Soil Mineralogy Karen Wallace

NCDHHS Regional Soil Scientist

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

Sheet Representation

Silicon-oxygen sheet

Aluminum-oxygen sheet

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

1:1 Layer

1:1 Layer

Silicon-oxygen sheet

Aluminum-oxygen sheet Hydrogen atoms

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

THERE IS NO SUCH THING AS NON-EXPANSIVE CLAY

MINERALS!!!!

SLIGHTLY EXPANSIVE and EXPANSIVE ONLY!

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

Stickiness

Nonsticky Slightly Sticky Sticky Very Sticky

Mostly 1:1 clays

Mostly 2:1 clays

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

Plasticity

Nonplastic Slightly Plastic Plastic Very plastic

Mostly 1:1 clays

Mostly 2:1 clays

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

2:1 Clays are UNSUITABLE for a septic system