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Formation of Limestone and Marble

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Formation of Limestone and Marble

Contents and Brief
Full Article Reading Time - 15 mins


Brief - Formation of Limestone to form Marble

The journey in the formation of limestone to form marble is complex, however, very specific chemicals and organisms influence the mineral composition. Calcium carbonate is the primary chemical compound in crystalline forms (polymorphs) calcite and much less prominantly aragonite and vaterite depending upon magnesium content. Sediments in the oceans are primarily skeletal remains of organisms and fragments of eroded rock, the purity of marble depends on the content of these factors. The deposition of sediment is a build up of many layers, as they are compressed the layers will go through a process of lithification and diagenesis where physical and chemical changes convert sediments into fine grained sedimentary rocks, including limestone. The conditions required for limestone tranformation into marble (metamorphism) is high pressure and heat which can be found the deeper you go into the earth and is called the geothermal gradient, too much and the limestone will return to the start of the rock cycle as magma. The limestone once exposed to the ideal conditions for metamorphism will undergo chemical reactions where calcite crystals will recrystallize changing the texture and appearance of the rock to metamorphic rock marble.



Chemicals and Organisms

Magnesium (Mg) Influences Mineral Solubility

A large factor that influences mineral composition during Earths changes and climate periods is Magnesium. It plays an important role in enzyme reactions of living organisms including muscle, nerve and regulating pressure and also increases the solubility of minerals and absorption of magnesium in living organisms. It is a diabolical fact that this known process is not recognised by corporations and results in increased risks of illness through consumption of processed foods.

It can therefore be presumed that geological and climate variations of magnesium instigated marine organism migrations over large periods of time, changing mineral solid precipitates to vary in composition. It is evident that waters that created pure calcium carbonate (CaCO3) 'chalk limestone' were not marine ecosystems during the period of formation, but either freshwater or acidic systems with a higher level of 'Carbon dioxide (CO2)'. Therefore, less abundance of organisms that require and produce magnesium.

Marine ecosystems are distinguished by waters that have a high salt content 'sodium chloride (NaCl)' and are currently the largest aquatic ecosystems on earth.


Seawater Source of Magnesium

The source of magnesium in seawater that is essential for marine organisms to thrive most likely comes from the erosion of mafic igneous rocks that consitute oceanic crust. Mafic rocks have a high magnesium content and it is what give this rock its dark colouration. Upwelling mantle along the mid ocean ridges produces mafic igneous rock through partial melt, and form the tectonic plate boundaries, swelling outwardly as if the earth is expanding. The chemical reactions during these events provide elements and minerals that are essential for marine environment life to thrive.


Calcium Carbonate (CaCO3)

1x Calcium, 1x Carbon and 3x Oxygen Atoms form the soluble Chemical Compund Calcium Carbonate (CaCO3). Calcium carbonate comprises approx 4% of the earth's crust and is found worldwide in rocks as the minerals calcite and aragonite. Calcium Carbonate constitutes approximately 20% of the worlds sedimentary rocks and most commonly known to form as Chalk, limestone, marble, travertine, and tufa.


Calcium Carbonate Contributors

Marine waters are rich in calcium carbonate and the main contributors to this is marine organism excretions, fecal matter and remains. Most of these organisms are typical of shallow water environments where sunlight and filterable food are more abundant. Calcium carbonate is the main component of shells, seashells, skeletal structure, cell walls and spicules of marine organisms such as molluscs, Echinoderms, Microscopic Organisms, Algae and Filter feeders. When these organism die, the remains settle and either break down by ocean currents or compressed by the pressure of overlaying sediment over long periods of time.



Carbonate Minerals and Crystals

Calcium Carbonate Crystalline Forms

Significant constituents of the calcium cycle are the crystal forms of calcium carbonate as the polymorphs 'calcite', 'aragonite' and 'Vaterite'. The mineral calcite is important for biological mechanisms and life as we know it and has a trigonal crystal structure which is more stable than its more complex counterparts Aragonite (orthorhombic) and vaterite (hexagonal). The stability of the crystal structure is affected by magnesium in as far as the higher the magnesium ions present the more likely Calcite will form in place of vaterite and aragonite. The introduction of magnesium rich water will revert the crystalline forms vaterite and aragonite to the more stable trigonal crystal, calcite.

