Category: Geography Notes PPT

The Quaternary is a subdivision of geologic time. It is the Quaternary Period that covers almost the last 2.6 million years up to the present day of the earth. The Quaternary, as well as the Tertiary Periods both together, form the Cenozoic Era.

The quaternary meaning can be understood by the below-given image:

Quaternary Geography

The Quaternary is further subdivided into two epochs, the first is the Pleistocene which is up to about 11,700 years ago and the second is Holocene which is about 11,700 years ago to the present day. The Quaternary Period is one of the very extraordinary changes that took place in the global environment and also the period during which much of human evolution had taken place.

The Quaternary Period has also involved many dramatic climate changes in the planet, which has affected food resources and led to the extinction of many species. The period also saw the rise of a new predator that is known as man.

Climatic Conditions During the Quaternary Period 

Scientists from all over the world have evidence of more than 60 periods of glacial expansion that was interspersed with briefer intervals of warmer temperatures. The entire Quaternary Period which also includes the present is referred to as an ice age because of the presence of at least one permanent ice sheet i.e. Antarctica. However, studies say that the Pleistocene Epoch was much drier and also colder as compared to the present time.

The glacial advancement varied between continents,  but approximately 22,000 years ago, glaciers covered about 30 percent of the surface of the earth. In areas that are now Europe and North America, there existed huge grasslands known as the “mammoth steppes” and had a higher productivity rate with greater biomass than the modern grassland. The grasses were very dense and also highly nutritious. Whereas, winter snow cover was quite shallow.

Ascent of Man During the Quaternary Period

Homo erectus was the first-ever hominid species that widely used fire. There are two hypotheses about the species’ origin. The first hypothesis is that the species were initially originated in Africa and later dispersed throughout Eurasia, with the ability to exploit the colder regions using fire and tools. The second hypothesis claims that the Homo erectus migrated to Africa from the region of Eurasia. Excavations in Dmanisi, Republic of Georgia, have uncovered fossil evidence that Homo erectus species were successful hunters.

The existence of the Homo neanderthalensis was from about 200,000 years ago to about 30,000 years ago. Fossil evidence showed that the species lived in western Europe which also includes southern Great Britain, throughout central Europe and Ukraine, and as far south as Gibraltar and the Levant. However, Neanderthal fossils have not been found in Africa. Neanderthals were shorter as well as stockier than modern humans with longer, stronger hands and arms. They lived in shelters, made and wore clothing, and also used diverse tools made up of stone and bone.

According to the climatic conditions, there was a requirement for a heavy diet in animal protein so they were sophisticated hunters. A recent discovery indicates that they also cooked and ate plants. They buried their dead and made ornamental or symbolic objects. No earlier hominid species have been shown to practice any kind of behaviour that indicates the usage of language.

Evidence suggests that Homo sapiens originated in the continent of Africa and the oldest fossils of anatomically modern humans, found in Ethiopia which are approximately 195,000 years old. By 100,000 years ago they had dispersed as far north which is now known as modern Israel, but the oldest fossils of modern humans are also found farther north are only 40,000 to 60,000 years old. By this discovery, it is clear that Homo sapiens, as well as Homo neanderthalensis, were contemporaries for a time. Few dental evidence claims that H. sapiens matured later than Neanderthals. This suggests that a longer childhood led to more time for social development as well as transmission of knowledge and technology to new generations. 

This might have led to the division of labour allowing all the females as well as the young to forage for more diverse food sources. Diversification in diet could have been a species advantage for the Homo sapiens when the climate cooled again. The most recent Neanderthal remains are approximately 28,000 years old. Homo sapiens weathered the drastic climate changes and continued to disperse throughout the Earth whereas the Neanderthals became extinct.

Salinity means the saltiness or the amount of salt which is dissolved in a water body. The water body which has salt content in it is called the saline water body.

