THINGS YOU SHOULD KNOW ABOUT ROCKS

Earth’s crust is composed of rocks.

• Rock is a naturally occurring and coherent aggregate of one or more minerals.
• To geologists, a rock is a natural substance composed of solid crystals of different minerals that have been fused together into a solid lump.
• A Rock, by a simple definition, is a solid with more than one component of a mineral or mineraloid. A single crystal is not a rock; but two crystals that are joined together, even if they are the same mineral, are technically a rock. The minerals or mineraloids may be large enough to be easily identified (such as in a pegmatitic granite), barely distinctive grains (as in a schist), or in a mixture of microscopic grains such as in a slate. A rock does not even need to have crystals but may be in the form of a non-crystalline solid state or glass, an amorphous mixture in which the chemicals are not crystallized into minerals, such as in obsidian. Generally rocks are considered to only be natural objects, but sometimes man-made substances are included as rocks.

The minerals may or may not have been formed at the same time. What matters is that natural processes glued them all together.

Such aggregates constitute the basic unit of which the solid Earth is comprised and typically form recognizable and mappable volumes. Rocks do not have definite composition of mineral constituents. However, feldspar and quartz are the most common minerals found in rocks. Petrology is science of rocks. It is a branch of geology. A petrologist studies rocks in all aspects – composition, texture, structure , origin, occurrence, alternation and relationship with other rocks.

 

Important features to look for when writing descriptions and identifying rocks are:

• Texture: Refers to the shape, arrangement and distribution of minerals or grains / clasts within the rock – the texture in a geological sense does not refer to the roughness of the surface of the rock;
• Structure: Refers to broader features of a rock which may extend beyond the hand specimen into the outcrop; examples are bedding (in sedimentary rocks), foliation (in metamorphic rocks);

Note: Texture and structure, collectively referred to as fabric, are of primary importance in determining which major rock group a particular rock specimen belongs to.

• Grain size: Refers to the size of individual mineral crystals or clasts (pieces of pre-existing rock) in a rock. Grain size is useful for determining various rock types within the three major rock groups;
• Mineralogy: Refers to the minerals present within the rock, and also their relative proportions (especially important in the case of igneous rocks);
• Other features: Features such as colour, hardness / strength and specific gravity can also provide useful information, and should be included in a complete petrographic description. If a rock appears to be weathered this is an important feature to note as weathering can change physical characteristics markedly. The size of the sample should also be noted.

 

Some Rock-Forming Minerals

1. Feldspar: Half the crust is composed of feldspar. It has a light colour and its main constituents are silicon, oxygen, sodium, potassium, calcium, aluminium.
2. Quartz: It has two elements, silicon and oxygen. It has a hexagonal crystalline structure. It is uncleavaged, white or colorless. It cracks like glass and is present in sand and granite. It is used in manufacture of radio and radar.
3. Bauxite: A hydrous oxide of aluminium, it is the Ore of aluminium. It is non-crystalline and occurs in small pellets.
4. Cinnabar: It is mercury sulphide and mercury is derived from it. It has a brownish colour.
5. Dolomite: A double carbonate of calcium and magnesium. It is used in cement and iron and steel industries. It is white in colour.
6. Gypsum: It is hydrous calcium sulphate and is used in cement, fertilizer and chemical industries.
7. Haematite: It is a red ore of iron.
8. Magnetite: It is the black ore (or iron oxide) of iron.

There are three main classes of Rocks. They are classified according to how they originated.

1. Igneous Rocks

Igneous rocks, which have solidified from molten material called magma, a molten mixture of rock-forming minerals and usually volatiles such as gases and steam.

Molten materials are found below the earth crust and are normally subjected to extreme pressure and temperatures – up to 1200° Celsius.

Since their constituent minerals are crystallized from molten material, igneous rocks are formed at high temperatures. They originate from processes deep within the Earth—typically at depths of about 50 to 200 kilometres (30 to 120 miles)—in the mid- to lower-crust or in the upper mantle. Igneous rocks are subdivided into two categories: intrusive (emplaced in the crust), and extrusive (extruded onto the surface of the land or ocean bottom), in which case the cooling molten material is called lava.

• Intrusive Igneous Rocks

Intrusive igneous rocks are formed when the magma cools off slowly under the earth’s crust and hardens into rocks. Gabbro and granite are examples of intrusive igneous rocks. Intrusive rocks are very hard in nature and are often coarse-grained.