Calcium Carbonate is soluble and changes in environment increases the solubility of the mineral calcite allowing small cavities and channels (vugs) to form in exisiting limestone. Various crystal types include calcite, quartz, dolomite and barite may line these Vugs from the precipitates through diagenesis.


Primary Mineral Solid Calcite

Based on geographic locations and mineral composition of exisiting calcium carbonate deposits worldwide it is suggested that through chemical reaction the primary mineral solid (precipitate) formed in marine waters was the mineral 'calcite' that was low in magnesium content. Arroganite and high magnesium calcite are more prominant in more recent deposits. The conditions that change the mineral content of calcium carbonate depend wholly on climate cycles and geographic changes affecting the organisms that reside in the waters during those periods.


Three Crystalline Polymorphs of Calcium Carbonate
Trigonal Crystal Structure of Calcite
Trigonal Crystal System 'Calcite'

Calcite is the Primary mineral in Marble and belonging to the most stable Trigonal Crystal system. It is the main component of eggshells, snail shells, seashells and pearls. High-magnesium calcite retains the calcite mineral structure.

Orthorhombic Crystal Structure of Aragonite
Orthorhombic Crystal System 'Aragonite'

Frequently forms in near-surface environments at ambient temperatures and does not usually contain significant magnesium. Forms as columnar or fibrous or as branching helictitic forms called flos-ferri ("flowers of iron")

Hexagonal Crystal Structure of Vaterite
Hexagonal Crystal System 'Vaterite'

As an unstable polymorph it is rarely found and is most likely to be encountered as the mineral forming the inner shell of mollusks. Pure vaterite has been discovered on the leaves of alpine species saxifraga sempervivum.


Deposition of Sediment

What is Sediment made from?

Sediments are made up of grains formed primarily from the remains of marine organisms and microscopic animals leaving behind skeletal fragments and detritis that have sedimented to the bottom of continental shelves. This makes sediment in the oceans the largest source of calcium carbonate. Another contributing factor of sediment is fragments of rock through erosion ashore or from the seafloor. The build up of sediment (deposition) goes through a process of diagenesis in which is the sediment undergoes physical and chemical changes and/or Lithification, converting freshly deposited grains of sediment in to rock. Variable amounts of silica in the form of chert or siliceous skeletal fragments is also common in sediments.

Carbonate Rock Grain Types

Allochems and orthochemical are terms used when describing types of Carbonate rock grains that are found in sedimentary rocks and vary in size from microscopic to visible particles. Allochems are fragments or grains that have formed prior to settling as a sediment. Orthochemical are grains that form on location through chemical precipitation during the cementing process and generally consist of micrite or sparite. Lithification and diagenesis processes often work in tandem and according to the rate of the sediment forming, upper layers of sediment will produce several different grain types that are eventually cemented together by precipitates of these chemical reactions.

Bioclasts (Skeletal Fragments)

A majority of grains are skeletal fragments of marine organisms, invertabrates and their secretions with varying levels of magnesium.

Ooids / Ooliths

Grains of less than 2mm with layers of calcite or aragonite on a central quartz grain or on a carbonate mineral fragment.

Pisoliths

Similiar to ooids but irregular in shape and larger than 2mm in diameter, also known to form subaerial (open air or on the earth's surface)

Oncoliths

Grains of less than 2mm with radial layers of calcite or aragonite on a carbonate mineral fragment or quartz grain.

Peloids

structureless grains of microcrystalline carbonate - formation is unknown and only speculative.

Limeclasts

Eroded fragments of limestone or partially lithified sediments from within (intraclasts) or outside (extraclasts) the depositional area.


Physical and Chemical Changes in Sediment

Lithification

Lithification is a process whereby freshly deposited loose grains of sediment are rearranged under pressure driving out mineral rich fluids during compaction. Reactions between the dissolved minerals in the ejected fluids may form new minerals and can combine with minerals from elsewhere brought in by under water currents and are redistributed into nodules and concretions as cementation.