Salinity is considered to be an important factor that determines many aspects of the water. The chemistry of natural waters and that of the biological processes present within it, is a thermodynamic state which is variable along with the temperature and the pressure that governs the physical characteristics of water – density, heat capacity, and other characteristics. 

In this discussion, we will know more about the salinity distribution, the salinity of the ocean, also we will know about the salinity content in the red sea.  

Salinity Distribution

A discussion of salinity that means the salt content of the oceans requires an understanding of these important concepts: 

• The present-day oceans are considered to be in a state where it receives as much salt as they lose

• The salty oceans have been mixed with each other over such a long time period that their composition of sea salt is actually the same everywhere.  

This uniformity of salt content is the reason for the little variation of salinity over space or over the time period. The range of salinity that is being estimated is from 33 to 37 grams of salt per kg. Majorly, the observed departure of this mean value of approximately 35 PSU, is caused by the processes at the earth’s surface which locally add or remove the freshwater content in it. The regions which have high evaporation are of elevated surface salinities, while those regions where there is higher precipitation have depressed surface salinities limit within themselves. Near the shore the regions that are close to large freshwater sources, there the salinity may be lowered by the process of dilution. This is very true in those areas where the region of the ocean which receives the freshwater is isolated from the open ocean by the geographical land.

The Salinity of the Ocean 

Salt in the ocean originates two sources:

The rocks on the land surface are the major source of salts that are dissolved in seawater. The rainwater which falls on the land is a little acidic in concentration, so it erodes the rocks. This erosion of rocks releases the ions which are being carried away to the streams and to the rivers which eventually flow into the ocean. These dissolved ions are used by the organisms living in the ocean and thus they are removed from the water. While, others are not removed, so their concentrations increase over the time period.

There is another source of salts present in the ocean called hydrothermal fluids. This salt comes from the vents on the ocean floor. The ocean water seeps into these cracks in the seafloor and thereby it is heated by magma from the Earth’s core. This heat causes a series of chemical reactions. The water here tends to lose the oxygen, magnesium, and sulfate concentration and it picks up metals like iron, zinc, and copper from the surrounding rocks. The heated water is then released through the vents in the seafloor, thereby carrying the metals with it. The ocean salts which come from underwater volcanic eruptions, directly release the minerals into the ocean water.

Prevalent most ions present in the seawater – chloride, and sodium. Together these ions make up approximately 85 percent of all the dissolved ions in the ocean. While magnesium and sulfate make up for another 10 percent of the total. Other ions are found in very minute concentrations.

Red Sea Salinity 

The Red Sea is actually a sea-water inlet of the Indian Ocean which lies between the continents of Africa and Asia. 

Talking about this sea salinity, the Red Sea is known to be one of the saltiest bodies of water in the world, this owes to higher evaporation and lower precipitation rate, no significant rivers or streams drain into this sea. The southern connection to the Gulf of Aden is an arm of the Indian Ocean, which is quite narrow.

Vertical Distribution of Salinity 

1. The saltiness of ocean water changes with depth, but the way the salinity changes rely on the position of the sea.

2. Salinity which is present at the surface of the sea gets decreased by the input of fresh waters or it gets increased by the loss of water, as it forms ice, or by the process of evaporation.

3. Salinity present at the depth is fixed as neither water nor as salt which can be added to it.

4. There is a major difference in the salinity occurring between the surface zones and between the deep zones of the oceans.

5. The lower saline water saturates quite above the higher saline dense water.

6. The salinity usually increases with the depth and there is a distinct zone known as halocline, where the salinity increases too abruptly.

7. The increase in salinity of seawater causes an increase in the density of the water.

8. High salinity seawater, which usually, sinks below the lower salinity water leads to stratification by the amount of salinity.

What is the Salinity of Seawater?

The seawater salinity is about 35 parts per thousand. 35 parts per thousand are the average salinity which can be stated in another way, that is about 3.5 percent of the weight of the seawater comes from the dissolved salts. 

