Geology

Many of the medieval monuments in the more southerly counties of England are made of pale, easily carvable limestone of Jurassic age. The detailed geological character of these stones may point to both provenance and date of the monument, so an accurate petrological description is important. The first thing to determine is whether they are oolitic or non-oolitic limestones. This distinction causes much confusion, because some people think that the term oolitic limestone covers all limestones from the Jurassic limestone belt. In fact, it refers only to the specific geological character of some of them.

The great majority of the limestones that we meet in monuments or buildings started as shallow seafloor sediments, consisting of clean-washed sand-sized grains of calcium carbonate. Either these grains were derived from the breakdown of shells and other calcareous skeletons, or else they grew on the seafloor by precipitation of the mineral calcite around smaller particles, just as lime-scale may grow on the inside of a kettle. This second category constitutes the ooliths. Surprisingly, they have a rather limited size range (typically from 0.2-0.8 mm in diameter: when they reached the maximum size they abraded away as fast as they grew and enlarged no further). They are also characteristically very well rounded or spherical. When sliced through and viewed microscopically, the layered structure of the precipitated calcite around the central nucleus is seen. They look like the many layers within the gobstoppers that some of us remember from our youth.

Many Jurassic limestones consist of little other than ooliths, maybe with the odd fossil or fragment. Others have much more shell debris in them, either scattered through a mass of ooliths or (more usually) in debris-rich layers that alternate with oolith-rich layers. This pattern reflects the surging of currents during the initial accumulation of the sediment and is very evident in limestones from near Bath. Other limestones accumulated on the seafloor in areas where ooliths did not form, so the resulting limestones only contain shell fragments from the invertebrate fauna that originally lived there. Geologists refer to these as bioclastic limestones.

Because the smooth spherical character, size-range, and abundance of ooliths is highly distinctive, there should be no problem in recognising them on a cut or rubbed stone surface, especially with a magnifier (no-one should ever venture out without one). Even the concentric laminations and the nucleus may be clear on close inspection. Experience will soon allow distinction between an oolitic limestone and a limestone that is principally composed of comminuted shell debris.

However, there is a further complication that may confuse neophytes and cause one oolitic limestone to look rather different from another. This is the disposition of the natural mineral cement within the stone. Limestones acquire their hardness by growth, during burial, of calcite crystals in the minute holes (pore space) between the grains. This material is popularly called �spar�. Some oolitic limestones (e.g. Ketton and Portland) are held together by tiny dabs of spar, principally at the points where adjacent ooliths touch. This is quite enough to give the stone rigidity, and a close look will show the ooliths themselves looking like ball-bearings in a box with the spar not evident (A). But other oolitic limestones (e.g. Bath, B) have their original porespace completely filled with spar, so that the ooliths at the cut surface of the stone may fall away to reveal a surface texture dominated by the concave moulds in the enclosing spar � like close-packed egg-cups on a tray. The first type is referred to as a grain-prominent oolitic limestone, and the second as a spar-prominent one. Yet others have lots of spar, but the individual ooliths are stronger and tend to remain in their little sparry cups at the cut surface. Painswick stone (C) from north of Stroud frequently shows this character.

Each frame (below) is c. 2 cm wide and shows what you would expect to see with a x10 magnifier and a clean surface.

Why, then, are some of the most distinctive non-oolitic limestones that contain only shell debris (Doulting and the highly-important mediaeval Dundry limestone (D) come to mind) persistently and erroneously referred to as oolitic when they are nothing of the sort? Geologists are not completely blameless, because the major Jurassic limestone sequences that run south-west to north-east across England were, in the 19^th century, referred to as the Lower, Middle, and Upper Oolites. The Lower was the most widespread and economically important, comprising the Great Oolite of the Bath region, and the Inferior (= subjacent) Oolite near Cheltenham. These stratigraphic units contain many different varieties of limestone of which only some are oolitic, but the name (maybe because it is such a euphonious word) was applied to the whole sequence. Then the historians of architectural materials came along and found it convenient to refer to this swathe of country as the Oolite Belt, and the trap was sprung. Ergo an oolitic limestone is any limestone from the Oolite Belt. Except, of course, that it isn�t.

Tim Palmer

It was while I was putting out the rubbish that it occurred to me how I might lead into an explanation of marble as a material. Here in west Wales the plastic wheelie bin does not yet rule supreme, and we are still allowed to use the traditional dustbin. The galvanised surface is a thin layer of zinc that has been induced to grow on the iron vessel by electrolysis. The zinc is in the form of crystals that grew outwards along the iron surface until they met their advancing neighbours and stopped. The result, looked at closely, resembles a jigsaw in which the pieces have angular contacts that abut against each other, rather than interlocking ones. This is the essence of any mass of material that is made up of intergrown crystals. The ice on a frozen windscreen is another � but both ice and zinc galvanisation are two-dimensional crystalline layers. The same thing can also happen in three dimensions, though it is not so familiar to us in our day-to-day experiences. The solid coating of ice that builds up on the walls of an old-fashioned deep-freeze has this structure of intergrowing crystals, and so does marble.