Diagenesis

The Processe of Diagenesis is the physical and chemical changes in sediments after it has settled down (deposition) include and according to conditions: adsorption, dissolution, precipitation, microbial activity, and compaction. sediments are compacted and porosity decreases (except dissolution of minerals and dolomitization) as they are buried beneath successive layers of sediment deposition and cemented by minerals that precipitate from solution (precipitation). As the compacted sediment is carried deeper by further deposition above, its organic content is progressively transformed through decomposition into Kerogen (insoluble organic matter).


Fine Grained Sedimentary Rocks

Composed of mainly carbonate minerals and classified as sedimentary rocks the 2 primary rocks types are limestone and dolomite. Chalk and carbonate mud also contain a high percentage of carbonate minerals and maybe formed through abrasion and erosion of carbonate rocks and sediments.

Dolomite (Dolostone)

Magnesium rich waters present at the time sediment undergoes Lithification or diageneis, Recrystallisation (dolomitization) can occur where a proportionate amount of calcite ions are replaced by magnesium ions to form Dolomite. The recrystallisation process dissolves the calcite minerals and the resulting chemical reaction with the magnesium and combined with temperature exceeding 25 degrees C, magnesium quartz will form. This fine-grained sedimentary carbonate rock has an appearance varying from banding to pockets of large crystals and has higher resistance and is less soluble than limestone formed with calcite due to its larger magnesium quartz crystal content..

Chalk

Purpoted to have formed in the Cretaceous, Chalk, a fine grained sedimentary rock is classified as a soft, porous form of limestone composed mainly of the mineral calcite. The lack of erosion from nearby rocks provides some explanation to the high purity of chalk and the calm, mild climate during this period enticed mostly microscopic organisms that thrive in low magnesium conditions. Most likely, the lithification and diagenesis processes differed in that the chemical soup from decaying microscopic organisms of mainly calcite would of been a result of a period of rising ocean levels increasing compaction rate accompanied by mass population or migration.

Carbonate Mud Rocks

Mud rocks are fine grained siliclastic sedimentary rocks and constitue 50% of all sedimentary rocks, formed generally by erosion. Classification of so many variations can be 'muddy' (excuse the pun!) without extensive study, however, slate is derived from a shale type sedimentary rock and is the finest grained metamorphic rock. Carbonate mud or Calcilutite contains a high content of carbonate minerals of Calcite and aragonite.

What and how is carbonate mud formed?

Micrite (Microcrystalline Calcite) is a carbonate mud consisting of individual crystals of less than 5 microns. The micrite crystals are primarily of calcite and aragonite. Recrystallization of the micrite crystals produces microspar with grains of over 5 microns. The origin of Carbonate mud is uncertain, however, it is presumed that it is formed through precipitates from seawater, algae secretions or produced by abrasion or the dispersing of skeletal remains. It seems likely that the abrasion of forming sediments through higher energy currents will set down fragments as a carbonate matrix in sheltered locations prior to recrystallization.


Limestone

Sedimentary Rock Limestone is primarily made from calcite (Stable Crystal form of Calcium Carbonate CaCO3) and various rock fragments. As described previously, microscopic fragments of mircroorganism remains, organic matter and eroded rock fragments go through several chemical processes during compaction. The minerals as a result of the lithification and diagenesis processes arrange and crystalize to form the limestone as we see it today.

What Gives Limestone its Colours?

The colours in Limestone depends wholly upon the minerals it is made from, as limestone is primarily made from calcite and skeletal fossils, white to grey is most common. A higher content of other trace minerals making approx 5-10% by average such as quartz, clay and magnesium will give a more grey colouration. The various trace minerals in limestone will oxidise through weathering mostly through rain and draining water. Yellow and reds are caused by traces of Iron and manganese, organic matter will darken the limestone and can be almost black.

Limestone Hardness

Limestone as a sedimentary rock is relatively soft with a hardness of three on the mohs scale, however, it has a crushing strength by over four times that of todays concrete. It comes as no suprise then that our more evolved ancestors avoided concrete!