Sedimentation can be described as the tendency for the particles which are in suspension to settle out of the fluid content. Here they are entrained and then they come to rest against a specific barrier. This happens due to their own motion through the fluid which is in the response to all the forces that are acting on them. The forces can be because of the gravitational pull, due to the centrifugal caused by acceleration or electromagnetism.

[Image to be Added Soon]

In terms of geology, sedimentation is generally described as the opposite process of erosion that is the terminal end where the sediment transport. In this sense, it also includes the termination of the transport by the process of saltation or the true bedload transport.   

What is the Process of Sedimentation? 

Sedimentation is the process that allows the different particles which are in the suspended form in water to settle under the gravitational effect. The particles which settle out from the suspension become the sediment, while in water treatment this is known as sludge. When this thick sediment continues to settle, this is called consolidation. In consolidation, the sediment, or sludge, which is assisted by mechanical means is known as the process of thickening.

Sedimentation for water treatment might be used to reduce the concentration of the particles in suspension before the application of coagulation, in order to reduce the amount of coagulating which the chemicals need, or after the coagulation and, possibly after the flocculation. When the sedimentation is applied after coagulation, this purpose is done mainly to reduce the concentrated number of solids which are in suspension so that the subsequent filtration can function well.

Sedimentation is used as one of the other application methods prior to the process of filtration. While other methods include dissolving in air flotation and some other methods of filtration. The solids-liquid separation processes at times are referred to as clarification processes. While there are different processes of sedimentation like horizontal flow and others which we will discuss in our further section. 

Explain Sedimentation

The Method Involved

The simplest sedimentation method is to fill a jar or a tank with water, leave that jar or tank alone, undisturbed for a longer period of time. This will allow the particles to settle and then pour the resulting water into another can. This practice is rarely viable in treating the water for townships hence, sedimentation tanks are operated continuously.

Another simple method of sedimentation is done with the process of rectangular tanks which have a horizontal flow through them. The water with the particles which are in suspension is then introduced at one end of the tank, and then when the water flows to the other end of the tank the settlement of particles in the water occurs here. The aim is simply to settle the particles and to manage the reach of the tank floor before the water is drawn out of the tank at the end. This kind of horizontal flow tank is generally built with a floor that has slopes that are flowing gently down to the inlet end of a hopper. This tank is then fitted with a mechanism that helps to scrape the sediment from the outlet end back to the inlet end and then into the hopper from where the water can be discharged hydraulically. In this design of tanks, attention has to be given to the inlet and the outlet ends to make sure that the water flows from one end to another end as uniformly as possible.

Deposition Definition Science

[Image to be Added Soon]

A deposition is moreover a geological process where the sediments, soil, rocks get added to a landform or the landmass. The wind, ice, water, and gravity transport previously get weathered into the surface material, which in the kinetic form of energy in the fluid is being deposited and is built up in layers of sediment.  

Deposition occurs when the forces responsible for sediment transportation are no longer sufficient to overcome the forces of gravity and the process of friction. This creates a resistance to motion which is known as the null-point hypothesis. This deposition can also be referred to as the build-up of the sediment from the organic derivation of matter or via the chemical process. 

Types of Sedimentation Tanks 

Horizontal Flow Tanks

They are rectangular in shape which has more length twice their width. This structure is designed as they need to flow more the distance and then to settle all the suspended particles. 

Radial Flow Tanks

The section which is via a typical type of radial flow circular tank has the water entering this tank through the inlet pipe which is centrally located and is placed inside the deflector box.

Inclined Settling

The inclined setting device is designed to increase the settling capability of the basin. This design principle majorly utilizes the fact which the depth of a gravity settler has very little bearing on the settling capacity. These are of great importance for settling areas which are 140 square feet in each of the inclined plates.

Ballasted Sedimentation

In this type of sedimentation process, the density difference which occurs between the water and its particles produced in the water for the treatment by the coagulation, flocs, in general, is quite small. For this reason, they settle slowly. The methods of plain sedimentation (which are horizontal, radial, and inclined sedimentation) are then preceded by a slow mixing process known as flocculation.

