Page 42 - Sedimentology and Stratigraphy

Limestone 29

composed mainly of calcium carbonate. Limestones, contains 11% to 19% Mg. The hard parts of many
and sediments that eventually solidify to form them, marine organisms are made of high-Mg calcite, for
are referred to as calcareous (note that, although example echinoderms, barnacles and foraminifers,
they are carbonate, they are not ‘carbonaceous’: this amongst others (see 3.13). Strontium may substitute
latter term is used for material that is rich in carbon, for calcium in the lattice and although it is in small
such as coal). Sedimentary rocks may also be made of quantities (less than 1%) it is important because
carbonates of elements such as magnesium or iron, strontium isotopes can be used in dating rocks
and there are also carbonates of dozens of elements (21.3.1).
occurring in nature (e.g. malachite and azurite are
copper carbonates). This group of sediments and rocks Aragonite
are collectively known as carbonates to sedimentary
geologists, and most carbonate rocks are sedimentary There is no chemical difference between calcite and
in origin. Exceptions to this are marble, which is a aragonite, but the two minerals differ in their
carbonate rock recrystallised under metamorphic mineral form: whereas calcite has a trigonal crystal
conditions, and carbonatite, an uncommon carbon- form, aragonite has an orthorhombic crystal form.
ate-rich lava. Aragonite has a more densely packed lattice structure
and is slightly denser than calcite (a specific gravity of
2.95, as opposed to a range of 2.72–2.94 for calcite),
and is slightly harder (3.5–4 on Mohs’ scale). In
3.1.1 Carbonate mineralogy
practice, it is rarely possible to distinguish between
the two, but the differences between them have some
Calcite
important consequences (18.2.2). Many invertebrates
The most familiar and commonest carbonate mineral use aragonite to build their hard parts, including
is calcite (CaCO 3 ). As a pure mineral it is colourless or bivalves and corals.
white, and in the field it could be mistaken for quartz,
although there are two very simple tests that can be
Dolomite
used to distinguish calcite from quartz. First, there is a
difference in hardness: calcite has a hardness of 3 on Calcium magnesium carbonate (CaMg(CO 3 ) 2 )isa
Mohs’ scale, and hence it can easily be scratched with common rock-forming mineral which is known as
a pen-knife; quartz (hardness 7) is harder than a knife dolomite. Confusingly, a rock made up of this mineral
blade and will scratch the metal. Second, calcite is also called dolomite, and the term dolostone is now
reacts with dilute (10%) hydrochloric acid (HCl), sometimes used for the lithology to distinguish it from
whereas silicate minerals do not. A small dropper- dolomite, the mineral. The mineral is similar in
bottle of dilute HCl is hence useful as a means of appearance to calcite and aragonite, with a similar
determining if a rock is calcareous, as most common hardness to the latter. The only way that dolomite can
carbonate minerals (except dolomite) will react with be distinguished in hand specimen is by the use of the
the acid to produce bubbles of carbon dioxide gas, dilute HCl acid test: there is usually little or no reac-
especially if the surface has been powdered first by tion between cold HCl and dolomite. Although dolo-
scratching with a knife. Although calcite sometimes mite rock is quite widespread, it does not seem to be
occurs in its simple mineral form, it most commonly forming in large quantities today, so large bodies of
has a biogenic origin, that is, it has formed as a part dolomite rock are considered to be diagenetic
of a plant or animal. A wide variety of organisms use (18.4.2).
calcium carbonate to form skeletal structures and
shells and a lot of calcareous sediments and rocks
Siderite
are formed of material made in this way.
Magnesium ions can substitute for calcium in the Siderite is iron carbonate (FeCO 3 ) with the same
crystal lattice of calcite, and two forms of calcite are structure as calcite, and is very difficult to distinguish
recognised in nature: low-magnesium calcite (low-Mg between iron and calcium carbonates on mineralogi-
calcite), which contains less than 4% Mg, and high- cal grounds. It is rarely pure, often containing some
magnesium calcite (high-Mg calcite), which typically magnesium or manganese substituted for iron in the