Geology Reference
In-Depth Information
tion that emphasizes textural criteria and allows subse-
quent staining, which is one of the most useful tech-
niques in carbonate rock studies. Comparing staining
techniques with other methods, e.g. X-ray diffraction,
may exhibit discrepancies in estimates of dolomite in
limestones (Gensmer and Weiss 1980). The success of
etching and staining depends on (a) the strength and
temperature of the reagents, (b) etching and staining
times, (c) the pre-treatment of the sample, and d) the
composition and fabric. An advantage of staining is the
possibility using the method already used during field
studies (Warne 1962).
Staining assists in (1) identifying minerals common
in carbonates (aragonite, Mg-calcite, calcite and dolo-
mite, gypsum and anhydrite, feldspars), (2) studying
dedolomitization (Evamy 1963), (3) recognizing micro-
porosity (Yanguas and Dravis 1985).
3.2.3.2 Stereoscan Microscopy
Utilization of Steroscan Electron Microscopy (SEM)
in the study of ancient and modern carbonate rocks has
brought about major breakthroughs in our understand-
ing of depositional and diagenetic products and pro-
cesses (Welton 1984; Trewin 1988).
The chief advantages of the SEM are the great depth
of focus, producing excellent photographs of extremely
small (micron- and submicron-sized) three-dimensional
surfaces, the high power of resolution and the possibil-
ity of very high magnifications in the range between
x 10 up to x 100 000.
Transmission Electron Microscopy (TEM) of repli-
cas, however,which has been used prior to SEM in the
study of carbonate rocks (Fischer et al. 1967), is still a
valuable tool in carbonate investigation (e.g. biomin-
eralization modes, microbial contribution to the forma-
tion of carbonates).
Today, SEM studies are a part of integrated analy-
ses which combine SEM with energy dispersive X-ray
(EDX) data, back-scattered electron images and image
analysis, cathodoluminescence microscopy and conven-
tional studies of (stained or etched) peels and thin sec-
tions. Peels and thin sections can be used directly for
SEM studies. EDX studies show element composition
and element distribution. Element mapping is used to
distinguish mineralogical phases and to elucidate Mg
and Sr distributional patterns in cements and fossils.
Carbonate SEM samples should be unaffected by
weathering. Nonporous limestones and dolomites are
studied using untreated fractures of rock chips or slices
(size about 5 by 10 by 10 mm) which are polished and
etched with dilute hydrochloric acid or formic acid, or
Titriplex-III. Samples are mounted on stubs by double-
sided sticky tape or various glues. Porous carbonates
may need specific sample treatment (Trewin 1988).
3.2.3 Microscopy
The microscopic study of thin sections of carbonate
rocks provides the basic microfacies data. The combi-
nation of microfacies analysis with the criteria derived
from fluorescence, photoluminescence, cathodolum-
inescene and fluid inclusion studies provide key infor-
mation on sedimentary and diagenetic processes.
3.2.3.1 Petrographic Microscopy
Good polarizing microscopes and binocular micro-
scopes are essential tools for microfacies studies. Micro-
scopes should have photographic equipment low and
high power magnification and should be equipped with
mechanical or computer-interfaced stages for examin-
ing modal composition and fabrics (Kobluk and Vyas
1989).
The speed, thoroughness and accuracy of thin-sec-
tion analysis can be increased by using integrated com-
puterized systems (Kobluk and Kim 1991). To provide
an overview of the whole thin section, the use of digi-
tal film scanners (De Keyser 1999) and microfiche in-
struments is strongly recommended. Microfacies ob-
servations should always start with low-power exami-
nations and continue to high-power magnification. Pri-
mary textural features in dolostones and recrystallized
limestones can be made visible by using a simple light
diffuser (translucent plastic or plain white paper), placed
directly under the thin section (Delgado 1977).
There are many excellent and colored texts on min-
eral identification in thin sections (e.g. Scholle 1979;
Adams et al. 1984; Carozzi 1993).
The major applications of SEM in investigating car-
bonate rocks include studies of the
• matrix of fine-grained limestones (micrite, chalk)
and the composition of modern carbonate muds,
• composition and ultrastructure of fossils and non-
biogenic carbonate grains,
• breakdown of fossils, including the importance of
endolithic borings in the destruction of grains and
the formation of micritic coatings,
• type, sedimentological and biostratigraphical role of
micro- and nannofossils,
• distribution and geometry of micropores,
• mineralogy, morphology and spatial relations of car-
bonate cements,
• stages of dolomitization,
