There are two things that define marbles,
in the broad understanding of the name. The first is their
composition. They are made predominantly of the calcium carbonate
mineral calcite, though they may contain smaller amounts of
other materials as impurities, which sometimes add colour.
Such colour may be generally distributed throughout the rock,
or concentrated in veins. Greens, pinks, reds and greys are
the usual colours arising from these impurities, and their
origins often lie in a variety of iron bearing and clay minerals.
But this description of composition could
equally well apply to limestones. The essential distinction
between these two important rock types is 'polishability'.
Marbles will take a good polish whereas many limestones will
not. Those that do are often referred to as marbles (Purbeck
and Frosterly marbles are examples) and it is to these hard,
polishable limestones that geological purists begrudge the
True marbles are metamorphic rocks. Just
as muddy sea-floor sediments may re-crystallise to form slates
when buried and exposed to (geologically) moderate amounts
of heat and pressure, so lime sediment and soft limestones
re-crystallise under the same conditions to form marbles.
The re-crystallisation, i.e. an alteration in the texture
of the calcite crystals that make up the rock, is the most
important change in the formation of marbles. Small amounts
of new minerals may grow from any impurities in the original
rock, resulting in coloured streaks and veins. But the enlargement
of the calcite crystals is the main change. This is particularly
favoured by heating the parent rock in the presence of water
(of which there is an abundance in the original sediment).
Such heating is sometimes provided by burial
to the warm depths of the Earth, but frequently the heat source
is a hot igneous rock that is pushed up from deep in the Earth
through a pile of overlying limestone, resulting in what is
referred to as contact metamorphism.
In some parts of the world, true marbles
are rare because our principal limestone deposits are not
old enough to have been subjected to the sort of deep burial,
squeezing, heating and igneous activity that happen when piles
of marine sediment are caught between colliding continents.
The exception is in north and west Scotland, where our oldest
limestones (the Durness limestone of Cambrian and Ordovician
age about 500 million years old) has locally been sufficiently
altered, as in the case of the beautiful and exotic Ledmore
marble from north of Ullapool.
Countries bordering the Mediterranean have
been affected by the continental collision that created the
Alps in the geologically recent past (in the past 60 million
years or so), marbles are common, hence forming such an important
part of the historical tradition of Greece and Italy.
The actual process of transformation of
limestone to marble involves an increase in the sizes of the
crystals present and the infilling of any pore spaces present
in the original rock.
Unaltered limestone can be thought of as
a mixture of different sizes of calcite crystals, mostly of
less than a micron (0.001 mm) to a few tens of microns across.
Between them are minute pore spaces filled with water or organic
compounds. The effects of heat and pressure both compress
the stone, reducing the pore spaces, and cause some of the
smaller crystals to dissolve and re-precipitate on the larger
At the same time, some new calcite may be
introduced in the internal water. As a result, the calcite
crystals are reduced in number, but increase in size to give
a texture rather like a sugar lump, in which individual crystals
of granulated sugar are pressed together, growing and interlocking
closely to give increased hardness and strength.
The shapes and sizes of the original grains
in the limestone (such as shells and ooliths) are usually
obscured in the process.
The close-textured rock now has virtually
no internal porosity - in fact, the texture is rather like
a fine-grained granite with the adjacent crystals interlocking.
So when it is cut the whole surface forms a mosaic of large
calcite crystals, which take a good polish on their sliced
faces, and have no gaps in between.
But there are other ways than extreme heat
and pressure of filling in the pore spaces and enlarging the
sizes of the crystals in a limestone. They take place over
longer periods of time in the quiet and unspectacular conditions
of normal burial in a pile of sedimentary rock. But if they
are allowed to go to completion, they may also give rise to
a strong and polishable hard limestone, which we may refer
to as a sedimentary marble.
The simplest scenario is when groundwaters
containing lime in solution slowly trickle or diffuse through
the rock, growing calcite cement crystals in any pores. The
weight of the overlying rock helps because it puts pressure
on grains that are sitting on one another. This both presses
them together and causes some dissolution where the contact
pressure is greatest. The dissolved calcite can then diffuse
into the immediate surrounds and re-grow as cementing crystals.
The zones where this sort of pressure solution has been particularly
localised often show up as zigzag lines of concentrated impurities
and are referred to as stylolites.
Even without new cement being introduced
to the sediment, the crystals that are there may grow slowly
in size at the expense of their smaller neighbours, forming
a more sugary fabric.
Some of the shells in many limestones are
made of a second, rather soluble, calcium carbonate mineral
called aragonite. These may re-crystallise by rearrangement
of the ions in the aragonite crystal lattice to form a mosaic
of interlocking calcite crystals, which bind tightly on to
the adjacent crystals of the natural cement within the rock.
Geologists refer to this sort of re-crystallisation as neo-morphism.
Limestones with these fabrics may be as hard and polishable
as a classic marble such as Carrara, but show more feature
because the boundaries of the individual grains in the rock
have not been so obscured. Purbeck marble, full of re-crystallised
snail shells, is an example.