Geology Reference
In-Depth Information
monly combined with petrographic and cathodo-
luminescence microscopy and isotope analysis (Sotak
and Lintnerova 1994). Fundamental techniques have
been described by Hollister and Crawford (1981), and
Goldstein (1993).
The most important carbonate minerals are Low-
Mg calcite (LMC), High-Mg calcite (HMC), aragonite
and dolomite (Fig. 3.8). The dividing line between Low-
Mg calcite and High-Mg calcite is commonly drawn at
4 mol % MgCO 3 . These groupings can be subdivided
into those containing a variable amount of MgCO 3
(4-12 mol %) and those containing a consistently high
amount of MgCO 3 (12-28 mol %, e.g. corallinacean red
algae, echinoderms). The latter group represents the
HMC sensu stricto (hHMC = high High-Mg calcite).
The amount of Mg substitution for Ca in the calcite
structure is determined by X-ray diffraction analysis.
The composition of carbonate sediments is studied
by petrographical microscopy and X-ray diffraction
(XRD), which is particularly useful for analyzing fine-
grained carbonate rocks. The qualitative and quantita-
tive methods of XRD whole rock analysis and clay min-
eral analysis are described by Hardy and Tucker (1988).
XRD data provide information on the chemical com-
position of carbonate minerals, allow carbonate miner-
als to be distinguished, and are used to determine the
ordering of dolomite crystals and the proportions of
calcite and dolomite. The 'carbonate bomb' (Müller and
Gastner 1971) offers the possibility of rapidly deter-
minating the contents of CaCO 3 and insoluble residues
in limestones.
Fluid inclusions in carbonates are a powerful tool
for the
history and timing of shallow and burial diagenetic
and low-grade metamorphic processes ( Brockerhoff
and Friedman 1987; Liedmann 1992),
conditions of dolomitization (Goldstein et al. 1990;
Montanaze and Read 1992),
migration processes of paleofluids (Burruss et al.
1985; Ricken 1992; Muchez et al. 1994),
uplift and subsidence history of sedimentary rocks
(McLimans 1987),
subaerial exposure and karst phenonema (Goldstein
et al. 1990),
data on pore water chemism and temperature dur-
ing the formation of geodes, vugs and veins.
3.2.4 Mineralogy and Geochemistry
Carbonate minerals include minerals composed of
CO 3 2- and one or more cations. Approximately sixty
minerals occur in nature with a CO 3 group in common.
Of these the trigonal CaCO 3 (calcite) and its orthor-
hombic polymorph (aragonite) are the most common
in modern sediments. Calcite and dolomite are by far
the most common carbonate minerals in ancient car-
bonate rocks. Several cations can substitute in varying
amounts for the Ca 2+ in the crystal structure: Mg 2+ ,
Fe 2+ and Mn 2+ are more readily accepted in the hex-
agonal calcite structure, and Sr 2+ and Ba 2+ by the ara-
gonite structure. The minor element composition of cal-
cite and aragonite is determined by biochemical and
physical-chemical processes.
3.2.5 Trace Elements
and Stable Isotope Analysis
Compare also Sect. 13.1 for the use of geochemical
studies of ancient limestones and the advantage of com-
bined investigations of microfacies, stable isotopes and
trace elements.
Trace elements: The elemental analysis of carbonate
rocks provides important data on the sedimentary and
diagenetic history. Atom Adsorption Spectrophotometry
System
Mol % MgCO 3
Stability
Calcite CaCO 3
trigonal
<4
stable
LowMg calcite (LMC)
Mgcalcite CaCO 3
trigonal
>4 to ~30
metastable
HighMg calcite
Mg content correlated
(HMC)
with water temperature
Aragonite CaCO 3
orthorhombic
very low
metastable, alters readily
in calcite under aqueous
conditions
Dolomite
trigonal
40 50
stable
CaMg(CO 3 ) 2
Fig. 3.8. Common carbonate minerals.
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