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Limestones

These are the most common sedimentary rocks from the non-clastic-group and are composed chiefly of carbonate of calcium with subordinate proportions of carbonate of magnesium.

LIMESTONES

 

Definition

 

     These are the most common sedimentary rocks from the non-clastic-group and are composed chiefly of carbonate of calcium with subordinate proportions of carbonate of magnesium.

 

       They are formed both bio-chemically and mechanically.

 

Composition

       In  terms     of  chemical  composition,  limestone' s  are  chiefly  made  up  of CaO  and          CO2, Magnesium Oxide is a common impurity in most limestone' s; in some its percentage may    exceed 2 percent, the rock is then called magnesian limestone.

Other oxides that may be  present in limestone are: silicon dioxide, ferrous and     ferric oxides  (or  carbonates);  and  aluminium  oxide.  Strontium  oxide  is  also     present in some. limestone' s as a trace element.

Texture.

 

     The most important textural feature of limestone' s is their fossiliferous nature.

 

     Fossils in all stages of preservation may be found occurring in limestone' s.

 

        Other varieties of limestone' s show dense and compact texture; some may be

 

loosely packed and highly porous; others may be compact and homogeneous. Concretionary texture is also common in limestone' s. .

 

Types.

 

     Many varieties of limestone' s are known.

 

            Broadly speaking these can be divided into two groups: autochthonous and allochthonous.

 

     Autochthonous includes those varieties which have been formed by biogenic precipitation from seawaters.

 

     Allochthonous types are formed from the precipitated calcareous sediments that have been transported from one place to another where they were finally

deposited.

 

Following are common types of limestones.

Chalk.

 

It is the purest form of limestone characterised by fine-grained earthy textureCommon colour of chalk is white. Some chalks may be exceptionally rich in the remains of very small sea organisms called

foraminifera.

 

Shelly Limestone.

 

   Also

called  fossiliferous  limestone,  it  has  a  rich  assemblage  of  fossils  that  are

fully

or   partly preserved. When the limestone is made up entirely of fossils, it is

termed coquina.

 

Argillaceous Limestone

 

These limestones contain clay as a significant constituent and are clearly of allochthonous origin.

 

When the clay and carbonate factions are present in almost equal proportions, the rock is termed marl.

 

Lithographic Limestones

 

These are compact massive homogeneous varieties of pure limestones that find extensive use in litho- printing.

Kankar

 

     It is a common nodular or concretionary form of carbonate material formed by evaporation of subsoil water rich in calcium carbonate just near the soil surface.

     It is non-marine in origin.

 

Calc-Sinter.

  It is a carbonate deposit formed by precipitation from carbonate rich spring waters.

These deposits are also known as travertine or calc-tuffa and commonly occur around margins of Hot

 

Springs.

 

DOLOMITE

Definition.

 

     It is a carbonate rock of sedimentary origin and is made up chiefly more than 50 percent - of the mineral dolomite which is a double carbonate of calcium and magnesium with a formula of CaMg(C03h.

       Ferrous iron is present in small proportions in some varieties.

     Gypsum also makes appearance in some dolomites.

       But the chief associated carbonate is that of calcium, in the form of calcite.

 

Texture

 

          Dolomite  shows  textures  mostly  similar   to   limestones  to  which  it  is  very

 

often genetically related.

 

In other varieties, dolomites may be coarsely crystalline, finely crystalline or showing interlocking crystals.

Formation.

 

           Dolomites are formed in most cases from limestones by a simple process of replacement of Ca++

 

ions by Mg++ ions through the action of Mg++ ion rich waters.

       This ionic replacement process is often termed dolomitization

 

     The replacement may have started shortly after the deposition of limestone or quite subsequent to their compaction.

     Direct precipitation of dolomites from magnesium rich waters is also possible.

 

     Such directly precipitated deposits of magnesium carbonate occur in association with gypsum, anhydrite and calcite.

 

     It is believed that in such cases, it is the calcite, which is precipitated first, depleting the seawater of CaC03 and enriching it with MgC03.

     The CaMg(C03h precipitates at a later stage.

 

           Dolomitization  by replacement  method,  however,  is  believed  to  be  the  most

 

common method of formation of dolomites.

 

Occurrence

      Dolomite is a widespread sedimentary rock and is found commonly associated with

.limestones.

     It forms intervening layers between limestone formations spread over wide areas.

     Also, it may occur at the extended boundaries of many limestone deposits.

 

      These indicate locations where' magnesium rich ground waters could have an easy

 

access for the replacement process to take place in an original limestone" rock.

 

Dolomite is so closely related to limestone in composition, texture, structure and physical properties that it may not always be easily possible to differentiate between the two rocks in hand specimens.

 

7. Coals

Definition.

 

     These may broadly be defined as metamorphosed sedimentary rocks of carbonaceous character in which the raw material has mostly been supplied by plants of various groups.

 

        The original raw material passes through many biomechanical and biochemical

 

processes before it becomes a coal in technical terms;

Formation.

 

     In most cases coals represent carbonized wood.

 

     The process of coal formation involves a series of stages similar to formation of sedimentary rocks such as wastage of forests and transport of the wood material through different natural

agencies to places of deposition, accumulation of the material in huge formations.

 

     Its burial under clays and other matter and its compaction and consolidation under superimposed load.

 

     Biochemical transformation of the organic matter so accumulated starts and is completed under the influence of aerobic and anaerobic bacteria available at the place of deposition.