The smog that is also referred to as smoke fog is intense air pollution and its name is derived in the 20th century from both fog and smoke as “smog” due to its nature of being opaque and for its typical odor. Smog is a mixture of nitrogen oxides, sulfur oxide, ozone, smoke, and other particulates. The kind of visible man-made pollution is derived from industrial emissions, coal combustion emissions, vehicle emissions, forest and agriculture fires along with photochemical reactions of these emissions.

Winter smog and summer smog are the two categories of smoke that are often discussed. Summer smog is often associated with the photochemical formation of ozone. In summertime photochemical smog is the dominant type of smoke that is formed because the temperature is warmer and there is more sunlight that is present. As the temperature is colder during the winter months and atmospheric inversion is common, the usage of coal and other fossil fuels becomes common to heat up houses and other commercial places.  Thus the winter smog formation is due to the excessive emissions due to the combustion of coal and other fossil fuels along with the lack of atmospheric pollutant dispersion under inversions.  

Both the primary and the secondary pollutants are the cause of the formation of smog. The primary pollutants are the pollutants that are emitted directly from the source like the emission of sulfur dioxide directly from the combustion of coal. The secondary pollutants such as ozone are formed when the primary pollutants after the emission from the source undergo a reaction in the atmosphere. 

Photochemical smog is a type of air pollution that is obtained from the admission that is generated from the combustion of the fuels in the engines and the industrial fumes. The pollutants give rise to secondary pollutants when they react in the atmosphere with sunlight and then combine with the primary emission to form photochemical smog. Since 2002 in cities like Delhi the smoke severity is aggravated by subtle burning in neighboring agricultural areas.

Most of this photochemical smog in many of the cities is increasing due to the inversion effect of the atmosphere that traps the pollution close to the ground. This topic, therefore, deals with what smog is, its types, cause, and effects.  

Types of Smog

There are two types of smog as the cause behind the formation of smog is different. The two classifications of smog are as follows:-

1. Photochemical Smog is also known as Los Angeles Smog: Photochemical smog definition is referred to as “summer smog” that is a type of air pollution developed from the admission that is generated from the combustion of the fuels in the engines and the industrial fumes.  Thus the photochemical smog formation happens with the chemical reaction of sunlight, nitrogen oxides, and volatile organic compounds in the atmosphere and leaves the air-born particles and ground-level ozone. It depends both on the primary pollutants and the secondary pollutants. Nitrogen oxides, particularly nitric oxide (NO) and nitrogen dioxide([NO_{2}]), and volatile organic compounds are considered as the primary pollutants. Whereas,  peroxyacetyl nitrates (PAN), tropospheric ozone, and aldehydes are considered secondary pollutants. 

In the morning due to the high rush hours, a very high concentration of hydrocarbon and nitric oxide emissions mostly by the vehicles on the roads and also a small percentage due to the industrial emissions. Some of this volatile hydrocarbon rapidly oxidizes the OH group atoms into peroxy radicals, which convert nitric oxide (NO) to nitrogen dioxide ([NO_{2}]). These two then further react with a series of chemical reactions.  

Photochemical smog effects on environmental factors as well as human beings are huge. When the hydrocarbons in the atmosphere combine with the chemicals that are contained within the photochemical smog they form molecules that can cause eye irritation. The atmospheric radical stops the ground-level ozone from being eliminated by interfering with the nitrogen cycle. This ground-level ozone is very toxic for human beings. The other negative effects that are associated with the formation of photochemical smog is shortness of breath as well as decreasing vision.

