Geology Reference
In-Depth Information
rics or show fabrics within apparently homogeneous
limestones (Bromley 1981).
Peels:
The imprint of etched surfaces on transpar-
ent plastic films is the most rapid and simplest method
for studying cut, polished, and etched samples. Acetate
replicas can be made using a dry-peel technique
(McCrone 1963; Germann 1965) or by making peel
sheets by pouring solutions (ethyl cellulose in trichlo-
roethylene, nitrocellulose) on to glass plates (Davies
and Till 1968). The procedure of preparing peels in-
cludes (1) cutting, grinding (and polishing) the sample,
(2) drying and etching the cut surface, (3) flooding the
surface with a solvent and (4) pressing the film onto
the surface; the film will settle down into irregularities
in the etched surface and produce the replica. After pull-
ing off, the peel should be pressed or mounted between
glasses. Peels have many advantages:
•
etate films are isotropic; the main disadvantage of peels,
therefore, is that minerals can not be identified by opti-
cal properties, such as relief or birefrigence.
Thin sections:
The investigation of the composition
and fabric of surface and subsurface samples requires
standard thin sections with a thickness of approximately
30
m. These thin sections are usually cut perpendicu-
lar to the bedding. Initially, the thin sections should not
be covered, allowing later more detailed investigations
(staining etc.). Very fine-grained micrites are studied
in high-quality ultra-thin sections with a thickness be-
tween 0.5 and 5
m (Lindholm and Dean 1973), allow-
ing observations at magnifications of between x 150
and x 1000. Thin sections may be subject to etching
and staining (Friedman 1959; Houghton 1980;
Rassineux and Beaufort 1987). These procedures and
other more detailed studies (e.g. cathodoluminescence
microscopy: Mugridge and Young 1984) only can be
applied to uncovered thin sections. Rapid covering can
be done using a plastic spray (Moussa 1976). Micro-
facies thin sections are generally larger and thicker than
regular petrographic slides. Commonly used sizes are
from 5 x 5 up to 15 x 20 cm. Large thin sections are
necessary for studying reef limestones, coarse-grained
detrital limestones and carbonates deposited during
events (e.g. turbidites, tempestites).
• Note: Samples are generally cut perpendicularly to
bedding planes. Therefore, a thin section comprising a
vertical interval of 5 cm records a set of 'microsequences'
corresponding to shorter or longer time intervals within
a scale of hundreds, thousands or more years.
They offer a rapid and inexpensive method to ob-
tain a first observation of the characteristics of the
sample.
•
Peels can be used not only for carbonates but also
for cherts, siliceous clastics and sulfates rocks
(Mandado and Tena 1986).
•
Peels can be studied just like thin sections using a
binocular microscope or a simple microfiche equip-
ment and they can be placed in the negative carrier
of a photographic enlarger, producing large nega-
tive prints by projecting the image directly to bro-
mide paper. These prints may be used as working
copies exhibiting locations of areas of particular in-
terest.
•
Replicas can be made of large surfaces, even up to
several tenths and hundredths of square centimeters.
•
Serial peels can be produced by regrinding surfaces
after each peel, thus allowing the three-dimensional
reconstruction of open-space structures or fossils
only known from thin sections (Honjo 1963).
3.2.2 Casts, Etching and Staining
Casts:
Impregnation of carbonate rocks and fossils with
low viscosity epoxy resin under vacuum and under pres-
sure has a high potential in recognizing micron- and
submicron-scaled structures in porous but also moder-
ately porous carbonates and evaporites. Impregnation
of carbonate rocks with a mixture of fluorescent dye
and epoxy resin enhance the ability to study micro- and
macroporosity. SEM observations of artificial casts aid
in the study of
•
•
Staining peels prior to the flooding of rock surfaces
allows minerals and fabrics to be differentiated (Katz
and Friedman 1965). Since earlier stains can be re-
moved by repolishing, different stains indicating dif-
ferent minerals can be used with good success.
•
Peels are suited for high-magnification studies and
phase contrast microscopy (Frank 1965; Honjo and
Fischer 1965) and for SEM studies (Brown 1986).
microporosity and pore geometries of limestones and
chalks (Gardner 1980; Beckett and Sellwood 1991;
see Pl. 6),
•
Acetate peels have proved to be of value in studying
carbonate cements and textural criteria (e.g. grain-
size distribution: Gutteridge 1985).
•
modern and ancient microborings (Golubic et al.
1975; see Pl. 52/1-4).
Disadvantages of peels are the susceptibility of ac-
etate films to contamination and the danger that po-
rous samples might be difficult to work with, because
the film may bulge into the pores, causing blisters. Ac-
Etching and staining
: Etching of slices or thin sec-
tions with hydrochloric acid (1-5% by volume) or ace-
tic acid (about 20% by volume) causes partial dissolu-
