Article on biogenic and carbonate rock

LIMESTONE:-

Limestones are familiar and widespread rocks that form the peaks of mountains in the Himalayas, form characteristic karst landscapes and many spectacular
gorges throughout the world.
Limestone is also important in the built environment, being the construction material for structures ranging from the Pyramids of Egypt to many palaces and churches.
As well as being a good building stone in many places, limestone is also important as a source of lime to make cement,
and is hence a component of all concrete, brick and stone buildings and other structures, such as bridges
and dams. Limestone strata are common through much of the stratigraphic record and include some very characteristic rock units, such as the Late Cretceous Chalk, a relatively soft limestone that is found
in many parts of the world. The origins of these rocks
lie in a range of sedimentary environments: some
form in continental settings, but the vast majority
are the products of processes in shallow marine environments, where organisms play an important role in
creating the sediment that ultimately forms limestone
rocks.
Calcium carbonate (CaCO3) is the principal compound in limestones, which are, by definition, rocks composed mainly of calcium carbonate. Limestones,
and sediments that eventually solidify to form them,
are referred to as calcareous (note that, although
they are carbonate, they are not ‘carbonaceous’: this
latter term is used for material that is rich in carbon,
such as coal). Sedimentary rocks may also be made of
carbonates of elements such as magnesium or iron,
and there are also carbonates of dozens of elements
occurring in nature (e.g. malachite and azurite are copper carbonates). This group of sediments and rocks
are collectively known as carbonates to sedimentary
geologists, and most carbonate rocks are sedimentary
in origin. Exceptions to this are marble, which is a
carbonate rock recrystallised under metamorphic
conditions, and carbonatite, an uncommon carbon-
ate-rich lava.

Carbonate mineralogy

Calcite

The most familiar and commonest carbonate mineral
is calcite (CaCO3). As a pure mineral it is colourless or
white, and in the field it could be mistaken for quartz,
although there are two very simple tests that can be
used to distinguish calcite from quartz. First, there is a
difference in hardness: calcite has a hardness of 3 on
Mohs’ scale, and hence it can easily be scratched with
a pen-knife; quartz (hardness 7) is harder than a knife
blade and will scratch the metal. Second, calcite
reacts with dilute (10%) hydrochloric acid (HCl),
whereas silicate minerals do not. A small dropper-
bottle of dilute HCl is hence useful as a means of
determining if a rock is calcareous, as most common
carbonate minerals (except dolomite) will react with
the acid to produce bubbles of carbon dioxide gas,
especially if the surface has been powdered first by
scratching with a knife. Although calcite sometimes
occurs in its simple mineral form, it most commonly
has a biogenic origin, that is, it has formed as a part
of a plant or animal. A wide variety of organisms use
calcium carbonate to form skeletal structures and
shells and a lot of calcareous sediments and rocks
are formed of material made in this way.
Magnesium ions can substitute for calcium in the
crystal lattice of calcite, and two forms of calcite are
recognised in nature: low-magnesium calcite (low-Mg
calcite), which contains less than 4% Mg, and high-
magnesium calcite (high-Mg calcite), which typically contains 11% to 19% Mg. The hard parts of many
marine organisms are made of high-Mg calcite, for
example echinoderms, barnacles and foraminifers,
amongst others (see 3.13). SLIMESTONE:-
Limestones are familiar and widespread rocks that
form the peaks of mountains in the Himalayas, form
characteristic karst landscapes and many spectacular
gorges throughout the world. Limestone is also impor-
tant in the built environment, being the construction
material for structures ranging from the Pyramids of
Egypt to many palaces and churches. As well as being
a good building stone in many places, limestone is
also important as a source of lime to make cement,
and is hence a component of all concrete, brick and
stone buildings and other structures, such as bridges
and dams. Limestone strata are common through
much of the stratigraphic record and include some
very characteristic rock units, such as the Late Cretaceous Chalk, a relatively soft limestone that is found
in many parts of the world. The origins of these rocks
lie in a range of sedimentary environments: some
form in continental settings, but the vast majority
are the products of processes in shallow marine envi-
ronments, where organisms play an important role in
creating the sediment that ultimately forms limestone
rocks.
Calcium carbonate (CaCO3) is the principal compound in limestones, which are, by definition, rocks composed mainly of calcium carbonate. Limestones,
and sediments that eventually solidify to form them,
are referred to as calcareous (note that, although
they are carbonate, they are not ‘carbonaceous’: this
latter term is used for material that is rich in carbon,
such as coal). Sedimentary rocks may also be made of
carbonates of elements such as magnesium or iron,
and there are also carbonates of dozens of elements
occurring in nature (e.g. malachite and azurite are
copper carbonates). This group of sediments and rocks
are collectively known as carbonates to sedimentary
geologists, and most carbonate rocks are sedimentary
in origin. Exceptions to this are marble, which is a
carbonate rock recrystallised under metamorphic
conditions, and carbonatite, an uncommon carbon-
ate-rich lava.

Dolomite

Calcium magnesium carbonate (CaMg(CO3)2) is a
common rock-forming mineral which is known as
dolomite. Confusingly, a rock made up of this mineral
is also called dolomite, and the term dolostone is now
sometimes used for the lithology to distinguish it from
dolomite, the mineral. The mineral is similar in
appearance to calcite and aragonite, with a similar
hardness to the latter. The only way that dolomite can
be distinguished in hand specimen is by the use of the
dilute HCl acid test: there is usually little or no reac-
tion between cold HCl and dolomite. Although dolo-
mite rock is quite widespread, it does not seem to be
forming in large quantities today, so large bodies of
dolomite rock are considered to be diagenetic

Siderite

Siderite is iron carbonate (FeCO3) with the same
structure as calcite, and is very difficult to distinguish
between iron and calcium carbonates on mineralogi-
cal grounds. It is rarely pure, often containing some
magnesium or manganese substituted for iron in the lattice. Siderite forms within sediments as an early
diagenetic mineral trontium may substitute
for calcium in the lattice and although it is in small
quantities (less than 1%) it is important because
strontium isotopes can be used in dating rocks

Aragonite

There is no chemical difference between calcite and
aragonite, but the two minerals differ in their
mineral form: whereas calcite has a trigonal crystal
form, aragonite has an orthorhombic crystal form.
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
practice, it is rarely possible to distinguish between
the two, but the differences between them have some
important consequences Many invertebrates
use aragonite to build their hard parts, including
bivalves and corals.
Dolomite
Calcium magnesium carbonate (CaMg(CO3)2) is a
common rock-forming mineral which is known as
dolomite. Confusingly, a rock made up of this mineral
is also called dolomite, and the term dolostone is now
sometimes used for the lithology to distinguish it from
dolomite, the mineral. The mineral is similar in
appearance to calcite and aragonite, with a similar
hardness to the latter. The only way that dolomite can
be distinguished in hand specimen is by the use of the
dilute HCl acid test: there is usually little or no reac-
tion between cold HCl and dolomite. Although dolo-
mite rock is quite widespread, it does not seem to be
forming in large quantities today, so large bodies of
dolomite rock are considered to be diagenetic .