2. Sulfurous Smog is Also Known as London Smog- Sulfurous smog is also known as London smog because of a smog episode in London in the year 1952 to lead to the deaths of 4000 people. It is developed due to the high concentration of oxides of sulfur in the air. The emission of Sulfur dioxide by the natural source is estimated to be 24 persons whereas anthropogenic sources contribute to 76% of the emission of Sulfur dioxide. The fossil fuel sources that produce the sulfuric acid and a particular time lead to the choking mixture when it is incorporated into the droplets of fog. It is associated with actinic flux, high humidity as well as low temperature. During 1950 the legislation eliminated this form of air pollution due to the reduced emission of Sulfur dioxide and smoke before 1950 as it was considered a serious air pollutant that causes irritation to human eyes, nose, and lungs. Certain volcanoes also emit Sulfur dioxide in the atmosphere along with the fossil fuels like coal that bare sulfur.

Causes of Smog

The numerous sources such as factories, vehicles, and consumer products are the typical factors for the formation of pollutants that result in the development of smog. In the urban set up most of the smoke formation is the consequence of vehicle emission that sums up to 50% of smog. The relation between the weather patterns and heavy motor vehicle traffic, consumer products, and other industrial emissions is the main factor for the occurrence of smog. The consumer products include plastic packaging, paints, solvents, and sprays. Some of the main causes of the formation of smog are given below:-

1. The Use of Coal is Fuel- Coal that is used for heating or power-producing plants results in the emission of high concentrations of sulfur oxides in the atmosphere. Also due to the presence of the high level of suspended particulate matter in the atmosphere and dampness, the effects due to the conversion of the coal also worsened. A smoggy environment is developed as the coal generates a significant amount of smoke while burning.

2. Industrial and Vehicular Emissions- Fossil fuel combustion in cars, trucks, buses, motorbikes, and boats results in the emission from the transportation sector that becomes the chief contributor of smog. Most of the smoke formed in the urban setup is due to the emissions from the vehicles during peak traffic time. For the production of various materials and coals, the industrial process incorporates a large number of fossil fuels and resources that need to be extracted from nature. Therefore the industries also result in the emission of harmful gases and fumes that are released into the atmosphere which ultimately leads to the formation of smog.

3. Excessive Waste Production- Our excessive consumption results in the production of a large amount of waste. To get rid of this huge amount of waste much of it is burned which leads to the emission of harmful gasses in the atmosphere which later transforms into smog.

4. Fireworks- Though the use of fireworks is very limited and the occasions are very rare, a single night of firework display results in enormous air and particulate pollutants that leads to a significant amount of smog. This is associated with new year eves for Diwali where large amounts of fireworks are used. This results in the formation of a dense layer of smoke the very next day.

5. Burning the Agricultural Material- In many of the countries the burning of the old crops are the waste materials that are generated from the farming practices is still prevalent. This leads to the major forms of smog. But the farmers still do this practice because it is convenient to do so. In capitals like Delhi, most of the smoke formation is due to crop fires. Every year thousands of farmers who are residing in North Indian states like Haryana Punjab in Uttar Pradesh bless their rice crop fields to grow wheat. The conversion of the agricultural materials also results in the burning of substances like fertilizers, ammonia, and pesticides that release certain gasses in our atmosphere which in later stages turn into smog.

6. Activities in Construction- In the areas of high construction density, a large amount of dirt and dust particles enter into the air causing smoke that particularly orcas from construction activities.

7. Natural Causes- Due to volcanic eruption and some specific plant life effects smog is developed due to natural causes as well. A high concentration of Sulphur dioxide is released during volcanic eruptions, particularly in the air, which is one of the primary constituents of smog formation. The radiocarbon amount of some of the plant’s life also results in the formation of smog in some areas.

Understanding the stratigraphy meaning becomes quite simple when you get familiar with the stratigraphy principle of geology. Stratigraphy is a branch of geology that deals with the description of rock or interpretation of geologic time scale. It also renders insight into the geologic history of strata. As a geological discipline, stratigraphy takes into account the spatial location and temporal sequence of rock bodies. With the help of stratigraphy, bodies of rock are dated and interlinked with each other. Stratigraphic studies are mainly conducted to study sedimentary and volcanic layered rocks.

