-surface freshwater process (James and
Choquette 1983). Some evidence exists however, for
skeletal aragonite dissolution on shallow sea floors
(Palmer et al. 1988) and in hypersaline waters (Sun
1992). In deeper waters of the open oceans aragonite is
dissolved at the aragonite lysocline and finally at the
aragonite compensation depth (Box 2.6; Sect. 2.4.5.6).
Experiments with ooids point to a relationship be-
tween the presence and composition of dissolved and
adsorbed OM and the formation of specific structures
(e.g. radial structures). Studies of mineral-organic ma-
trix relationships of organisms indicate that specifically
composed membranes tend to nucleate carbonate and
control the mineralogy and ultrastructural arrangement
of the biocrystals.
Mg-calcite (High-Mg calcite, HMC):
Many biogenic
calcites and non-biogenic calcites contain variable
amounts of MgCO
3
. These calcites with more than a
few mol% MgCO
3
are generally referred to as magne-
sian (or magnesium) calcites. The boundary between
Low-Mg calcite and High-Mg calcite is a matter of dis-
cussion (Richter 1979, 1984) as is the distinction be-
tween High-Mg calcite and high High-Mg calcite
(Sect. 3.2.4). Chave (1954, 1964) used 4 mol% as a
boundary. Many Mg-calcites have an average compo-
sition of about 14 mol% MgCO
3
(Bathurst 1975) but
biogenic Mg-calcite varies strongly in the amount of
MgCO
3
contents (e.g. calcareous red algae 10-30%,
echinoderms 10-15%). In modern environments mag-
nesian calcite occurs in invertebrate and algal skeletons,
cements and ooids; in non-marine environments in
freshwater tufas and cements formed in terrestrial and
aquatic settings. Modern marine inorganic Mg-calcite
cements form predominantly in warm low-latitude
waters that are supersaturated with respect to calcium
carbonate.
The Mg/Ca ratio of magnesian calcites varies di-
rectly with the Mg
2+
/Ca
2+
ratio and the temperature of
the ambient water. Temperature and latitudinal controls
are evident for most shallow-marine organisms form-
ing Mg-calcite skeletons (calcareous red algae, benthic
foraminifera, bryozoans, echinoderms and barnacles;
see Fig. 4.9). A marked compositional change occurs
from the tropics to the poles. Higher latitude calcites
formed in cold waters (as well as cements formed in
deeper waters of the oceans) contain less magnesium
than lower latitude calcites in tropical warm seawater,
7.1.2 Common Carbonate Minerals
A few criteria of common carbonate minerals were sum-
marized in Sect. 3.2.4 and in Fig. 3.8. Fig. 4.9 outlines
the mineralogical composition of skeletal grains. Car-
bonate mineralogy and chemistry are discussed by
Reeder et al. (1983) and Morse and Mackenzie (1990).
Aragonite:
Orthorhombic aragonite differs from cal-
cite in its higher specific gravity
and hardness, lack of
cleavage, and a high Sr/Ca ratio that varies with water
temperatures. In skeletal aragonites, physicochemical
processes play a major role in the incorporation of Sr
into aragonite during biomineralization. Mg occurs only
in low concentrations. Aragonite occurs in modern sedi-
ments in the form of skeletal material and cement. Ma-
rine aragonite often precipitates as needlelike crystals
forming in isopachous and circumgranular rims around
grains, as an intergranular mesh of crystals, and as
botryoids. In recent carbonates and Cenozoic rocks ara-
gonite is abundant, but is less common in Mesozoic
rocks, and almost absent in Paleozoic rocks. The de-
crease in aragonite with geologic age is in part due to
diagenetic processes, but probably also reflects secular
mineralogical changes during time (see Sect. 7.1.5.1).
