Soil Mineralogy - PDF document

Soil Mineralogy Karen WallaceNCDHHSRegional Soil Scientist What is Soil Mineralogy? Actually referring to clay mineralogyThe chemical makeup and arrangement of atoms and molecules into sheets that give clay (.002 mm) its ability to shrink and swell upon drying and wetting. What is a Sheet? A flat array of compounds with the same chemical arrangement joined by shared oxygen atoms.2 types of sheetsOxygen and siliconOxygen and aluminumMultiple sheets make up layers Sheet Representation Siliconoxygen sheet Aluminumoxygen sheet Layers Sheets joined by sharing oxygen atoms2 types1:1 1 sheet of silicon and oxygen and 1 sheet of aluminum and oxygen2:1 2 sheets of silicon and oxygen and 1 sheet of aluminum and oxygen 1:1 Clays 1 silicon and oxygen sheet 1 aluminum and oxygen sheetLayers are joined by hydrogen bondingBonds are strong so the space between layers is fixedLittle shrink or swell upon drying or wetting 1:1 Layer 1:1 Layer Siliconoxygen sheetAluminumoxygen sheet Hydrogen atoms 2:1 Clays 2 silicon and oxygen sheets1 aluminum and oxygen sheetLayers are joined by weak oxygenoxygen and cationoxygen linkagesBonds are loose so other compounds such as water can enter the structure and push the layers apart Shrink and swell upon drying and wetting 2:1 Layer 2:1 Layer Siliconoxygen sheetAluminum oxygen sheet Siliconoxygen sheet Calcium (cation) 2:1 Layers + Water Siliconoxygen sheetAluminumoxygen sheetSiliconoxygen sheetCalcium (cation) 2:1 Layer Water Clay Particles 0.002 mm in diameterMade up of many layersEither 1:1 or 2:1Mostly made of 1:1 clay layersSLIGHTLY EXPANSIVEMostly made of 2:1 clay layersEXPANSIVE THERE IS NO SUCH THING NONEXPANSIVECLAY MINERALS!!!! SLIGHTLY EXPANSIVE andEXPANSIVE ONLY! Why is this important? Shrinkswell potential effects more than just the physical movement of clay layersRelated to soil porosity (space available for water and air)If a 2:1 clay gets wet what happens? What happens if those pores are closed up? Relate to Septic System The nitrification field gets dosedSoil gets wet and pores close upAnother dose is released into the nitrification fieldWhere does that effluent go?Surface? .1941 (3) Slightly expansive SUITABLE1:1Fixed interlayer spacingLittle shrink and swell of claysExpansive UNSUITABLE2:1Variable interlayer spacing Lots of shrink and swell of clays Testing Methods In the labray diffractionAtterberg Limits: liquid limit, plastic limit, plasticity indexIn the fieldMoist consistencyWet consistencyPlasticityStickiness Moist Consistency Moisten sample if not already moistPlace between thumb and forefingerPress until the sample breaksAmount of pressure required to break the sample determines firmness Friability Very FriableFriableFirm Very Firm Extremely Firm Mostly 1:1 clays Mostly 2:1 clays Suitable Moist Consistencies Loose sandlikeVery Friable crushes under very gentle pressureFriable crushes under gentle or moderate pressureFirm crushes under moderate pressure but resistance is noticeable Unsuitable Moist Consistencies Very Firm crushes under strong pressure, barely crushableExtremely Firm crushes only under very strong pressure; cannot be crushed between thumb and forefinger Stickiness Thoroughly wet a sample of soilPlace between thumb and forefingerGently pull apart fingers Amount of soil on both thumb and forefinger determines the stickiness Stickiness NonstickySlightly StickySticky Very Sticky Mostly 1:1 claysMostly 2:1 clays Suitable Stickiness Nonsticky practically no soil adheres to thumb or forefingerSlightly Sticky soil adheres to both fingers but comes off of one rather cleanlySticky soil adheres to both fingers and tends to stretch somewhat then pull apart Unsuitable Stickiness Very Sticky soil adheres strongly to both fingers and decidedly stretches with fingers are separated Plasticity Thoroughly wet a sample of soilRoll between hands to form a wire (snake)Wire should be 4 cm longPick up the wire from one end and evaluate where is breaksThe thinness of the wire determines plasticity Plasticity NonplasticSlightly PlasticPlastic Very plastic Mostly 1:1 claysMostly 2:1 clays Suitable Plasticity Nonplastic no wire formsSlightly Plastic 4 cm long wire forms with diameter between 6 and 4 mm and supports its own weightPlastic 4 cm long wire forms with diameter between 4 and 2 mm and supports its own weight Unsuitable Plasticity Very Plastic 4 cm long wire forms with a diameter of 2 mm and supports its own weight In the field, how do we know we have 2:1 clays? Moist consistencyVery firmExtremely firmWet consistencyStickiness: very sticky Plasticity: very plastic 2:1 Clays are UNSUITABLE for a septic system