Geo-Chemistry By Engr. Asadullah Memon B.E. (Petroleum & Natural Gas)

Introduction to Geochemistry The field of geochemistry involves: 1.Study of the chemical composition of the Earth and other planets, 2.Chemical processes and reactions that govern the composition of rocks, water, and soils, 3.The cycles of matter and energy that transport the Earth's chemical components in time and space, and their interaction with the hydrosphere and the atmosphere. Some subsets of geochemistry are: a)Isotope geochemistry: Determination of the relative and absolute concentrations of the elements and their isotopes in the earth and on earth's surface. b)Examination of the distribution and movements of elements in different parts of the earth (crust, mantle, hydrosphere etc.) and in minerals with the goal to determine the underlying system of distribution and movement. c)Cosmochemistry: Analysis of the distribution of elements and their isotopes in the cosmos. d)Biogeochemistry: Field of study focusing on the effect of life on the chemistry of the earth. e)Organic geochemistry: A study of the role of processes and compounds that are derived from living or once-living organisms. f)Water Geochemistry: Understanding the role of various elements in watersheds. g)Regional, environmental and exploration geochemistry: Applications to environmental, hydrological and mineral exploration studies.

The main focus of geochemistry is to:  Understand the principles governing the distribution and re- distribution of elements, ionic species and isotope ratios in earth materials, so that we can interpret the formation of mineral assemblages: conditions (P, T, etc.), processes (magmatic crystallization, weathering, chemical precipitation, metamorphism, etc.), and even the age.  Predict changes in mineral assemblages (minerals, concentrations of elements, isotopic ratios) if a given mineral assemblage is subjected to different conditions (T, P, interaction with a fluid, etc.)  Geochemistry plays an important role in forecasting the quality of crude oil in the accumulation. Geochemistry = chemistry of the Earth (i.e., of earth materials — minerals and rocks)

THE EARTH'S CHEMISTRY  The bulk of the Earth is made from iron, oxygen, magnesium and silicon.  More than 80 chemical elements occur naturally in the Earth and its atmosphere.  Mostly Earth is composed of three parts 1.Crust 2.Mantle (Upper & Lower) 3.Core  The Earth's crust is a thin layer of rock that floats on the mantle. The crust is made mostly from oxygen and silicon (silicate minerals such as quartz), with aluminium, iron, calcium, magnesium, sodium, potassium, titanium and traces of 64 other elements.  The upper mantle is made up of iron and magnesium silicates; the lower is silicon and magnesium sulphides and oxides.  The core is mostly iron, with little nickel and traces of sulphur, carbon, oxygen and potassium.

Fig.- This diagram shows the percentages of the chemical elements that make up the Earth. Fig.- This diagram shows the Earth interior.

EARTH'S INTERIOR  The Earth's crust is a thin hard outer shell of rock. Under the crust, there is a deep layer of hot soft rock called the mantle.  The crust and upper mantle can be divided into three layers according to their rigidity: 1.The lithosphere (The lithosphere is the upper, rigid layer of the Earth. It consists of the crust and the top of the mantle and it is about 100 km thick ), 2.The asthenosphere (Below the lithosphere, in the Earth's mantle, is the hot, soft rock of the asthenosphere. The boundary between the lithosphere and the asthenosphere occurs at the point where temperatures climb above 1300°C), 3.The mesosphere.  Beneath the mantle is a core of hot iron and nickel. The outer core is so hot (4500°C °C) that it is always molten. The inner core is even hotter (up to 7000°C) but it stays solid because the pressure is 6000 times greater than on the surface.  The inner core contains 1.7% of the Earth's mass, the outer core 30.8%; the core - mantle boundary 3%; the lower mantle 49%; the upper mantle 15%; the ocean crust 0.099% and the continental crust 0.374%.