In marble�s case, the material of which the stone is composed is the mineral calcite, the most common of the crystalline varieties of calcium carbonate and one of the 2 or 3 most important minerals in the stones that are used by architects and carvers. The purest white marbles, such as statuary Carrara, are 100% pure calcite. In contrast, most marbles, including other varieties of Carrara, have other minerals (usually mixtures of clays or iron-rich compounds) mixed in with them, and this gives many of them a streaked or grey appearance, or some other colour or pattern. Marked variation in colour and texture is even more evident in the sedimentary marbles, which may also contain fossils. Purbeck is a familiar example.

All marbles started their geological life as limestones, which are also predominantly composed of calcite, and the formation of marble from limestone principally involved a textural change. The original limestone sediment was a mix of grains and limey mud. Shell fragments and ooliths (see the last Newsletter) often featured. Each of these components was itself a jumble of carbonate crystals, of widely differing shapes, sizes, and orientations. In addition, there were originally minute spaces between the constituents. A thought-experiment helps: imagine a bowl of sugar (a crystalline substance), assembled as a mixture of all the different varieties that can be found in the kitchen cupboard: granulated and caster; powered icing sugar and large hard lumps of coffee sugar. Air fills the tiny pore-spaces between the grains. If we dampen this mixture, the surfaces of the grains dissolve: on drying, the grains stick together as lumps because the dissolved sugar crystallises and forms a bond where adjacent grains touch. The finest particles may dissolve completely and end up as additions to the surfaces of the larger crystals. This is the analogy to the growth of a true metamorphic marble like Carrara. In a damp, warm environment buried within the earth, small crystals of calcite go into solution and the atoms (more correctly ions, because they carry an electrical charge) migrate a short distance to the surfaces of large crystals on which they settle and add to the solid crystal structure. Over time, the smallest crystals disappear and larger ones grow at their expense until they meet their neighbours and cannot enlarge further. Typically the average size of the crystals at this stage is 0.5 - 1 mm across. Fossils and individual grains in the parent limestone become less distinct as they dissolve and reprecipitate. If there are impurities in the limestone, they may undergo chemical alteration and form darker streaks, wisps and veins. This is the process by which a porous limestone becomes a dense marble of solid interlocking crystals as seen in the Figure below (the similarity to the galvanised texture is obvious though the crystal size is much smaller and the fabric is 3-dimensional). The warmer the burial environment, the faster (geologically speaking) the alteration process � an example of metamorphism � occurs. That is why limestones that are buried deeply or close to hot volcanic rocks readily become altered to marble. The parent limestone does not have to be very old: the limestones that recrystallised to give Carrara marble were about the same age as the famous fossil-bearing rocks of Lyme Regis.

In contrast, a sedimentary marble like Purbeck has undergone recrystallisation by a rather different process. The famous pond-snails that provide the main constituent of Purbeck Marble lived in a coastal freshwater lagoon, and its floor was composed of a 2 foot deep layer of the dead shells. These shells were made of aragonite, another calcium carbonate mineral that commonly makes up the shells of molluscs. Calcite and aragonite have exactly the same atoms in them in the same proportions, but aragonite is more soluble than calcite in the damp buried environment that persisted as the lake floor became covered by later deposits above it. The aragonite shells started to be replaced by calcite crystals, which seeded and grew outwards at the expense of their more soluble aragonite neighbours. Soon the advancing calcite crystals, as in Carrara, met one another and the replacement of aragonite by a mosaic of calcite crystals was complete. So both types of marble show a fabric of tightly abutting calcite crystals that with little space between them (see the figure below), but the origins or the fabrics are rather different in the two cases. In Purbeck Marble, spaces between the shells also filled up with a natural cement of intergrown calcite crystals so that the whole rock became dense and crystalline. Minor additional iron minerals coloured it blue-grey, red, or green.

When a marble is polished, it is the individual calcite crystals that are cut and smoothed to a high-gloss finish. Many ordinary limestones can never achieve this because, at a microscopic level, they are full of pores. Only when a limestone loses these can a glossy finish be achieved. Some older limestones (Hopton Wood from the Carboniferous of Derbyshire comes to mind) have gradually lost their pores through natural calcite cement growing in the original pore-spaces, and have achieved enough closely-grown large calcite crystals to become polishable. Anglesey and Egglestone marbles are other examples of Carboniferous hard limestones that will take a polish.

Geologists often get rather stuffy about applying the term marble to the polishable limestones, asserting that the word should only be used for the true metamorphic product. I don�t take this view: after all, the term marble has been applied to polishable limestone since well back into the Middle Ages, whereas the subject (and the name) of Geology only started in the 18^th century.