The word ‘granite’ is the worst offender, but ‘marble’ takes the silver medal. Some of the names that are used for different types of stone provide the stone industry’s own Tower of Babel, seemingly designed to cause mutual confusion by meaning rather different things to a geologist than to someone closer to the heart of the industry. However, being a bit pedantic is one of the little temptations of all professions and I would be happy to try and resist it by using the term marble more widely. There are, after all, a range of intermediate forms between hard limestones and the true metamorphic calcareous rocks to which geologists prefer to limit the term.
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 marble name.
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 Britain, 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.
Further south in Europe, however, where the 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 crystals.
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. Our own Purbeck marble, full of re-crystallised snail shells, is an example.
Copyright Â© Dr Tim Palmer
Dr Tim Palmer (firstname.lastname@example.org) is a Chartered Geologist and consultant on the petrography of building stones and limes with a particular interest in historic buildings and landscapes. He researches and teaches geology part-time in the Earth Sciences Institute at the University of Wales, Aberystwyth, SY23 3DB. Tel: 01970 627107.