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Soil Mineralogy Karen Wallace
NCDHHS Regional Soil Scientist
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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.
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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
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Sheet Representation
Silicon-oxygen sheet
Aluminum-oxygen sheet
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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
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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
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1:1 Layer
1:1 Layer
Silicon-oxygen sheet
Aluminum-oxygen sheet Hydrogen atoms
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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
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2:1 Layer
2:1 Layer
Silicon-oxygen sheet
Aluminum-oxygen sheet Silicon-oxygen sheet
Calcium (cation)
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2:1 Layers + Water
Silicon-oxygen sheet
Aluminum-oxygen sheet Silicon-oxygen sheet
Calcium (cation)
2:1 Layer
Water
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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
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THERE IS NO SUCH THING AS NON-EXPANSIVE CLAY
MINERALS!!!!
SLIGHTLY EXPANSIVE and EXPANSIVE ONLY!
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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?
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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?
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.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
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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
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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
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Friability
Loose Very Friable Friable Firm Very Firm Extremely Firm
Mostly 1:1 clays
Mostly 2:1 clays
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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
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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
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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
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Stickiness
Nonsticky Slightly Sticky Sticky Very Sticky
Mostly 1:1 clays
Mostly 2:1 clays
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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
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Unsuitable Stickiness
Very Sticky – soil adheres strongly to both fingers and decidedly stretches with fingers are separated
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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
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Plasticity
Nonplastic Slightly Plastic Plastic Very plastic
Mostly 1:1 clays
Mostly 2:1 clays
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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
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Unsuitable Plasticity
Very Plastic – 4 cm long wire forms with a diameter of 2 mm and supports its own weight
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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
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2:1 Clays are UNSUITABLE for a septic system