• Extrusive Igneous Rocks

Extrusive igneous rocks are formed when molten magma spill over to the surface as a result of volcanic eruption. The magma on the surface (lava) cools faster on the surface to form igneous rocks that are fine grained. Examples of such kind of rocks include pumice, basalt, or obsidian.

When lava cools very quickly, no crystals form and the rock looks shiny and glass-like. Sometimes gas bubbles are trapped in the rock during the cooling process, leaving tiny holes and spaces in the rock.

Typically, granite makes up large parts of all the continents. The seafloor is formed of a dark lava called basalt, the most common volcanic rock.

There are more than 700 known types of igneous rocks and most of them are formed under the earth’s crust since volcanic events are not all that frequent. On this basis, we are going to look at the commonly identified types of igneous rocks, both intrusive and extrusive. They include:

• Granite

Granites are the light-colored and coarse-grained igneous rocks. They are intrusive rocks and they contain three major minerals including feldspar, mica and quartz. They appear pinkish, gray or tan depending on the grain sizes and concentrations and grain sizes of the three minerals. Granites are often used in construction activities because of its strength and presence in great quantities.

• Gabbro

Gabbros are the dark-colored and coarse-grained igneous rocks. They are intrusive rocks and they are made up of mineral elements including pyroxene, feldspar, and sometimes olivine. They appear grey in color with minute dark spots all over them.

• Basalt

Basalts are the dense dark-grey colored and fine-grained igneous rocks. They are extrusive and are chiefly composed of plagioclase and pyroxene. They are the commonest type of solidified lava. Basalts are also frequently used in building and construction.

• Diorite

Diorites are the coarse-grained igneous rocks just like the Gabbros and Granite. They are intrusive and contain a mixture of minerals including hornblende, pyroxene, feldspar and sometimes quartz. They appear light-colored with some dark spots.

• Andesite

Andesites are light grey-colored and fine-grained igneous rocks. They are extrusive rocks which are mainly made up of plagioclase minerals mixed up with biotite, pyroxene, and hornblende.

• Obsidian

Obsidians are the dense dark-colored and smooth igneous rocks. They are the granitic or volcanic glass formed by the rapid cooling of lava without crystallization. Obsidians usually appear dark, but transparent in thin pieces.

• Pegmatite

Pegmatite is a form of igneous rock consisting of extremely coarse granite resulting from the crystallization of magma rich in rare elements. Pegmatite is light-colored and often contains rare minerals that are not present in other parts of the magma chamber.

• Pumice

Pumice is a light-colored vesicular and porous like igneous rock that forms as a result of very fast solidification of molten rock material. The vesicular and porous like texture is due to the gas trapped within the melt during the rapid solidification. Pumice stones are commonly used as abrasive materials.

• Peridotite

Peridotite is a dark coarse-grained igneous rock consisting principally of olivine. It is an intrusive rock and may also contain some small amount of minerals such as feldspar, quartz, amphibole, pyroxene, or quartz.

• Rhyolite

Rhyolites are light-colored and fine-grained igneous rocks. They are extrusive igneous rocks and they are typically composed feldspar and quartz minerals. They often have a smooth surface.

• Welded Tuff

Welded Tuffs are extrusive igneous rocks made up of the materials discharged during a volcanic event. The discharged materials fall on the earth surface after which they lithify into rocks. Welded Tuffs are mostly composed of volcanic ash and at times include large size pieces such as clinkers.

• Fire Opal

Fire opals are the igneous rocks that are flaming orange or yellow and red in color. At times they are found in rhyolite and are categorized as one of the most spectacular igneous geological rocks. They are formed long after the cooling of rhyolite whereby silica-rich ground water moves into the rocks and sometimes deposit gems such as topaz, agate, opal, jasper, or red beryl in the rock’s cavities.

• Scoria

Scoria is a dark-colored vesicular and porous like igneous rock. It is an extrusive rock and the vesicles or porous like composition is due to the gas trapped within the melt during cooling and solidification. Scoria usually forms as a bubbling crust on top of the lava as it flows down from the volcanic vent and eventually solidifies with some air trapped inside.

 

Based on the presence of acid forming radical, silicon, igneous rocks are divided into Acid Rocks and Basic Rocks.

