Page 43 - Sedimentology and Stratigraphy

30 Biogenic, Chemical and Volcanogenic Sediments

lattice. Siderite forms within sediments as an early There is an alternative to making thin-sections of
diagenetic mineral (18.2). rocks made up primarily of carbonate minerals. It is
possible to transfer the detail of a cut, flat surface of a
block of limestone onto a sheet of acetate by etching the
3.1.2 Carbonate petrography surface with dilute hydrochloric acid, then flooding the
surface with acetone and finally applying the acetate
All of these carbonate minerals have similar optical film. Once the acetone has evaporated, the acetate is
properties and it can be difficult to distinguish peeled off and the imprint of the rock surface can then be
between them in thin-section using the usual optical examined under the microscope. These acetate peels
tests. Their relief is high, and the birefringence col- are a quick, easy way to look at the texture of the
ours are high-order pale green and pink. The cleavage rock, and distinguish different clast types: the rock
is usually very distinct, and where two cleavage can also be stained in the same way as a thin-section.
planes are visible they can be seen to intersect to
form a rhombohedral pattern. Dolomite can be iden-
tified by adding a dye to the cut surface before a glass 3.1.3 Biomineralised carbonate sediments
cover slip is put on the thin-section: Alizarin Red-S
does not stain dolomite, but colours the other car- Carbonate-forming organisms include both plants
bonates pink. A second chemical dye is also com- and animals. They may create hard parts out of cal-
monly used: potassium ferricyanide reacts with cite, in either its low-Mg or high-Mg forms, or arago-
traces of iron in a carbonate to stain it blue, and nite, or sometimes a combination of both minerals.
on this basis it is possible to distinguish between The skeletal fragments in carbonate sediments are
ferroan calcite/aragonite/dolomite and non-ferroan whole or broken pieces of the hard body parts of
forms of these minerals. The two stains may be used organisms that use calcium carbonate minerals as
in combination, such that ferroan calcite/aragonite part of their structure (Figs 3.1 & 3.2). Some of
ends up purple, ferroan dolomite blue, non-ferroan them have characteristic microstructures, which can
calcite/aragonite pink and non-ferroan dolomite be used to identify the organisms in thin-sections
clear. (Adams & Mackenzie 1998).

Fig. 3.1 Types of bioclast commonly
found in limestones and other sedimen-

tary rocks.