| The History of Limestone | Acantha Lifestyle Background |
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| By David Smith, Petrology Curator, Mineralogy Department, The Natural History Museum, London. The images are courtesy of The Natural History Museum. |
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| By definition, a limestone is a sedimentary rock made of calcium carbonate, CaCO3. All limestones began as a sediment on the sea floor of shallow tropical seas and were lithified (turned to stone) through the binging of the constituent grains with calcite. Sea water contains carbon dioxide (CO2) which escapes as the water is warmed up by the sun. As this happens, calcium and bicarbonate icons in the water combine to form calcium carbonate, in much the same way as limescale forming in a kettle.
The amount of calcium carbonate within a limestone may vary from 100% as in the Chalk of the southeast of England, to 70% in the Magnesian Limestones and to as little as 30% in sandy limestones, such as Ham Hill stone. Limestones, exemplified by warm tones of Ketton stone and many of the ‘Bath nucleus of a minute sand grain. Ooids can be seen forming today in the warm tropical seas of the Bahamas, where the gentle currents roll the grains across the sea floor as the calcium carbonate is deposited uniformly around the nucleus. Skeletal limestones, on the other hand, are those whose grains are dominated by the remains of marine animals. That is to say, they contain fossils. Shelly limestone include stones such as Portland and Purbeck Stone, and Portuguese Beige. Coral-rich limestones include Rouge Royal of Belgium and the colourful limestones of the Torquay are of Devon (e.g. Ashburton and Petitor). The ‘Birdseye’ and Derbydene limestones of Derbyshire contain fossils of crinoids, a plant-like animal that used to live on the sea floor about 300 million years ago. |
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| From the type and amount of unbroken fossils it is possible to iner the kind of environment in which the stone was formed. Intact corals preserve in life-position in a stone clearly indicate formation within a reef environment. But a limestone of broken skeletal material suggests the animals had been moved from their life position by ocean currents. Broadly speaking the more broken and disaggregated the fossils, the closer the rock was formed to the high-energy waves of the shoreline. These currents kept the shells in constant motion, swirling around and knocking against each other resulting in abraded fragments aligned in layers at an angle to the bedding. At the other end of the ‘energy spectrum’ you have limestones that were formed in the deep ocean by the slow accumulation of a carbonate ‘rain’ of microscopic organisms. Larger organisms that died and fell to the sea floor were rapidly buried and preserved relatively. |
 Oolitic limestone Concentric growth rings of ooids held in a calcite cement |
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Strongly defined fabric has implications on whether the rock is best cut parallel or perpendicular to the bedding but one needs to consider its intended purpose. The gradual accumulation of carbonate mud in the deep ocean entombs and preserves the shells of floating creatures that have reached the end of their life cycle and settled to the sea floor, producing a hard, finely laminated limestone with qualities that make the Jura limestones suitable for flooring. Whereas the gentle environment in which ooids form and are natural graded according to grain size produces a rock whose uniformity in all directions makes it perfect for carving-a freestone! Selection of a limestone may be purely based on its colour. For example, the uniform black of Noir Belge makes a bold statement for a fire surround. The colour is acquired early on in the formation of the limestone. The cream, buff, brown, orange and red tints are all caused by varying amounts of iron oxides dissolved in the fluids within the sediment. The greys and blacks are due to the presence of very fine particles of organic matter and varying amounts of iron sulphide. |
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| As more and more sediment settles to the seafloor, the grains in lower layers become compacted together, the fluid is squeezed out and the sediment hardens. As the migrating fluids move through the rock, they may precipitate a fine calcite cement that holds the grains together. Over time the fine cement gives way to coarser crystals that allow the stone to take a good polish. Interestingly, it is not the cement that gives limestones such as Portland and Ketton strength, but the ooids. Used in buildings under compression it does not matter that they lack good cement providing that they have strong grains. In fact, such stones have the added benefit of being relatively easy to cut and carve. Conversely, limestones such as Bath stone grain their strength from the cement and not the ooids, which are soft and weak and prone to weathering. |
 Petitor - As good as any foreign stones, the coral-rich limestones of Devon were once very popular |
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Similar fluids to those that precipitate a cement may also dissolve the weaker grains leaving casts of their ornate form and opening up the porosity of the rock. This is typified by the Portland Roach in which the ‘corkscrew’ holes render it useless for external use bu adds an interesting texture to sculptures. From a building point of view, the most important aspect of stone is the pores, particularly those less than 0.005mm across. Fine-grained muddy limestones contains a high proportion of interconnecting micropores that can carry moisture far back into the stone, making it susceptible to frost and pollution attack. Thus in selecting stone it is important to look at it under a microscope and out it through several standard tests to determine the stone’s suitability for the intended use. So just how does a limestone that was being buried on the sea floor end up being quarried several tens, or even hundreds, of metres above sea level? Such uplifting movements of the Earth’s crust are possible as the jigsaw pieces that form the rind of the Earth move independently and collide with each toher. The collision of Africa with Europe over the past 50 million years has resulted in some highly sought after limestones that take a good polish. As the crust was buckled and folded to form the Alpine Mountains chain, some of the limestones became buried and heated causing the growth of large calcite crystals. They retained some of their original features from when they were sediment, unlike the true marbles which are the result of recrystallisation of limestone under higher temperatures. |
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| During burial, limestone can obtain other characteristic features. The wispy, zig-zag lines seen in limestones such as Jaune Jaspe and Trani are the consentration of muddy particles from within the limestone, and are called stylolites. Cracks in the rock are often filled with white calcite. However the presence of impurities can colour these weins red, green and even gold as in the stunning black Italian stone, Port Oro.
Limestones used today for construction or decoration exhibit features that, to the trained eye, tell a long story of their formation. The type of fossils, how intact they are, and their arrangement within the stone reflect, to some degree, where in the ocean the limestone sediment was formed. The extent of cementation, the crystal size of the cement and the presence of veins indicates processes that have occurred since the limestone was buried and the uplifted to it’s present position. So next time you come across a limestone out your face right up to it and look at the finer details. See if you can determine the events that gave it the qualities suitable for the purpose intended. |
 Porto - Striking gold veins in Portoro due to the presence of iron |
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