Fig.- The main layers that form the Earth.  Our knowledge of the Earth's interior comes mainly from studying how earthquake waves move through different kinds of rock.  Analysis of how earthquake waves are deflected reveals where different materials occur in the interior. S (secondary) waves pass only through the mantle. P (primary) waves pass through the core as well. P waves passing through the core are deflected, leaving a shadow zone where no waves reach the far side of the earth.  The speed of earthquake waves reveals how dense the rocky materials are. Cold, hard rock transmits waves more quickly than hot, soft rock.

Geo Chemical Classification of Elements  There are several trials to classify elements on geochemical basis.  Names such as siderophile, chalcophile, lithophile, hydrophile, thalassophile, atmophile are commonly used to denote particular geochemical affinity of elements.  chemical affinity is the electronic property by which dissimilar chemical species are capable of forming chemical compounds or Affinity is the tendency of a molecule to associate with another..  Chemical affinity can also refer to the tendency of an atom or compound to combine by chemical reaction with atoms or compounds of unlike composition.

Geochemical Affinity In the classification scheme of Goldschmidt, elements are divided according to how they partition between coexisting silicate liquid, sulfide liquid, metallic liquid, and gas phase…defined by examining ore smelting slags and meteorites Silicate Liquid Sulfide Liquid Metallic Liquid Gas Phase Siderophile Chalcophile Lithophile Atmophile H, He, N, Noble gases Alkalis, Alkaline Earths, Halogens, B, O, Al, Si, Sc, Ti, V, Cr, Mn, Y, Zr, Nb, Lanthanides, Hf, Ta, Th, U Cu, Zn, Ga, Ag, Cd, In, Hg, Tl, As, S, Sb, Se, Pb, Bi, Te Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Mo, Re, Au, C, P, Ge, Sn To first order, the distribution of elements between core and mantle resembles equilibrium partitioning between metal liquid and silicates…confirmed by iron and achondrite meteorites (but at high P, no separate sulfide phase) Melting a chondrite gives 3 immiscible liquids plus vapor:

What makes an element siderophile or lithophile? Notably, the Goldschmidt categories are well- grouped in the periodic table of the elements:

Chemical Weathering

 Weathering is a process of the disintegration and degeneration of rocks minerals or soils as a result of direct contact with the atmosphere of the Earth OR The disintegration, or breakdown of rock material is called Weathering.  Three types of weathering: 1)Chemical weathering: breakdown as a result of chemical reactions (CaCO 3 +CO 2 +H 2 O ---> Ca HCO 3- ) 2) Mechanical Weathering: No change in chemical composition, just disintegration into smaller pieces 3) Biological Weathering: Can be both chemical and mechanical in nature (For Example: Tree throw).  The rate of weathering differs with variation in the chemical composition and structure.

Chemical Weathering Definition: Transformation/decomposition of one mineral into another  Mineral breakdown carbonate dissolves primary minerals --> secondary minerals (mostly clays)  Water is the main operator:  Dissolution Many ionic and organic compounds dissolve in water Silica, K, Na, Mg, Ca, Cl, CO 3, SO 4 water + carbon dioxide + calcite dissolve into calcium ion and bicarbonate ion H 2 O + CO 2 + CaCO 3 --> Ca HCO 3 -  Acid Reactions Water + carbon dioxide carbonic acid Water + sulfur sulfuric acid H + effective at breaking down minerals

Chemical Weathering  Oxidation Oxygen dissolved in water promotes oxidation of sulfides, ferrous oxides, native metals  Organic Activity Plant material makes H + ions available  Hydration Attachment of water molecules to crystalline structure of a rock, causing expansion and weakness  Hydrolysis combination of hydrogen and oxygen in water with rock to form new substances

Factors that Influence Chemical Weathering The factors that influence Chemical weathering are,  The climate of the place ( Temperature and moisture characteristics ),  The vegetation ( Most effective in areas of warm, moist climates – decaying vegetation creates acids that enhance weathering )  The physical nature of the rock. In case of Chemical weathering water plays a major role as is evident from the description of methods, therefore in the absence of water, chemical weathering is nearly impossible.

TYPES OF WEATHERING REACTIONS