Granite, with its beautiful range of colours and patterns, is becoming much more familiar both for exterior work and for polished interiors. A huge range of different types is now available, with colours ranging from black and dark olive green, through speckled pinks and reds with a silver sparkle, to almost white. To a geologist, not all of these types of rock are strictly granites, but they do all have a similar geological origin, which is quite different from the sandstones and limestones which started out as soft sediments on some ancient sea or river bed.
Tens to hundreds of kilometres below the surface of the Earth, the rocks are very hot and under huge pressure from the weight of the overlying crust. Under these conditions they slowly circulate by convection like molten toffee cooking in a pan. Cracks in the rigid, cooler surface of the Earth's crust are opened up by the force of the heaving mass beneath. In this way, the continents are slowly moved around on the surface (at a rate of a few centimetres a year) and the gaps between the plates of the moving crust are filled with upwelling material from deep in the Earth. On the opposite edges of the plates, sands and clays that have been eroded off the land and carried into the sea by rivers get pushed back deep into the Earth under the continents and via the deep trenches along the edges of the oceans.
The deep material (called the mantle) is made up of minerals that only form in the conditions of extreme high temperature and pressure found at great depths in the Earth. As convection causes it to move towards the surface, the pressure is released and this causes a partial melting, even though the temperature does not get any higher. As the so-called magma moves even closer toward the surface and starts to cool, crystals of different minerals form.
In different parts of the deep Earth, the chemical composition of this molten magma is rather different. Where sediment gets recycled back into the earth it forms magma that is less dense than the surrounding rocks, so it starts to push its way slowly back up to the surface.
Magma cools as it slowly gets closer to the surface, forming a solid interlocking 3-D mass of crystals, usually of not more than three or four main mineral types. So rocks of this type are called crystalline.
There are two main things that control what the resulting rock-type looks like. First, the chemical composition of the magma. Crystalline rocks that form under the oceans contain lots of iron and magnesium and many of the minerals that are found in them(such as pyroxene) are heavy and coloured. These black granites (strictly they should be called gabbros) are particularly familiar in graveyards.
In contrast, the rocks that ascend from the bowels of the Earth underneath continents contain much ore silica and are characterised by light coloured minerals such as quartz, potassium-containing feldspars and the sparkling micas. The feldspars are particularly variable in colour and the reds, salmons, and whites of the granites that are familiar as kerbstones, work-tops, and cladding get their colours from crystals of feldspar that differ only in the trace amounts of iron and some other minor elements.
These are the true granites, geologically speaking. The mica they contain may be silver or dark brown in colour. Both types have a characteristic flaky structure and their sparkle comes from the light reflected off the surface of each flake.
These two types of crystalline rock are the extremes. There are many that are of intermediate composition because different types of magma got mixed or sorted at depth. Some rocks get squeezed and folded during their journey upwards and concentrations of different minerals can get smeared out and stirred into one another to give streaked and swirling patterns, like chocolate sauce stirred into ice cream.
The other thing that affects the appearance of crystalline rocks is how fast they approached the surface and how quickly they cooled down. If cooling was rapid (the extreme case of this is magma that spews onto the surface of the earth as volcanic lava) the crystals did not have much time to grow before the rock became solid. In this case it is only possible to see crystalline structure in a slice of the rock seen through a microscope.
The most decorative granites are the ones where the magma cooled slowly and large crystals had time to grow. The true granites of Cornwall and Shap are of this type.
The first crystals of feldspar started to grow when the temperature of the magma was still about 8000C. As some of the chemical elements in the melted mass got locked up in the feldspars, the composition of the surrounding liquid gradually altered, and with further cooling other minerals, such as glassy quartz and mica, grew in the remaining space. If you look carefully at a polished granite counter-top, the three intergrown types of mineral crystals can easily be spotted.