Limestone Weathering and Acid Degradation

Acid degradation and Weathering cause limestone to exhibit different colors based on the minerals that it is constituted. As limestone is made primarily from calcite crystals from calcium carbonate it is partially soluble, this is evident in the many stark erosional landforms seen worldwide from rain and draining water. Acid rain though dubbed as purely man made, is a natural process where rain mixes with carbon dioxide, nitrous oxide and sulfuric dioxide in the atmosphere and also the draining water mixes with acids from decaying organic matter. Our contribution to this with our environmentally unfriendly and quite frankly uneccessary processes disrupt nature. The draining water and sometimes acidic water (acid degradation) will over long periods of time dissolve the calcium carbonate peeling away to show the sediment layers and enlarge cracks to form karst lansdscapes, caves and gorges etc...

Karst Landscapes (Limestone Bedrock)

As limestone is a soft soluble rock that will slowly erode through years of weathering to form Karst landscapes. Underground streams, sinkholes and caves are typical examples of what can be found in a Karst landscape.

Malham Cove Karst Platform

Karst Landscape - Malham Cove

Half a mile north of the village of Malham, in the Yorkshire Dales National Park, a large limestone formation 'Malham Cove' features a curved cliff and karst platform. it is presumed to have been formed by glacier meltwater at the end of the last ice age.


Limestone Precipitates

Dissolved calcium bicarbonate in the run off water from limestone can form stalactites as it drips from the ceiling of a cave. Some of the calcium bicarbonate precipitates back into limestone as it comes into contact with the air.

Mother Shipton's Petrifying Well

Petrifying Well - Mother Shipton's Cave

Mother Shipton's Cave in Knaresborough, North Yorkshire, has a what they call a 'petrifying well', where limestone water run off coats teddys and other random items. The precipitated calcium bicarbonate can cover a small teddy with limestone in as little a few months.

What is Limestone Used for

Limestone is used widely for a variety of purposes and include construction, agricultural, smelting, desulfurisation, glass making and calcium supplements.

Limestone Infrastructure

Architecture, escpecially megalithic and tartarian architecture, which still stands today and outlast our modern post 19 century buildings. Town halls, train stations, banks, libraries, churches, castles and notable buildings of our modern infrastructure were generally made totally or with a large quantity of limestone. Todays architecture uses powdered limestone and clay heated to form cement which is the main ingredient in mortar and concrete.

Calcium Carbonate Additives

Calcium Carbonate is widely used as an additive. In foods, calcium carbonate is added as a source of calcium, acid reducer, white food colouring, caking agent and to strengthen dough. The list of uses for calcium is almost endless and is also fed to livestock as a supplement, added to our tap water and used within most pharmaceutical products! The calcium carbonate additives are in the form of limestone dust which has been proven to cause mild respiratory issues, be a skin irritant and cause sillicosis or cancer. Excessive consumption can cause hypercalcemia and disgestive issues.

FACT: Nature has its own way of providing sufficient calcium within the foods we intake if produced organically, its little wonder why so many people suffer from illness, even if they are not as severe as those mentioned above. I suggest a shopping challenge for those who believe that the amount of calcium we ingest in our foods alone is negligible - simply check the ingredients of all the products you purchase!

CacO3 and Co2

As a major and widely used industrial raw material due to it economically viable mining. Demand for limestone remains extremely high and is now classified as a critical raw material. Concerns over its future availability has escalated over recent years forcing alternatives to be sourced. Obviously, economic viability has to be the largest contributing factor. It must also be noted that the processing of limestone for all consumption is a very large contributing factor to Co2 emissions.


Limestone Transformation into Marble

Modern day geologist and scientists according to their academic status, will provide a complex explanation for the processes earlier described in the formation of limestone, with an equally more complex theory for the transformation of limestone into marble. Taking into considering the classical elements and processses involved will help to understand how the rock cycle works, and I feel that this is important to cover this in brief before jumping straight to metamorphosis of limestone in to marble.