• Acid Rocks

These are characterized by high content of silica—up to 80 per cent, while the rest is divided among aluminium, alkalis, magnesium, iron oxide, lime etc.. These rocks constitute the sial portion of the crust. Due to the excess of silicon, acidic magma cools fast and it does not flow and spread far away. High mountains are formed of this type of rock. These rocks have a lesser content of heavier minerals like iron and magnesium and normally contain quartz and feldspar. Acid rocks are hard, compact, massive and resistant to weathering.

Basic Rocks

These rocks are poor in silica (about 40 per cent); magnesia content is up to 40 per cent and the remaining 40 per cent is spread over iron oxide, lime, aluminium, alkalis, potassium etc.

Due to low silica content, the parent material of such rocks cools slowly and thus, flows and spreads far away. This flow and cooling gives rise to plateaus. Presence of heavy elements imparts to these rocks a dark colour. Basalt is a typical example, others being gabbro and dolerite. Not being very hard, these rocks are weathered relatively easily.

 

Economic Significance of Igneous Rocks

• Since magma is the chief source of metal ores, many of them are associated with igneous rocks.
• The minerals of great economic value found in igneous rocks are magnetic iron, nickel, copper, lead, zinc, chromite, manganese, gold, diamond and platinum.
• Amygdales are almond-shaped bubbles formed in basalt due to escape of gases and are filled with minerals.
• The old rocks of the great Indian peninsula are rich in these crystallised minerals or metals.
• Many igneous rocks like granite are used as building material as they come in beautiful shades.

 

2. Sedimentary Rocks

Sedimentary rocks are called that because they were once sediment. Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently naturally transported (or not).

All rocks of earth are exposed to actions of denudational agents and are broken in various sizes of fragments. Such fragments are transported by different exogenous agencies and deposited.

The compaction effect due to the weight of the piling layers of materials reduces the porosity of the rocks formed and intensifies the cohesion between the grains. At times, fossil fuels and organic matter may settle within the sediments leading to cementation. Cementation is the gluing of the rock pieces together either by salt compounds or organic matter. When these materials eventually harden, the mixture is transformed into a rock.

These deposits through compaction turn into rocks. This process is called “Lithification“. This is the process of formation of sedimentary rocks.

Some retain their characteristics even after Lithification like sandstone, shale, etc. Therefore, we can see various layers of different thickness in these sedimentary rocks.

Sedimentary rocks are called secondary, because they are often the result of the accumulation of small pieces broken off of pre-existing rocks.

Sedimentary rocks are of three basic types. These include clastic, chemical, and organic sedimentary rocks.

A. Clastic Sedimentary Rocks

Clastic sedimentary rocks are formed from the buildup of clatics: small pieces of fragmented rocks deposited as a result of mechanical weathering then lithified by compaction and cementation. Examples of Clastic sedimentary rocks include sandstone, shale, siltstone, and breccias.

• Conglomerate

A rock made from cemented gravel. A sedimentary rock with a variable hardness, consisted of rounded or angular rock or mineral fragments cemented by silica, lime, iron oxide, etc. Usually found in mostly thick, crudely stratified layers. Used in the construction industry.

• Sandstone

Sandstones are clastic sedimentary rocks made up of cemented sand grains. Sandstones vary from fine-grained to coarse grained are readily distinguishable by the naked eyes. Mature sandstones or quartz sandstones are light-colored and majorly consist of rounded and well-sorted quartz grains. Graywackes or immature sandstones consist of angular grains of diverse minerals. Sandstones are generally white, red, gray, pink, black, or brown in color.

• Shale

Shale consists of clay minerals or clay-sized pieces that have been compacted by the weight of the overlying rock materials. Shale belongs to clastic sedimentary rocks and they tend to split into fairly flat pieces. Shales are of many colors including gray, red, brown, or black depending on their composition of iron oxides and organic materials. They are generally a good source of fossils and are mostly found at the bottom of lakes or oceans.

• Siltstone

Siltstones are composed of small-sized rock particles which are finer than sand grains but coarser than clay. It is among the clastic sedimentary rocks which are the most difficult to identify since it appears almost similar to fine-grained sandstone or a coarse shale. They normally occur in a wide variety of colors.

• Breccia

Brecia are clastic sedimentary rocks made up of angular rock broken parts that are cemented together. The angular shape means that the broken parts haven’t traveled far from their pre-existing materials. The broken pieces are similar to conglomerate because of their large pea-sizes. Breccias are commonly found along fault zones and they take any color.