Sometimes, rising magma tore off flakes of the overlying rocks as it pushed up through them, mixing and melting the fragments to form small local patches of other minerals. These fragments are discoloured nuisances to the producer, but fascinating to geologists. Occasionally they produce granites with a completely new texture, such as the famous Australian orbicular granite with golf-ball like dark masses set in a light matrix.
These coarse-crystalled rocks were still deep in the earth, sometimes several kilometres down, when they finally became completely solid. They only became exposed at the surface by the gradual erosion of overlying rock layers over millions of years.
Some of the minerals that they contain are unstable at the low temperatures and pressures of the surface, or where they meet air and moisture. Some feldspars and dark minerals like pyroxene slowly rot into clay, staining brown as iron is released. This can be a source of discoloration on granite used for exterior work.
The word 'granite' is the worst offender, but 'marble' takes the silver medal when it comes to an abused term. 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.
Compacted mud is an excellent building material in the right place, like a hot, dry desert, but it is quite unsuitable for the wet, northern European climate unless covered and kept dry. Rocks that are made of mud and silt are the Cinderellas of the useful sedimentary stones, overshadowed by their more glamorous relations, limestone and sandstone. Usually they are soft and weak, weathering easily and soon breaking down under the influence of ice and rain to their constituent particles of clay.
But mudrocks may be transformed from something soft, grey and dull into a versatile and attractive material that has the property of being much harder and easy to split into thin sheets. It can be used in flooring, walling, cladding and (literally above all) roofing. This splendid material is slate. The story of slate starts in the ancient seas that once covered much of the land around us. Rivers that drained the land carried with them a sediment load of silt that dropped out onto the sea floor, building deltas and barrier islands where the rivers met the sea. Close to the shore the coarser sediments (composed of the larger grains) formed sandstones, but the smaller silt and clay particles were carried further away from the shoreline and settled out more slowly into the deeper water.
Eventually these fine-grained sediments built up into thick piles of mud on ancient sea-beds, slowly compacting in thickness as the water was squeezed out of them by the increasing weight of the accumulating sediment pile. The grains in muds are of two main sorts. Some are minute particles of quartz, much smaller than the quartz sand-grains in a sandstone. Thin layers (laminations) enriched in quartz particles sometimes show up as slightly lighter-coloured layers in a slate and give away the original direction of the natural sedimentary bedding of a rock.
Most of the particles, however, are made up of clay minerals - a geological mixed-bag of chemically similar but distinct silicate minerals that are the end products of the weathering reactions of the minerals that composed the rocks of the land surface. Besides their small size (only a few thousandths of a millimetre across), clay mineral particles have one other important property. The minute individual crystals of which they are made are cut by planes of weakness in one direction, in the direction in the crystal structure along which they break easily, thus producing smaller grains that are flaky.
Deposition of muddy sediments on the seafloor sets the stage for the formation of slate, but they then have to be altered into something stronger and harder by the effects of heat and pressure - the process known to geologists as metamorphism. Although slate represents the results of a fairly low level and gentle metamorphic alteration, the temperature and pressure increases involved still require something more than just burial of an increasingly thick sediment pile on the sea-bed.
The lateral forces provided by the sideways movement of continents and the tectonic plates that support them across the surface of the globe can provide the necessary changes. Squeezing and folding by compression of formerly quiet, muddy sea floors, has often happened in geological history. Not only is direct squeezing pressure applied, but the buckled sediments may become deeply buried to where pressures and temperatures are higher, until some such time as erosion brings them to the surface again. Often, such zones of squeezing mark the lines where ancient continents, formerly separated by an ocean, approached one another and collided.
The slate-belt of north Wales and southern Scotland, for example, marks the site of a former ocean, the Iapetus Ocean, that ceased to exist about 450 million years ago. In north Cornwall, as well as in Brittany and Spain, the same scenario happened, about 150 million years later. The increase in temperature and pressure that the muddy sediments are subjected to causes several changes, all of which combine to produce a harder rock with a well-developed rock cleavage, that is, a preferred direction of splitting.
