Clay Mineralogy and Soil Consistence

What is a Sheet?• A flat array of compounds with the samechemical arrangement joined by sharedoxygen atoms.• 2 types of sheets– Oxygen and silicon– Oxygen and aluminum• Multiple sheets make up layers

Sheet Representation• Silicon-oxygen sheet• Aluminum-oxygensheet

Layers• Sheets joined by sharing oxygen atoms• 2 types– 1:1 – 1 sheet of silicon and oxygen and 1sheet of aluminum and oxygen– 2:1 – 2 sheets of silicon and oxygen and 1sheet of aluminum and oxygen

1:1 Clays• 1 silicon and oxygen sheet• 1 aluminum and oxygen sheet• Layers are joined by hydrogen bonding• Bonds are strong so the space betweenlayers is fixed• Little shrink or swell upon drying orwetting

1:1 LayerSilicon-oxygen sheetAluminum-oxygen sheetHydrogen atoms1:1 Layer

2:1 Clays• 2 silicon and oxygen sheets• 1 aluminum and oxygen sheet• Layers are joined by weak oxygen-tooxygenand cation-to-oxygen linkages• Bonds are loose so other compounds suchas water can enter the structure and pushthe layers apart• Shrink and swell upon drying and wetting

2:1 LayerSilicon-oxygen sheetAluminum-oxygen sheetSilicon-oxygen sheetCalcium (cation)2:1 Layer

2:1 Layers + WaterSilicon-oxygen sheetWaterAluminum-oxygen sheetSilicon-oxygen sheetCalcium (cation)2:1 Layer

Clay Particles• < 0.002 mm in diameter• Made up of many layers– Either 1:1 or 2:1• Mostly made of 1:1 clay layers– SLIGHTLY EXPANSIVE• Mostly made of 2:1 clay layers– EXPANSIVE

THERE IS NO SUCH THINGAS NON-EXPANSIVE CLAYMINERALS!!!!SLIGHTLY EXPANSIVE andEXPANSIVE ONLY!

Why is this important?• Shrink-swell potential effects more thanjust the physical movement of clay layers• Related to soil porosity (space availablefor water and air)• If a 2:1 clay gets wet what happens?• What happens if those pores are closedup?

Relate to Septic System• The nitrification field gets dosed• Soil gets wet and pores close up• Another dose is released into thenitrification field• Where does that effluent go?• Surface?

.1941 (3)• Slightly expansive - SUITABLE– 1:1– Fixed interlayer spacing– Little shrink and swell of clays• Expansive - UNSUITABLE– 2:1– Variable interlayer spacing– Lots of shrink and swell of clays

Testing Methods• In the lab– X-ray diffraction– Atterberg Limits: liquid limit, plastic limit,plasticity index• In the field– Moist consistency– Wet consistency• Plasticity• Stickiness

Moist Consistency• Moisten sample if not already moist• Place between thumb and forefinger• Press until the sample breaks• Amount of pressure required to break thesample determines firmness

Friability• Loose• Very Friable• Friable• Firm• Very Firm• Extremely FirmMostly 1:1 claysMostly 2:1 clays

Suitable Moist Consistencies• Loose – sand-like• Very Friable – crushes under very gentlepressure• Friable – crushes under gentle ormoderate pressure• Firm – crushes under moderate pressurebut resistance is noticeable

Unsuitable Moist Consistencies• Very Firm – crushes under strongpressure, barely crushable• Extremely Firm – crushes only under verystrong pressure; cannot be crushedbetween thumb and forefinger

Stickiness• Thoroughly wet a sample of soil• Place between thumb and forefinger• Gently pull apart fingers• Amount of soil on both thumb andforefinger determines the stickiness

Stickiness• Nonsticky• Slightly Sticky• Sticky• Very StickyMostly 1:1 claysMostly 2:1 clays

Suitable Stickiness• Nonsticky – practically no soil adheres tothumb or forefinger• Slightly Sticky – soil adheres to bothfingers but comes off of one rather cleanly• Sticky – soil adheres to both fingers andtends to stretch somewhat then pull apart

Unsuitable Stickiness• Very Sticky – soil adheres strongly to bothfingers and decidedly stretches withfingers are separated

Plasticity• Thoroughly wet a sample of soil• Roll between hands to form a wire (snake)• Wire should be 4 cm long• Pick up the wire from one end andevaluate where is breaks• The thinness of the wire determinesplasticity

Plasticity• Nonplastic• Slightly Plastic• Plastic• Very plasticMostly 1:1 claysMostly 2:1 clays

Suitable Plasticity• Nonplastic – no wire forms• Slightly Plastic – 4 cm long wire formswith diameter between 6 and 4 mm andsupports its own weight• Plastic – 4 cm long wire forms withdiameter between 4 and 2 mm andsupports its own weight

Unsuitable Plasticity• Very Plastic – 4 cm long wire forms with adiameter of 2 mm and supports its ownweight

In the field, how do we know wehave 2:1 clays?• Moist consistency– Very firm– Extremely firm• Wet consistency– Stickiness: very sticky– Plasticity: very plastic

2:1 Clays are UNSUITABLEfor a septic system