Chemistry of Soil (CHM031) PDF

Chemistry of Soil Learning Objectives Discuss the different types of soil colloids and their properties.  Greek: geo (earth), sphaira (ball/ globe) portion of the earth that includes the earth's interior, rocks and minerals, landforms and the processes that sh...

Chemistry of Soil Learning Objectives Discuss the different types of soil colloids and their properties.  Greek: geo (earth), sphaira (ball/ globe) portion of the earth that includes the earth's interior, rocks and minerals, landforms and the processes that shape the earth's surface. http://kidspressmagazine.com/science-for-kids/misc/misc/layers-earth.html 3  (Mohorovičić discontinuity) http://clarkscience8.weebly.com/earth-layers.html 4 http://clarkscience8.weebly.com/earth-layers.html 5 http://clarkscience8.weebly.com/earth-layers.html 6 ELEMENT ATOM % MASS % Oxygen 53.3 49.5 Silicon 15.9 25.7 Hydrogen 15.1 0.9 Aluminum 4.8 7.5 Sodium 1.8 2.6 Iron 1.5 4.7 Calcium 1.5 3.4 Magnesium 1.4 1.9 Potassium 1 2.4 All others 3.7 1.4 TOTAL 100 100 7 Lithosphere is mainly rocks and minerals. Prominent among these are: a. Silicate minerals (compounds of metals with Si and O) b. Carbonate minerals (metals combined with C and O) c. Oxide minerals (metals combined with O only) d. Sulfide minerals (metals combined with S only) 8 Some non-silicate minerals of economic importance MINERAL TYPE NAME CHEMICAL FORMULA SOURCE AND/OR USE 1. Oxide Hematite Fe2O3 iron ore; pigment Magnetite Fe3O4 iron ore Corundum Al2O3 gemstone; abrasive 2. Sulfide Galena PbS lead ore Chalcopyrite CuFeS2 copper ore 3. Carbonate Calcite CaCO3 cement; lime 9 ▪ The upper layer of earth in which plants grow ▪ typically black, dark brown or reddish material consisting of a mixture of organic remains, clay and rock particles ▪ Natural product formed from weathered rock by the action of climate and living organisms 10  Weathering – alteration and breakdown of rock minerals and rock masses when they are exposed to the sun, wind, rain or organisms Physical weathering – mechanical disruption of rocks (granular disintegration, exfoliation, etc.) Chemical weathering – caused by OH-, mild acids, as well as H2O & temperature Biological weathering – due to vegetation, animals or microorganisms Erosion – the removal or transport of weathered/broken rock materials downslope, and away from the original site of weathering; may be aided by gravity, wind or water 11  zone of leaching zone of accumulation https://www.pmfias.com/soil-profile-soil-horizon-soil-types-sandy-clayey-loamy/ 12 ▪ Most active portion of the soil and determine the physical and chemical properties of a soil ▪ They are made up of the smallest particles of humus and clay in the soil ▪ Only particles less than 0.001 mm are officially defined as colloids ▪ Humus particles are tiny pieces of organic matter and known as organic colloids ▪ Clay particles are known as inorganic colloids Clay – any particle that is less than 0.002 mm in size 13 1. Crystalline silicate clays Phyllosilicates → tetrahedral and octahedral crystal sheets 2. Non-crystalline silicate clays (Andisols) Dominantly amorphous clays (allophane and imogolite) 3. Iron and aluminum oxides (Oxisols) Dominantly gibbsite (Al oxide) and goethite (Fe oxide) 4. Organic (humus) colloids (Histosols) Non-crystalline colloids dominated by long C-chain molecules 14 Phyllosilicates Also called sheet silicates Group of minerals that includes the micas, chlorite, serpentine, talc and clay minerals Basic structure of phyllosilicates is based on interconnected six member rings of SiO4-4 tetrahedra that extend outward in infinite sheets 3 out of the 4 oxygens from each tetrahedral are shared with other tetrahedral, which leads to a basic structure of Si2O5-2 15 Phyllosilicates Most phyllosilicates contain the hydroxyl ion, OH-, with the OH- located at the center of the 6-membered rings Thus, the group becomes Si2O5(OH)-3 https://www.tulane.edu/~sanelson/eens211 /phyllosilicates.htm 16 Phyllosilicates Examples of Phyllosilicates Talc (soapstone) Mica group: muscovite, biotite Chlorite group Phyllosilicates : TALC also called soapstone has the formula Mg3Si4O10(OH)2 Occurrence: found in low grade metamorphic rocks that originated as ultrabasic to basic igneous rocks Rocks composed almost entirely of talc have a greasy feel and referred Talc mine in Pakistan to as soapstone 18  e.g. granite, obsidian e.g. marble, gneiss e.g. limestone, sandstone http://www.cet.edu/ete/modules/msese/earthsysflr/rock.html 19 Phyllosilicates : MICA GROUP Mica Group: Muscovite and Biotite Muscovite (Al2K2O6Si) Common constituent of Al-rich medium grade metamorphic rocks Also found in siliceous, Al-rich plutonic igneous rocks (muscovite granites) but has not been found as a constituent of volcanic rocks Has white to sometimes light brownish color 20 Phyllosilicates : MICA GROUP Mica Group: Muscovite and Biotite Biotite (K(Mg,Fe) 3(AlSi 3O 10)(F,OH) 2) Found in hydrous ultrabasic rock like kimberlite Common in trachytic volcanic rocks, granitic plutonic rocks and a wide variety of metamorphic rocks Brown to black in color 21 Phyllosilicates : CHLORITE Chlorite – has a complicated formula: (MgFeAl)3(SiAl)4O10(OH)6 Common mineral in low grade metamorphic rocks, where it occurs in association with minerals like actinolite, epidote and biotite In hand specimen, it is characterized by its green color; may be green to pale yellow 22 Andisols Andisols – non-silicate crystalline clays; e.g. allophane and imogolite Volcanic ash is chemically/mineralogically distinct from most other soil parent materials. Composed largely of vitric or glassy materials containing varying amounts of Al & Si. It lacks a well-defined structure (i.e. amorphous), and is quite soluble. 23 Andisols Allophane Allophane and Imogolite are common early- stage residual weathering products of volcanic glass and both have poorly-ordered structures. Allophane forms inside glass fragments where Si concentration and pH are high and has a characteristic spherule shape. Imogolite tends to form on the exterior of Imogolite glass fragments under conditions of lower pH and Si concentration, and has a characteristic thread-like morphology. 24 Sesquioxides or Oxisols Sesquioxides – iron and aluminum oxides Dominantly gibbsite (Al-oxide) and goethite (Fe- oxide) Gibbsite Found in many soils Especially important in highly weathered soils of warm humid regions Consist of mainly either Al or Fe atoms coordinated with O atoms; Some, such as gibbsite (Al-oxide) and goethite (Fe- oxide), form crystalline sheets; others form amorphous coatings on soil particles Goethite 25 Sesquioxides or Oxisols The surface plane of covalently bonded hydroxyl groups gives these colloids the capacity to strongly adsorb certain ions 26 Histosols Humus – also called organic soil colloids; non-crystalline colloids dominated by long C-chain molecules 1. Important in nearly all soils. They are not mineral or crystalline in nature 2. Consist of long convoluted chains and rings of carbon bonded to H, O and N 3. Very high capacity to adsorb water 4. Have high amounts of both positive and negative charges, but net charge is always negative 27 Histosols 28