The compression rotates some of the clay particles so that their flat, flaky faces are perpendicular to the direction of stress. Other clay particles start to re-crystallise and grow with a different orientation, again with their flat faces aligned in the same way. Some new clay minerals, such as chiorite, and other flaky minerals such as mica, may also form and grow with similar orientations. All these flaky minerals growing in an aligned direction also have their internal planes of crystal weakness minerals growing in an aligned direction and the effect is that the whole rock splits easily in a direction of 90 degrees to the original squeezing stress.
Of course, this direction need not have any relation to the direction in which the original bedding in the mud ran, so faint bedding laminations sometimes cross the planar surfaces where the rock has split along the cleavage. Slates may be quite varied in colour because of the different minerals that grew during the metamorphic alteration. Chlorite is green, so gives a greeny tint to the grey of the slate. Some slates contain quite a lot of iron, and are pink or purple.
Locally, gobstopper-sized blobs within purple slates have undergone alteration of the iron to a green form that is chemically reduced, giving the famous spotted slates of north Wales. The green blobs were there before the squeezing started, because they have also been stretched out and deformed. Rarely, fossils are found in slates that have been stretched on the geological rack in the same way - the Delabole Butterfly (actually a brachiopod that lived on the muddy sea floor) is an example.
A few slates started as segments other than simple muds. Some of the Lake District slates formed from the ash that was spewed out of the volcanoes that lined the shores of the Iapetus Ocean all those millions of years ago. The re- crystallisation of minerals during the metamorphic alteration of slate can sometimes be a potential source of trouble. The original muds varied in the proportion of fine particles of organic matter that they contained. Organic-rich muds tended to develop iron sulphides (pyrites) as a waste product of the activities of bacteria that thrived in them on the seabed.
Metamorphism can cause the pyrites to grow as quite large crystals which are hard and shiny when the slate is freshly split. But beware! Pyrites are notoriously unstable when exposed to air and rain and will soon rot away to a rusty stain, leaving a hole. Some pyritic slates will rot away completely. Some of the slates imported into Britain in recent years have turned out to be expensive mistakes for just this reason.
The surface of the earth may seem permanent when measured against the duration of a human life span, but over geological time the combined action of wind, rain, ice and plant action can reduce even the toughest rocks to dust. None of the rocks exposed in the land around us can resist the inexorable effects of erosion.
Erosion is partly physical in its effect and partly chemical. The action of the freezing and thawing of water in cracks within rock is the most severe physical effect, eventually causing the surface to split and fragments to fall off as particles of sand and gravel.
The pressure of roots forcing the cracks apart may also contribute, and so may expansion under the heat of the sun in some parts of the world.
Chemical effects arise because some of the minerals of which rocks are made up are unstable over long periods of time when exposed to a mixture of water, oxygen and carbon dioxide that falls as rain. They dissolve or react chemically to form new chemical compounds that are soluble and which are carried into the ground in solution. Also, plants can contribute when their decay makes the water in the ground more acidic.
However, not all the minerals found in rocks are equally susceptible to erosion and decay. Above all, crystals of the mineral quartz (a hard and chemically unreactive form of silica dioxide) put up such a resistance that the other minerals in quartz-rich rocks weather away around them to leave a residue of quartz grains.
These grains become sorted by winnowing into a pure sand which becomes the main component of the sediment that is carried along the beds of streams and rivers, eventually to be taken to the sea.
A look at the sediment in the bottom of a stream on Dartmoor illustrates the point. The rock is granite, rich in quartz as well as less resistant minerals such as feldspar and mica.
On top of the moor, the stream sediments are full of large, angular fragments of quartz and partly broken and rotten lumps of feldspar a few millimetres in diameter. Further downstream, the feldspars become less common because they have broken down further and changed to clay and the quartz crystals are worn down and smoothed, becoming smaller and less angular and sorted into grains of similar size. The sediment is said to be becoming more mature.