 

     The degree of carbonification depends to a great extent on the time and type of environment in which the above processes have operated on the source material giving rise to different varieties

of coal.

 

Types

Peat.

 

It is the lowest grade coal that consists of only slightly altered vegetable matter. It may not be even considered as a coal. It has very low calorific value, high percentage of moisture and is rich in volatile matter.

 

Lignite.

 

     It is also known as brown coal and forms the poorest grade of coal with calorific value ranging between 6300-8300 B.th.V.

          It is compact and massive in structure with an upper specific gravity of 1.5 and hardness of 2.5 on

 

Mohs' Scale of Hardness.

 

      Some varieties of lignite may still show to a good extent the traces of original vegetable structure.

 

Bituminous Coals

 

      These form a broad group of common coals having essential properties varying within wide limits.

 

         The fixed carbon ranges between 69-78 per cent and the calorific value between 9,500 B.th.V to

 

14,000 B.th.V.

 

     Their common character is that they contain enough volatile matter, which makes them quite soft on heating, and they start agglomerating.

Some of bituminous coals may contain volatile matter to such a high extent as 30 per cent of their bulk.

 

Anthracite

 

        It is considered the highest-grade coal with fixed carbon ranging between 92-98 per cent.

 

       It has highest calorific value in coals and burns almost without any smoke, as the

 

volatile matter is negligible.

 

Occurrence

 

     Coals of different varieties are found to occur almost in all countries of the world, though in varying proportions.

       Coals form all-purpose fuels, some varieties being more suitable for specific industrial uses.

 

8. Iron Ores of Sedimentary Origin

 

      The iron ores form beds or layers of variable thickness that occur interstratified with other sedimentary rocks.

 

      Sedimentary iron deposits are regarded having formed chiefly as chemical precipitates in the form of oxides, carbonates and silicates from marine waters rich in corresponding salts.

 

      Metasomatic replacement has also been suggested as another important process for formation of many iron ore deposits.

           It is also suggested that certain type of bacteria play considerable role in the precipitation of iron.

 

Gypsum

 

            It is a sedimentary rock composed of the mineral of the same name-gypsum, which has a composition of CaS04.2H2O.

 

            Its common colour is white but it may also occur in other shades such as yellow, red or dark grey due to impurities present in the rock.

                Gypsum is formed in nature as a result of evaporation from sea-waters rich in sulphate salts.

 

ANHYDRITE is a granular aggregate of mineral anhydrite, CaS04' and is genetically related to the mineral gypsum: hydration of anhydrite results in gypsum.

 

These rocks are commonly associated in occurrence

 

Uses: Gypsum finds extensive uses in many industries, e.g.

 

(i) as a raw material in the manufacture of fertilizers;

(ii) as an essential ingredient in the manufacture of Cement;

(iii) in the manufacture of Plaster of Paris.

 

(iv) as fire proofing component of gypsum boards.

 

10.           Rock Salt

     It is also a sedimentary rock composed of mineral halite (NaCl).

 

     The texture of rock salt varies from coarse-grained crystalline to fine-grained massive.

 

The purest rock salt is white in colour but it may occur in various other shades as grayish and reddish due to presence of impurities

 

           Rock salt occurs in many parts of the world interbedded with other sedimentary formations.

 

     It is commonly associated with other evaporites.

 

     Subsidence of the basin of deposition during the process of evaporation has been suggested by some as a possible explanation.

11.            Flint and Chert

 

     Flint is a dark coloured sedimentary rock of siliceous composition consisting chiefly of halcedony and extremely fine-grained quartz.

     It occurs commonly as concretions or nodules in chalk (limestone) deposits.

 

     Chert is also a sedimentary rock composed of cryptocrystalline silica showing great variety of colours.

      It is more common in occurrence compared to flint and occurs in the form of

 

beds or layers within limestones and other deposits.

 

Their origin may be due to any of following two causes: (a) Primary Precipitation.

 

     It is believed that under special environments chert gets precipitated inorganically from seawater rich in amorphous silica.

 

       The  theory  is  yet  considered  inadequate  because  modem  seawaters  are

 

generally quite under saturated with amorphous silica.

(b) Replacement.

 

     Waters containing amorphous silica from siliceous skeletal sources are thought to have replaced lime stones forming concretions and nodules of flint by the process of replacement.

 

12.           Tillite

     It is a sedimentary rock of glacial origin.

 

It is characterised by a structure less matrix that has fragments of various sizes, shapes and composition embedded in it.

 

     Most of these embedded fragments bear striations and other evidence of their transport by glaciers before their deposition and compaction.

 

     The name is derived from the fact the rock is merely a compacted and consolidated form of the glacial debris called till.

 

     The matrix or ground mass of the till is generally of grey to greenish appearance whereas the embedded fragments are of extremely heterogeneous character.

 

 

ENGINEERING IMPORTANCE

 

      Sedimentary rocks cover a great part of the crust of the earth; they make up more that 75 percent of the surface area of the land mass.

       It is with these types of rocks that an engineer has to deal with in majority of cases.

 

      It is, therefore, essential for a civil engineer to know as much as is possible about the salient features of these rocks.

He has to see, for instance, if such rocks would withstand loads under heavy construction and also, if they could be trusted in cuts and tunnels in highway construction and also as reservoirs.

 

They are the most important rocks to act as natural reservoirs of oil and ground water supplies.

 

 

 

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