 

B. Chemical Sedimentary Rocks

Chemical sedimentary rocks are formed when the water components evaporate, leaving dissolved minerals behind. Sedimentary rocks of these kinds are very common in arid lands such as the deposits of salts and gypsum. Examples include rock salt, dolomites, flint, iron ore, chert, and some limestone.

• Rock salt

Rock salt is chemical sedimentary rocks often made up of the mineral sodium chloride. Salt is colorless or white and might be colored when mixed with impurities such as clay or iron oxide. It is easily identified by its salty taste and it is also water soluble. It also has the mineral name ‘halite.’

• Dolomite

Dolomites are chemical sedimentary rocks that almost resemble calcite. The similarity is because dolomite at first begins to take shape as limestone but they are later chemically altered through the substitution of some of its calcium by magnesium.

• Chert

Cherts are chemical sedimentary rocks formed due to the deposition of cryptocrystalline quartz. Cherts are of dull brown or gray in color and are often found as nodules firmly enclosed in limestone which protrude out of the limestone when the limestone is slowly immersed in water. Jasper is red, bright yellowish brown or reddish brown chert. Flint on the other hand is chert with a waxy luster.

• Limestone

Limestones are chemical sedimentary rocks made up of the mineral calcite. They may be hard to visually identify. Their colors vary from brown, dark gray, to light gray. The common types of limestone include fossiliferrous limestone rich in fossils, lithographic limestone that is very fine-grained, coquina limestone composed of broken shell fragments, encrinal limestone composed of crinoid fragments, and travertine deposited by the forces of moving surface water.

• Gypsum

Gypsum belongs to chemical sedimentary rocks. It is soft and can be easily bruised. It is usually white in color and is used to produce plaster of Paris.

 

C. Organic Sedimentary Rocks

Organic sedimentary rocks are formed from the accumulation of any animal or plant debris such as shells and bones. These plant and animal debris have calcium minerals in them that pile on the sea floor over time to form organic sedimentary rocks. Examples include rocks such as coal, some limestone, and some dolomites.

• Coal

Coal belongs to organic sedimentary rocks are made up from the buildup of decomposed plant material in a swampy environment. Coal is combustible in nature and is frequently extracted for use as fuel. It ranges from brown to black in color and its concentration depends on the compaction and alterations of the pre-existing organic materials. Examples of coal in order of the degree of compaction and alteration include peat, lignite, bituminous, and anthracite.

• Amber

Amber is an organic sedimentary rock and is naturally plastic and is light-weight compared to majority of the typical stones. Amber is simply a hardened tree sap and its colors ranges from transparent yellow to creamy yellow or red to dark brown.

 

Economic Significance of Sedimentary Rocks

• Sedimentary rocks are not as rich in minerals of economic value as the igneous rocks.
• But important minerals such as hematite iron ore, phosphates, building stones, coals, petroleum and material used in cement industry are found.
• The decay of tiny marine organisms yields petroleum. Petroleum occurs in suitable structures only.
• Important minerals like bauxite, manganese, tin are derived from other rocks but are found in gravels and sands carried by water. Sedimentary rocks also yield some of the richest soils.

 

3. Metamorphic rocks

The word metamorphic get their name from “meta” (change) and “morph” (form).

Metamorphic rocks are formed under the surface of the earth from the metamorphosis (change) that occurs due to intense heat and pressure (squeezing). The rocks that result from these processes often have ribbonlike layers and may have shiny crystals, formed by minerals growing slowly over time, on their surface. The recrystallization that takes place does so essentially in the solid state, rather than by complete remelting, and can be aided by ductile deformation and the presence of interstitial fluids such as water.

Any rock can become a metamorphic rock. All that is required is for the rock to be moved into an environment in which the minerals which make up the rock become unstable and out of equilibrium with the new environmental conditions. In most cases, this involves burial which leads to a rise in temperature and pressure. The metamorphic changes in the minerals always move in a direction designed to restore equilibrium. Earth movements can cause rocks to be deeply buried or squeezed.

In turn, the movements subject other rocks to extreme pressure and heat which contributes to changes and assemblage of some minerals. The changes typically modify the rock’s crystal type and sizes and may also subject the rocks to further radical changes. Metamorphic processes come about at heats between 150° and 795° Celsius. The changes can be chemical (compositional) and physical (textural) in character.