Just these sorts of erosion and stream-transport events, happening at times in the far geological past, wore away long-vanished granites (and other types of quartz-rich rock) and eventually dropped the quartz-rich load to form sandstones.
Close to the source, the immature sediments gave rise to sandstones in which the grains were easily visible to the naked eye and in which some feldspar was also present. Such coarse sandstones are often known as grits - the famous Carboniferous Millstone Grit of Derbyshire is an example.
Further downstream, the more mature sediments formed the sand banks that were destined to become pure sandstones which, by definition (to a geologist), are made up of grains ranging from 1/16mm to 2mm in diameter.
Not all the sand carried by rivers made it as far as the sea. Generally speaking, rivers cut down and erode the underlying rock in their fast flowing upper reaches, but slow down and start to meander across so-called alluvial plains as they approach their mouths. The sand bars on the inside of meandering river channels and the deltas at river mouths are two of the sites where sandstone may start to form, first as sand-banks but later, when buried, becoming hardened into rock.
River sandstones of this sort form beds which may reach several metres in thickness, but which may rapidly taper out along a quarry face, as is seen in many of the Yorkshire sandstones. Often they show an internal lamination that is oblique to the natural bed, showing that they were deposed as small, moving underwater dunes.
Occasionally, the rivers flooded and broke their banks, spilling out a fast-moving torrent of water and suspended sand onto the surrounding flood plain. These sands settled quickly and show thin, horizontal laminations internally, along which the stone splits easily. This is the origin of York paving.
Much of the sandstone in Britain comes from the Carboniferous period (some 300 million years ago) when the climate was wet and large rivers flowed across the northern counties into shallow seas.
Locally, the same conditions existed at other times: the North York Moors are made of much younger Jurassic sandstones, a mere 150 million years old.
At yet other times, land erosion took place in a dry desert climate and the quartz grains were blown by strong desert winds, accumulating and moving as large dunes. These desert sandstones are easily recognisable. They have large-scale, sweeping, oblique internal laminations, just like the modern sand dunes of the Sahara, and the individual quartz grains are all much the same size and spherical because they have been rubbing together in the driving desert winds. The New Red Sandstone of Cheshire and the west Midlands, formed in Permian times following the drying u p of the Carboniferous rivers and coal swamps, and the geologically older Old Red Sandstone of Scotland, are examples.
The sandstone story doesn't finish until the soft sediments become hardened into a usable rock, and this happened deep in the earth, after the banks or dunes had been buried. Minerals of natural cement, carried in solution in the ground water that fill the pore spaces in all sedimentary rocks at depth, precipitated as crystals and glued neighbouring grains together.
The type of cement reflects the chemical composition of the pore water. Lime or clay minerals give rise to softer sandstones. The hardest sandstones, sometimes referred to as quartzites, have a natural cement that has the same quartz composition as the original grains. Under the microscope, it is difficult to see where the original sedimentary grain stops and the cement begins, though sometimes a thin strain of iron minerals marks the site of the original grain surface.
Many desert sandstones are pink or red, because the cement (or one of the cementing minerals) is the red iron oxide, haematite. Some older books show pictures of what these ancient deserts are thought to have looked like, with bright pink sand glowing in the Permian sunshine. The truth, alas, is less spectacular. The sand was sand-coloured. The red haematite only formed as the water table moved upward, bringing dissolved iron into contact with the oxygen in the air.
I would like to be able to refer readers to a book that bridges the gap between the academic study of limestones and the interests of the construction industry, but unfortunately I do not know of one. The information I have given here is well known to geologists, but tends to be published in rather obscure publications.
Calling a rock a limestone is saying nothing more than that it is calcium carbonate (CaCO3). The look of that calcium carbonate under a scanning electron microscope (SEM) can be quite different in limestones from different quarries.