The Stratigraphic Principle

Stratigraphy organizes bodies of rock chronologically and spatially in accordance with their contained characteristics. It puts indirectly connected rock units together in a relationship. Stratigraphy is the substructure for remodelling the Earth’s history. It also plays a part in solving general geological questions.

The stratigraphic principle was initially introduced in 1669 by Nicolaus Steno in his documented work ‘Dissertations prodromus’. It has been founded and formulated on the foundation that, with a smooth sequence of sedimentary layers, the layer in the footwall (below) is older (matured) than the layer in the hanging wall (above).

Types of Stratigraphy

Following are the most important sub-disciplines in stratigraphy with their elements of study:

1. Lithostratigraphy (Lithostratigraphic unit)

2. Biostratigraphy (Zones)

3. Chronostratigraphy [(Chronostratigraphic units) {Age, Period, Epoch}]

4. Magnetostratigraphy (Reversals, chrons)

5. Sequence stratigraphy (Allostratigraphic units)

6. Pedostratigraphy – (Pedostratigraphic unit)

7. Geo Chronostratigraphy — (Geochronostratigraphic unit)

8. Chemostratigraphy (Isotope zones)

Stratigraphic Relationship

There are two types of contact in stratigraphy i.e.: conformable and unconformable.

1. Conformable: Non-fragmented accumulation, no breakages or hiatus (interruption or impairment in the continuity of the geological record). The resulting surface strata are known as conformity. Further, there are two forms of contact between conformable strata: abrupt contacts (directly isolated beds of distinctly distinguished lithology, minor depositional break, referred to diastems) and gradational contact (steady change in deposition, mixing zone).

2. Unconformable: Period of weathering/non-deposition. The surface stratum resulting is known as an unconformity. There are further

Four Kinds of Unconformity:

Angular Unconformity: younger sediment lies upon a weathered surface of folded or slanted older rocks. The older rock steeps at a different angle from the younger.

Disconformity: the contact between older and younger beds is noted by apparent, non-uniform weathering surfaces. Paleosol might form right above the disconformity surface due to the non-deposition setting.

Nonconformity: comparatively young sediments are accumulated right above older igneous or metamorphic rocks.

Paraconformity: the bedding planes below and above the unconformity run parallel to one another. A time gap exists, as depicted by a faunal break, but there is no weathering, just a period of non-accumulation.

Stratigraphic Subdivision

1. Concept of Zone

With respect to stratigraphic subdivision, there is the concept of stage. A stage is a crucial subdivision of strata, each minutely following each other while bearing a unique, distinctive assemblage of fossils. Thus, stages can be described as a group of strata consisting of the same major fossil assemblages. French palaeontology Alcide d’Orbigny is felicitated for the introduction and implementation of this concept. He named stages after geographic localities with specifically finer sections of rock strata that carry the characteristic fossils on which the stages are established.

2. Concept of Zone

The zone is an elementary biostratigraphic unit. The thickness of the unit ranges from a few to hundreds of metres, and its extent ranges from local to global. Biostratigraphic units are further classified into 6 principal kinds of biozones that are as below: 

1. Assemblage Biozones: These are strata that consist of a special correlation of three or more taxa.

2. Abundance Biozones: These are strata in which the abundance of a specific taxon or group of taxa is considerably higher than in the adjacent part of the section.

3. Concurrent Range Biozone: It includes the coincident, concurrent or overlapping part of the range of two particular taxa.

4. Interval Biozone: It includes the strata between two particular biostratigraphic surfaces. It can be established on either the highest or lowest occurrences.

5. Lineage Biozone: These are strata consisting of species demonstrating a particular segment of an evolutionary lineage.

6. Taxon Range Biozone: It depicts the known stratigraphic and geographic range of formation of a single taxon.

Sequence Stratigraphy

Sequence stratigraphy is an evaluation of sedimentary deposits in a time-stratigraphic aspect. It generally includes subdividing a sedimentary basin fill into individual sequences of accumulation (thus the name), which can then be associated with alterations in the two elementary parameters of sediment supply and shelter (the amount of space available for accumulation). A key purpose of this is to reconstruct how sediments filled a basin and thus, how the stratigraphy occurred through time and space. This can enable geologists and scientists to identify many significant aspects like finding out where fine and coarse-grained sediments are located and how sea level changed.