Heat from magma and the heat from friction along fault lines is the major contributor of heat that brings about the rock changes. Even though the rocks do not actually melt, some mineral groupings redistribute the elements within the original minerals to form new forms of minerals that are more stable at the new temperatures and pressures. As a result, the original rocks are transformed into metamorphic rocks.

Over time, because of the movement of the crust, these metamorphic rocks are pushed to the surface where you can find them every day.

Metamorphism occurs when rocks are forced down to lower levels by tectonic processes or when molten magma rising through the crust comes in contact with the crustal rocks. Metamorphism is a process by which already consolidated rocks undergo recrystallization and reorganization of materials within original rocks.

In the process of metamorphism in some rocks grains or minerals get arranged in layers or lines. Such an arrangement is called foliation or lineation.

They can be divided into many categories, but they are typically split into:

• Foliated metamorphic rocks; where pressure squeezes or elongates the crystals and they have a clear preferential allignment.
• Non-foliated metamorphic rocks; where the crystals have no preferential allignment. Some rocks, such as limestone are made of minerals that simply don’t elongate, no matter how much stress you apply.

Banding: When minerals and materials of different groups are arranged into alternating thin to thick layers appearing in light and dark shades, they are called structures with banding, and rocks displaying banding are obviously called banded rocks.

Causes of Metamorphism

• Orogenic (Mountain Building) Movements

Such movements often take place with interplay of folding, warping, crumpling and high temperatures. These processes give existing rocks a new appearance. Lava Inflow The molten magmatic material inside the earth’s crust brings the surrounding rocks under the influence of intense temperature pressure and causes changes in them.

• Geodynamic Forces

The omnipresent geodynamic forces such as plate tectonics also play an important role in metamorphism.

 

On the basis of the agency of metamorphism, metamorphic rocks can be of two types:

• Thermal Metamorphism

The change of form or re-crystallisation of minerals of sedimentary and igneous rocks under the influence of high temperatures is known as thermal metamorphism. There may be various sources of the high temperatures—hot magma, hot gases, vapours and liquids, geothermal heat etc.

A magmatic intrusion causing thermal metamorphism is responsible for the peak of Mt. Everest consisting of metamorphosed limestone. As a result of thermal metamorphism, sandstone changes into quartzite and limestone into marble.

• Dynamic Metamorphism

This refers to the formation of metamorphic rocks under the stress of pressure. Sometimes high pressure is accompanied by high temperatures and the action of chemically charged water.

The combination of directed pressure and heat is very powerful in producing metamorphism because it leads to more or less complete recrystallisation of rocks and the production of new structures. This is known as dynamothermal metamorphism. Under high pressure, granite is converted into gneiss; clay and shale are transformed into schist.

 

The most common metamorphic rocks are:

• SCHIST: A metamorphic uneven-granular, medium to coarse grained, crystalline with prominent parallel mineral orientation. Goes from silvery white to all shades of gray with yellow to brown tones depending on the mineral concentration. Some schists have graphite and some are used as building stones.

 

• GNEISS: A metamorphic uneven granular medium to coarse grained crystalline with more or less parallel mineral orientation. Colors are too variable to be of diagnostic value. Due to physical and chemical similarity between many gneisses and plutonic igneous rocks some are used as building stones and other structural purposes.

 

• QUARTZITE: A metamorphic or sedimentary rock with crystalline texture, consists of rounded quartz grains cemented by crystalline quartz, generally white, light gray or yellow to brown. Same uses as sandstone.

 

• MARBLE: A metamorphic even-granular grain to medium grained and may be uneven granular and coarse grained in calc-silicate rock. The normal color is white but accessory minerals act as coloring agents and may produce a variety of colors. Depending upon its purity, texture, color and marbled pattern it is quarried for use as dimension stone for statuary, architectural and ornamental purposes. Dolomite rich marble may be a source for magnesium and is used as an ingredient in the manufacture of refracting materials.

 

Rock cycle

A rock can begin as one type and can change many times. In fact, rocks are always changing. However, the changes happen so slowly that they are difficult to see. We have seen above that heat and pressure can change rocks, which then break down by weathering and move by erosion. It can take thousands of years for rocks to weather and erode. This process of change is called the rock cycle.

The rock cycle simply moves from the igneous to metamorphic to sedimentary rocks and the process repeats itself over and over. Igneous rock can change into sedimentary rock or into metamorphic rock. Sedimentary rock can change into metamorphic rock or into igneous rock. Metamorphic rock can change into igneous or sedimentary rock. However, the process takes thousands to millions of years. The process begins when rocks thrust upwards by tectonic forces, and eroded by wind or water.