SEMs produce useful microscopic pictures because they appear three dimensional, like the world around us. Often the rocks are first impregnated with plastic, the limestone etched away with acid and the plastic studied under the microscope, when the bits that slick out are actually the holes in the original rock.
All limestones start as floors of shallow tropical seas. The famous stones of Portland and Bath were laid down 150million years ago when what is now that area of England was more like the Bahamas are today. Limestone is still being formed in parts of the tropics, where some perfectly good, hard stone is no more than 30 or 40 years old. It has even been known to contain impurities like cola bottles.
Sea water contains carbon dioxide (CO2), which escapes when the water is warmed by the sun. When this happens, calcium and bicarbonate ions in the water combine to form calcium carbonate, just like scale forming in a kettle. This crystallises as calcite.
The famous ooliths (tiny spheres) in limestones are grains of sand or pieces of shell around which calcium carbonate which precipitated from the sea has stuck. The cement that sticks the ooliths together also consists of calcite crystals, which grew either on the sea floor or later in the rock when it was buried.
Shells of sea animals form grains in limestones, and some of them, such as pieces of starfish and sea lilies, promote the growth of cement crystals around themselves particularly well and result in limestones which are especially strong.
Some limestones, such as Portland and Ketton, take their strength from their ooliths. They have little cement in between, but that does not mean they are inferior stones because in a building they are used under compression and as long as the ooliths are strong the lack of cement does not matter. In fact, such stones have the additional benefit of being relatively easy to cut and carve.
Other limestones, such as Bath stone, gain their strength not from the ooliths, which are soft and weak, but from the calcite cement in between the grains.
The larger the crystals of the cement, the stronger that cement will be. It will also be smoother, which means the stone will take a polish. Sea lilies, sea urchins and starfish promote growth of large crystals around themselves. Since there are large amounts of these in some carboniferous limestones such as Griffeton Wood, Swale Dale and the Irish blue limestones, these stones will take a good polish.
Over time, large crystals will grow at the expense of smaller crystals in any limestone, but especially if the stone is buried and warms up. And if it happens to be buried next to where granite is being forced up from deep in the earth near to the edge of a continental plate (when the granite will be at 800-900°C), the limestone will re-crystallise and become marble.
One of the reasons why some of the French limestones take a good polish is that they got mixed up in the formation of the Alps resulting from Africa bumping into Europe over the past 50million years. As the land became buckled and folded, some of the limestones became buried and heated, causing the growth of large crystals in the stone.
From a building point of view, the most important aspect of the stone is the pores (the holes in the stone), particularly those of less than 0.005mm across. These will affect the way the stone weathers.
Some of the tests regularly used in Europe to predict stone durability, such as the saturation coefficient, the capiliarity and the effective porosity, are specifically designed to give an indication of the small pores (the microporosity) of a stone.
In this country, the salt crystallisation test can accurately distinguish the more microporous stones if it is carried out and interpreted accurately The test consists of 15 cycles of soaking stones in a salt solution and drying them. Salt crystals will grow in the pores which may cause some stone to break away It is this loss of stone which the test measures. But the test is easy to get wrong and is highly susceptible to slight variations in temperature, salt concentration and drying time between soakings.
A problem which can occur with some limestone is the presence of mud between the grains. The mud contains a high proportion of micropores which interconnect and can carry moisture far into the stone or even right through it, making it susceptible to frost or pollution attack. A lot of pores also means a large internal surface area, which weakens the stone.
The way to a fuller understanding of stone is through a mixture of petrography (description of the stone, especially with photographs) and the sort of information which comes from some Continental tests. Such a combination should allow the producer, the specifier and the end user to predict really quite well how the stone will perform in use.
SEM photographs are particularly useful and the results are easier to interpret than those obtained from thin sections (0.03mm thick slices of rock viewed down an ordinary microscope) that geologists traditionally use. SEMs are becoming standard pieces of equipment and will not add a lot to the cost of tests.