Uses of Sequence Stratigraphy

Over the years, sequence stratigraphy has transpired to be an extensively used, methodological framework that confines many contexts of sedimentology and stratigraphy and has different useful applications and anticipating capacities.

Tectosilicate, also known as framework silicate, is any element of a group of compounds having a structure that has silicate tetrahedrons(each of which includes central silicon atoms surrounded by four oxygen atoms at the corners of a tetrahedron). Each of the four oxygen atoms of a given tetrahedron is joined with another tetrahedron. Each tetrahedron is further connected to four others. Tectosilicate, along with the quartz and other silicate minerals having a chemical formula consisting of some multiples of SiO₂.

What is Tectosilicate?

Tectosilicate or Framework silicate is the substantial group of silicates consisting of approximately 75% of the earth’s crust. Framework silicates are identified by the three-dimensional structure of silica tetrahedra. Tectosilicate examples are the members of the quartz, feldspar, and zeolite group of minerals. Excluding the quartz group, they are considered as aluminosilicates with the general chemical formula AlXSiyO2(x + y))x-(1:2 ratio of Si to O atoms). 

The tectosilicate can include extra cations if some of the silicates are substituted by additional lower charge cations such as aluminum to give an overall negative charge. This replacement can also occur in other forms of silicates.

Some sparse minerals may consist of more than one type of anion coinciding in their crystal structure, or they may include complex anions that are halfway between the types written above.

Tectosilicate Definition

Tectosilicate is defined as the polymeric silicates in which silicon oxygen tetrahedral groups are correlated by splitting all their oxygen atoms with other groups so as to form three dimensional structure or network.

Tectosilicate  Fórmula

The general formula of three-dimensional or tectosilicate or framework silicate is (SiO₂)n. All the oxygen atoms of Si04 are united with other tetrahedra and hence forming the three dimensional network.

Eg SiO₂- quartz, tridymite, cristobalite- These oxide minerals are the crystallized form of silica. 

Three-dimensional  aluminosilicates are formed when Sio44- gets replaced with AlO45-.

Eg. Feldspar, zeolites  and Ult etc.

Tectosilicate Structure

Tectosilicate structure is made up of interconnected tetrahedra moving externally in all directions forming an intricate framework. All the oxygen is combined with other tetrahedra in the lower groups. In the nearly pure form of oxygen and silicon, the most predominant mineral found is quartz(SiO2). 

Aluminum can easily be substituted for the silicon ion in the tetrahedron. In other classes, this occurs to a certain degree but it is the biggest factor of the diversified structure. 

While tetrahedron is approximately the same as aluminum at its center, the charge is now a negative (-5) instead of a negative (-4). As the charge in a crystal must be balanced, additional cations are required in the tectosilicate structure and this is the basic reason for the maximum variations in the lower groups.

Tectosilicate Examples

The tectosilicate group includes the most abundant minerals composing the Earth crust. These includes

Quartz – 12%

Plagioclase Feldspar – 39%

Orthoclase Feldspar – 12%

Some Tectosilicate Examples Include: 

Orthoclase – KALSi₃O₈

Anorthite – CaAl₂Si₂O₈

Labradorite – (Ca, Na)(Al, Si)₄O₈

Albite – NaAlSi₃O₈

Quartz – SiO₂

Did You Know?

• Tectosilicate is a group of silicates that consists of all four oxygen atoms from each tetrahedron sharing themselves.

• The tectosilicates that contain Al are commonly known as plagioclase or feldspar.

• Feldspar is the name of the group of rocks forming tectosilicate minerals that comprise as much as 60% of the Earth’s crust.

• Tectosilicates represent almost 60% of the rock crust of the Earth. The Silicate group and feldspar group are the two most important groups.