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What is Soil Mineralogy?
Actually referring to clay mineralogy The chemical make-up and arrangement
of atoms and molecules into sheets that give clay (< 0.002 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 sheets – Oxygen and silicon – Oxygen and aluminum
Multiple sheets make up layers
Layers
Sheets joined by sharing oxygen atoms 2 types
– 1:1 – 1 sheet of silicon and oxygen and 1 sheet of aluminum and oxygen
– 2:1 – 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 sheet Layers are joined by hydrogen bonding Bonds are strong so the space between
layers is fixed Little shrink or swell upon drying or
wetting
2:1 Clays
2 silicon and oxygen sheets 1 aluminum and oxygen sheet Layers are joined by weak oxygen-to-
oxygen and cation-to-oxygen linkages Bonds 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
Silicon-oxygen sheet
Aluminum-oxygen sheet Silicon-oxygen sheet
Calcium (cation)
2:1 Layers + Water
Silicon-oxygen sheet
Aluminum-oxygen sheet Silicon-oxygen sheet
Calcium (cation)
2:1 Layer
Water
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
Why is this important?
Shrink-swell potential effects more than just the physical movement of clay layers
Related 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 dosed Soil gets wet and pores close up Another dose is released into the
nitrification 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 the
sample determines firmness
Friability
Loose Very Friable Friable Firm Very Firm Extremely Firm
Mostly 1:1 clays
Mostly 2:1 clays
Suitable Moist Consistencies
Loose – sand-like Very Friable – crushes under very gentle
pressure Friable – crushes under gentle or
moderate pressure Firm – crushes under moderate pressure
but resistance is noticeable
Unsuitable Moist Consistencies
Very Firm – crushes under strong pressure, barely crushable
Extremely Firm – crushes only under very strong pressure; cannot be crushed between 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 and
forefinger determines the stickiness
Suitable Stickiness
Nonsticky – practically no soil adheres to thumb or forefinger
Slightly Sticky – soil adheres to both fingers but comes off of one rather cleanly
Sticky – 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 soil Roll between hands to form a wire (snake) Wire should be 4 cm long Pick up the wire from one end and
evaluate where is breaks The thinness of the wire determines
plasticity
Suitable Plasticity
Nonplastic – no wire forms Slightly Plastic – 4 cm long wire forms
with diameter between 6 and 4 mm and supports its own weight
Plastic – 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 consistency – Very firm – Extremely firm
Wet consistency – Stickiness: very sticky – Plasticity: very plastic