Geology Reference
In-Depth Information
quartz), and
recrystallization
(changes in crystal size,
shape and crystal lattice without a change in mineral-
ogy). The last term is often used rather loosely, since it
is not always possible to demonstrate whether or not a
mineralogical change has occurred. Neomorphic pro-
cesses lead to crystal enlargement or diminution (Box
7.11).
The transformation of aragonite and High-Mg cal-
cite grains and mud to Low-Mg calcite is one of the
most important processes in carbonate diagenesis be-
cause it controls the ultimate petrophysical properties
of limestones and their geochemical composition (par-
ticularly stable isotopes and Sr content: Al-Asam and
Veizer 1986a, 1986b; Banner 1995; Maliva 1998). Re-
placement of aragonite by calcite involves gradual dis-
solution of the original mineral and precipitation of cal-
cite, so that relicts of original shell or cement struc-
tures may be preserved in the neomorphic calcite.
Box 7.11.
Terms introduced by Folk (1965) in order to
differentiate neomorphic processes.
Aggrading neomorphism
: A kind of coalescive neo-
morphism or recrystallization in which the crystal size
increases and finer crystal mosaics are replaced by
coarser crystal mosaics of the same mineral or its poly-
morph without the intermediate formation of visible
porosity (Bathurst 1975). Occurs chiefly in fine-grained
limestones and results in microsparitic patches, laminae
and beds. See Morse and Mackenzie (1990) for a dis-
cussion of aggrading neomorphism. Pl. 38/2, 3.
Coalescive neomorphism:
Larger crystals grow at the
expense of smaller crystals or small crystals grow within
a larger crystal.
Degrading neomorphism:
A kind of coalescive neo-
morphism in which the crystal size decreases; smaller
crystals grow within a larger crystal. Not common, but
may occur under stressed or low-metamorphic condi-
tions.
to solution than carbonate grains. Carbonate grains ex-
hibit strong variabilities with regard to pressure solu-
tion potential.
The only feature common to all case studies is that
echinoderm grains seem to be the most resistant grains,
and micritic particles (e.g. peloids) and argillaceous mi-
critic matrix are the least resistant parts of carbonate
rocks.
7.6 1 Recrystallized Carbonate Rocks:
What to do?
Recrystallized limestones are predominantly char-
acterized by changes in the size, shape and arrange-
ment of crystals, and by the obliteration of the original
depositional textures and constituents. Recrystalliza-
tion of micritic limestones may be controlled by the
clay content of limestones; a clay content > 2% seems
to inhibit or prevent crystal coarsening (Bausch 1968).
Enlargement of micrite may create
microspar
(see
7.5.3 Significance of Compaction and
Pressure Solution
Box 7.10 lists important effects of the mechanical and
chemical compaction of carbonate rocks during burial
as well as the influence of dolomitization. It also dis-
cusses the economic importance of compaction and
pressure solution.
7.6 Neomorphic Processes:
Alteration and Recrystallization
Carbonate sediments and carbonate rocks are affected
by diagenetic processes that alter mineralogical com-
position and/or crystals and crystal fabrics. The term
neomorphism
(Folk 1965) relates to transformations of
minerals taking place in the presence of water and in-
cludes processes of
replacement
(the dissolution of one
mineral and simultaneous formation of another min-
eral, e.g. silicification of carbonates),
inversion
(the re-
placement of a mineral by its polymorph; e.g. arago-
nite -> calcite (calcitization); opal-A -> opal-CT ->
Fig. 7.19.
Recrystallized coral limestone
. Arrows point to
centers of former corallites of a colonial coral. Note faint
radial structures representing the recrystallized coral septa.
Aggrading neomorphism has replaced cement crystals be-
tween coral calices by a medium to coarse-grained inequi-
granular, hypidiotopic calcite fabric. Crystals were enlarged
at the expense of other crystals (note left side of the figure in
particular). Jurassic: Argentina. Scale is 5 mm.
