Geology Reference
In-Depth Information
where MgCO
3
mol% increases with increasing tem-
perature (Videtich 1985; Morse and Mackenzie 1990).
High-Mg calcite is rarely preserved in the geologi-
cal record. Exposure of Mg-calcite to meteoric water
leads to dissolution and eventual replacement by stable
Low-Mg calcite, processes which may occur within
geologically short time ranges of only a few 100 000s
of years (Friedman 1964; Land et al. 1967; Richter
1979). Both incongruent dissolution in the stabiliza-
tion of HMC skeletons without evidence of textural
modification of skeletal microstructures and congruent
dissolution of HMC and the precipitation of LMC with
defined textural changes have been reported. The pro-
cess of Mg loss during stabilization of Mg-rich cal-
cites is not well understood (Oti and Müller 1985;
Bischoff et al. 1993).
Dolomite:
The mineral dolomite, CaMg(CO
3
)
2
, can
be distinguished from calcite by its commonly euhe-
dral rhombic crystal form in thin sections, X-ray dif-
fraction, and staining techniques using titan yellow al-
izarine S, or potassium ferricyanide. Small amounts of
iron produce brown or tan weathering colors of many
dolomite and dedolomitized rocks.
7.1.3 Diagenetic Processes and Controls
Major diagenetic processes affecting carbonate sedi-
ments and rocks are micritization (Sect. 4.2.3), disso-
lution and cementation, compaction, neomorphism, do-
lomitization, and the replacement of carbonate grains
and matrix by non-carbonate mineralogies (e.g. silici-
fication and chertification; see Sect. 13.1.2.1).
Dissolution:
Undersaturation of pore fluids with re-
spect to carbonate leads to dissolution of metastable
carbonate grains and cements. Dissolution is particu-
larly effective in shallow near-surface meteoric envi-
ronments, in deep burial and cold waters (Steinsund
and Hald 1994) as well in the deep sea (Berelson et al.
1994), where seawater becomes undersaturated with
respect to aragonite and Mg-calcite (Morse 2002).
How to estimate dissolution effects? Use fossils as
reference base. Many fossils (e.g. mollusks, corals, cal-
careous algae) have well-defined skeleton mineralogies
(Box 4.9) and microstructures. Use the alterations in
these criteria to reveal dissolution processes (Sect. 4.2.1;
Fig. 4.10).
Cementation
comprises processes leading to the pre-
cipitation of minerals in primary or secondary pores
and requires the supersaturation of pore fluids with re-
spect to the mineral.
Compaction
and
pressure solution
(stylolitization)
refer to mechanical and chemical processes, triggered
by the increasing overburden of sediments during burial
and increasing temperature and pressure conditions.
Neomorphism
(Folk 1965) is a term summarizing
all transformations taking place in the presence of wa-
ter through dissolution-reprecipitation processes be-
tween one mineral and itself or a polymorph.
Recrystallization
refers to changes in crystal size,
crystal shape and crystal lattice orientation without
changes in mineralogy. The term is often used in a very
loose manner (Sect. 7.6).
Examples for
inversion
are the gradual replacement
of aragonite by calcite through solution and in-situ pre-
cipitation in an aqueous environment or the replace-
ment of dolomite by calcite (see Sect. 7.8.3). Cemen-
tation as well as neomorphism produce calcite spar fab-
rics. For diagnostic criteria see Sect. 7.7.
Calcite (Low-Mg calcite, LMC):
Low-Mg calcite is
traditionally regarded as a stable carbonate phase with
little tendency toward intracrystalline alteration of its
chemistry, but dissolution-cementation processes may
occur and produce intracrystalline porosity by partial
dissolution (Pedone et al. 1994).
Low Mg/Ca ratios of meteoric waters favor the pre-
cipitation of LMC in meteoric and burial diagenetic
environments. In marine environments LMC carbon-
ates form at platform margins, on slopes and on the
deep-sea floor by primary LMC cements or rapid con-
version of High-Mg calcite to calcite (Schlager and
James 1978).
LMC is a major constituent of pelagic chalks, lime-
stones and marbles. In deeper waters of the open oceans
calcite dissolution takes place at the calcite lysocline
and the calcite compensation depth (Sect. 2.4.5.6 and
Box 2.6 ).
Calcites may contain trace amounts of Mn
2+
and
Fe
2+
. Calcite containing ferrous iron Fe
2+
(up to sev-
eral thousand ppm) is called
ferroan calcite.
Ferroan
and non-ferroan calcites (as well as ferroan dolomite:
ankerite) can be distinguished by staining with potas-
sium ferricyanide (Dickson 1966; Adams and Mac-
Kenzie 1998) and their luminescence and intensity.
Ferroan calcite occurs as cement and as fissure filling,
replacement of calcitic matrix or replacement of ara-
gonitic or Mg-calcitic skeletons. Transformation of ara-
gonite to ferroan calcites requires the existence of an
open system, e.g. late-stage void filling by sparry cal-
cite in former aragonite molds. Ferroan calcite cements
are often of late diagenetic origin and can indicate deep
burial reducing conditions, because shallow settings of
carbonate sequences favor oxidization, whereas ferroan
calcite cements precipitates under reducing conditions
in the pH range 7 to 8.