Influencing Classical Elements of Change

The change in states of minerals is akin to the classical elements influencing change. To consider the classical elements 'earth', 'fire', 'air' and 'water' and fifth element 'aether' is to understand the basic proccess of cycles and changes that we see in our physical world. Older and labelled as less advanced, past civilisations used these terms to explain the basics of our physical world with aether also being described as a natural phenomena of many different names. In respect of the Rock Cycle, various attributes contribute to the breaking down and formation of rocks of all types and directly correspond to these elements. These elements almost always work in combination with each other and include the processes required for life and its cycle.

Platonic Solids

Plato and possibly Pythagoras designated each element with a geometric shape known as the platonic solids, corresponding to the basic buidling blocks of our physical reality. The shapes were chosen according to their movability. Corealations are extensive and include colour, frequency, mathematics, chakras, Metatron Cube even substances under pressure will take on particular geometric properties. Despite fundamental and so many obvious corealations, the classical elements are not supported by modern science and a complex 'atomic theory' is used removing aether.

Fire

Heat from ambient to extremely hot conditions. A method of transmutation in combination with Earth and other elements.

Attributes: Thawing, Crystallization, Gassing, Melting, Convection

Platonic Solid: Tetrahedron - 4 faces, Plasma, Yellow. Dual - Inverted Tetrahedron

Platonic Solid Tetrahedron

Earth

The platform that we consider the physical world as we see it today, all the particles and elements that make up everything we see.

Attributes: Container, Medium, Order, Geometric matrix

Platonic Solid: Cube - 6 faces, Solid, Red. Dual - Octahedron

Platonic Solid Cube

Air

Atmospheric pressure providing a space for above and below and a medium for transportation of Earth and other elements.

Attributes: Oxidization, Wind, Abrasion, Erosion.

Platonic Solid: Octahedron - 8 faces, Gas, Green. Dual - Cube

Platonic Solid Octahedron

Aether

The unseen electrical transfer and frequency response in chemical reactions and contact between Earth and other elements.

Attributes: Charge, Frequency and Vibration (Cycles of time), Magnestism (attract and repel)

Platonic Solid: Dodecahedron - 12 faces, Universe, Purple. Dual - Icosahedron

Platonic Solid Dodecahedron

Water

Water is the carrier of information (minerals), four different states enter in to Earth to provide minerals and chemicals required for change.

Attributes: Freezing, Dissolving, Evaporation, Abrasion / Erosion, Absorption, Precipitation

Platonic Solid: Icosahedron - 20 faces, Orange. Dual - Dodecahedron

Platonic Solid Icosahedron

The Rock Cycle

A concept that explains the changing state of rocks in relation to its exposure to the elements over geological time. The three major rocks types will over time transform and recycle in an ongoing process called the rock cycle. All aspects of the elements will have an impact on the building and breaking down of rocks, including weathering and the movement of the earth through what we see as volcanos and earthquakes etc.. These events influence the chemical and biological (aether) changes required to alchemically transform one into another.

Igneous : From Magma - Molten rocks cooled and crystallised below or at the earths surface.

Sedimentary: Compaction and cementation of sediment from fragments of rock and organism remains

Metamorphic : A hard and more dense crystallised rock resulting from a process that transforms igneous or sedimentary rocks.


Metamorphism and Conditions Required for Change

Conditions required for metamorphism to change sedimentray rock into Marble involve several factors, all of which involve enough pressure and heat to cause a partial melt or extreme heating of the parent rock (protolith). Too much heat or pressure and the rock will fully melt and return to magma restarting its journey on the rock cycle. The protolith can be igneous, sedimentary or an already metamorphosed rock. Metamorphism can be caused by compression from overlying rock, tectonic stress, extreme heating from magma, or alteration by fluids. The geothermal gradient is a term that describes the increase in temperature the deeper you go inside the earth, if sediment layers are deep enough they will experience temperatures high enough for metamorphism. Tectonic movements can also force rocks into the earth deep within the geothermal gradient. Stress and pressure can be exerted on the rock in several ways, the stresses that the rock has undergone can be seen in the formations in most rocky outcrops and mountainous regions.

