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 Silicon-oxygen sheet Aluminum-oxygen sheet Hydrogen atoms 1:1 Layer 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 Silicon-oxygen sheet Aluminum-oxygen sheet Silicon-oxygen sheet Calcium (cation) 2:1 Layer 2:1 Layers + Water Silicon-oxygen sheet Aluminum-oxygen sheet Silicon-oxygen sheet Water Calcium (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 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 Mostly 1:1 clays Friable Firm Very Firm Mostly 2:1 clays Extremely Firm 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 Mostly 1:1 clays Sticky Very Sticky 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 Mostly 1:1 clays Plastic Very plastic 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 .