This processes of erosion by wind and water is referred to as erosion and transportation weathering. The rocks ejected from beneath the earth by the tectonic forces are known as igneous rocks. The rocks that are eroded are carried by wind or moving water until they deposited in other regions where they settle in layers. Here, they are squeezed subjected to pressure by the overlying weight to form sedimentary rocks. This process is termed as compaction and cementation.

Rocks and debris continue to pile in layers until pressure and heat changes the underlying layers to form metamorphic rocks. What’s more, eroded rocks may continue to press and squeeze into sedimentary rocks as the transformations due to pressure and heat take place. Due to plate tectonic activities, the rocks on the surface sink down the earth and are buried where they melt because of the heated molten materials.

This process is known as subduction and burial. When these materials rise to the higher surfaces above the molten materials or rejected through the same plate tectonic processes, they cool down and recrystallize into igneous rocks. This process is termed as cooling and crystallization. So, the pushing up and formations of igneous, sedimentary, and metamorphic rocks by the tectonic forces describes the rock cycle.

 

What are the uses of rocks?

1. COAL: A sedimentary rock, formed from decayed plants, is mainly used in power plants to make electricity.
2. LIMESTONE: A sedimentary rock, it is used mainly in the manufacture of cement, the production of lime, manufacture of paper, petrochemicals, insecticides, linoleum, fiberglass, glass, carpet backing and as the coating on many types of chewing gum.
3. SHALE: A sedimentary rock, well stratified in thin beds. It splits unevenly more or less parallel to bedding plane and may contain fossils. It can be a component of bricks and cement.
4. CONGLOMERATE: A sedimentary rock with a variable hardness, consisted of rounded or angular rock or mineral fragments cemented by silica, lime, iron oxide, etc. Usually found in mostly thick, crudely stratified layers. Used in the construction industry.
5. SANDSTONE: A sedimentary rock more or less rounded. Generally thick-bedded, varicolored, rough feel due to uneven surface produced by breaking around the grains. Used principally for construction, it is easy to work, the red-brown sandstone of Triassic age, better known as “brownstone,” has been used in many eastern cities.
6. GRANITE: An igneous-plutonic rock, medium to coarse-grained that is high in silica, potassium, sodium and quartz but low in calcium, iron and magnesium. It is widely used for architectural construction, ornamental stone and monuments.
7. PUMICE: An igneous-volcanic rock, it is a porous, brittle variety of rhyolite and is light enough to float. It is formed when magma of granite composition erupts at the earth’s surface or intrudes the crust at shallow depths. It is used as an abrasive material in hand soaps, emery boards, etc.
8. GABBRO: An igneous-plutonic rock, generally massive, but may exhibit a layered structure produced by successive layers of different mineral composition. It is widely used as crushed stone for concrete aggregate, road metal, railroad ballast, etc. Smaller quantities are cut and polished for dimension stone (called black granite).
9. BASALT: An igneous volcanic rock, dark gray to black, it is the volcanic equivalent of plutonic gabbro and is rich in ferromagnesian minerals. Basalt can be used in aggregate.
10. SCHIST: A metamorphic uneven-granular, medium to coarse grained, crystalline with prominent parallel mineral orientation. Goes from silvery white to all shades of gray with yellow to brown tones depending on the mineral concentration. Some schists have graphite and some are used as building stones.
11. GNEISS: A metamorphic uneven granular medium to coarse grained crystalline with more or less parallel mineral orientation. Colors are too variable to be of diagnostic value. Due to physical and chemical similarity between many gneisses and plutonic igneous rocks some are used as building stones and other structural purposes.
12. QUARTZITE: A metamorphic or sedimentary rock with crystalline texture, consists of rounded quartz grains cemented by crystalline quartz, generally white, light gray or yellow to brown. Same uses as sandstone.
13. MARBLE: A metamorphic even-granular grain to medium grained and may be uneven granular and coarse grained in calc-silicate rock. The normal color is white but accessory minerals act as coloring agents and may produce a variety of colors. Depending upon its purity, texture, color and marbled pattern it is quarried for use as dimension stone for statuary, architectural and ornamental purposes. Dolomite rich marble may be a source for magnesium and is used as an ingredient in the manufacture of refracting materials.