The three main ways that metamorphic rocks can form are Contact, Regional, and Dynamic...

Contact Metamorphism
Occurs when heat from magma at an igneous intrusion comes in contact with a body of rock (aureole). fluids and gasses from the magma and its surrounding environment begin to intract with the existing minerals. Contact metamorphism may contribute to the intense crystal bandings that are found in mountains.

Regional Metamorphism
Affecting a large area in mountainous regions, extreme pressure or compression (differential stress) from the subduction and uplifting of continental crust. Most likely to cause Lithostatic Pressure, where source is from the weight of rocks above that causes equal pressure exerted from all directions and uniformly is Lithostatic pressure.

Dynamic Metamorphism
Similar to regional metamorphism, however, the huge sudden or constant forces of heat and pressure will cause the rocks to partially melt enabling them to be bent, folded or sheared. Uneven pressure (differential) either stretches as it is being compressed and pushes in a single direction or sheered from two opposing forces and can lead to foliation which is seen in rocks with flat layers.


Recrystallization

Recrystallization is a process that occurs When sedimentary rocks limestone, chalk and dolomite are exposed to high temperatures and pressures, often as a result of crustal movements, they will go through a process of metamorphism in which the calcite crystals in the rock recrystalise. The recrystallization of calcite in these conditions makes them grow larger, forming a dense and interlocking lattice of equigranular calcite crystals. The original sedimentatry structures (grains and micro skeletal fossils) in limestone are dissipated by crystal expansion changing the texture and appearance of the rock. Limestone at the location of shifting continental plates will undergo 'contact metamoprhism' in which the hot magma sparks intense crystal growth. Other minerals in the rock such as clay and complex silicate structures will also metamorphose to produce micas, and gem minerals.

The mineral rich fluids (fluid phase) locked in the protolith play a key role in the chemical reactions that transform the mineral content and textures and may even transform the protolith so much that its original state is difficult to determine. The fluids will determine the chemical reactions that take place and rate of mineral crystal growth and deformation. Ocean water can penetrate cracked oceanic crusts and faults and circulate crustal areas including magmas transporting hot hydrothermal fluids loaded with minerals which is necessary for chemical reactions and may even cause metamorphism (hydrothermal metamorphism).


Metamorphic Rock Marble

Where does marble get its colours?

Pure white marble has a crystalline and sugary appearance and formed from very pure and low in silicate limestone or dolomite (low magnesium interruption). Other minerals including quartz, pyrite, graphite, iron oxides and clay incorporated with Calcite during metamorphism is what gives marble its wide varitey of colours, veins and swirls. Marble that contains impurities such as clay minerals, iron oxides can be bluish, gray, pink and yellow. Green, brown coloration is often due magnesium-rich limestone or dolomite forming the mineral serpentine.

Mistaken as Marble

Dense, massive limestone is sometimes described as "marble". For example, the famous Portoro "marble" of Italy is actually a dense black limestone. As quartzite has similar appearances, some marbles are mistaken as quartzite and can lead to very dissapointing results as marble is much softer that quartzite. Because travertine, serpentine and breccia can be brought to a high polish and are also a crystalline carbonate based sedimentary rock, they are sometimes reffered misleadingly and mistaken as marble.

Marble Deposits

Millions of tonnes of marble is mined and quarried every year from deposits hundreds of feet thick, mainly in mountainous regions. Even though marble is Found all over the world, Italy, China, Spain, and India account for 50% of its production. Ireland’s wild Atlantic coast has an exclusive and rare marble deposit 'Connemara marble'. Unique and striking with green of many hues in varying layers and patterns.

Marble Construction and Sculpture

Marble is crushed or cut into blocks or slabs as dimension stone and used for a variety of purposes in the construction industry. In its crushed form its used as aggregate, or in the block and slab form used for monuments, sculptures and decor such as floor tiles, worktops, windowsills. Carrara Marble from Carrara in Italy is a famous marble and was used by Michelangelo, Donatello, and Canova for their masterpiece sculptures.