Soils 230 Soil Mineralogy

1
Soils 230 Soil Mineralogy
2
Overview

• Introduction
• Clay Mineralogy
• Landscape and Site
• Laboratory Methods
• Field Methods
• Rule .1941 a3
• Soil Taxonomy
• Example Tale of Two Soils
• Lab exercise
• Field Excercise

3
Introduction

• Why do we care?

4
Clay Mineralogy Why is it important?

• The minerals in the clay-sized fraction of the
  soil affects the degree to which soils shrink
  (when dry) and swell (when wetted).
• As a clay soil swells the amount and size of pore
  space decreases and affects the ability to move
  water (effluent) through the soil.

5
Hydraulic conductivity decline from wastewater
6
Infiltration rate decline from wastewater
INFILTRATION RATE (cm/h)
LAUNDRY
TIME (h)
7
Why does this happen?

• Change in cations in solution
• Disperse some clay
• Clogs pores
• Result in swelling of some clay
• Closes pores

8
Expansive clay in thin section

• Bright colors due to the clay being oriented.
• Orientation due to stress (shrink-swell)
• Porosity limited

4 mm
9
Nonexpansive clay in thin section

• Less oriented clay
• Voids outlines with clay skins (argillans)

4 mm
10
Mixed clay in thin section

• Less oriented clay
• Thin argillans
• High iron content

4 mm
11
Expansive clay low hydraulic conductivity
micropore network
(Borchardt, 1986)
12
Introduction to Clay Minerals

• Alan Clapp, Dean Hesterberg, and David Lindbo
• Orange Co. Health Department
• Dept. of Soil Science, NC State

13
Outline

• Building blocks
• Classification
• Properties
• Identification
• Formation

14
Building Blocks of Clay

• Silica Tetrahedron-four sides, four oxygen
  molecules and one silica (Si4)
• Aluminum Octahedron-eight sides, six oxygen
  molecules and one Al3
• These are bound together by shared oxygen
  molecules into different layers

15
Tetrahedron and Octahedron
16
Clay Minerals

• Layer(s) of linked Si tetrahedra
• Layer(s) of Al octahedra

17
Sheets of Tetrahedron and Octahedron
18
Definitions

• Plane-plane of atoms, individual row in the
  composition and structure of the clay mineral
• Sheet-combination of planes
• Layer- combination of sheets i.e.. 11

19
Clay Mineral Classification

• 11 Clay Minerals
• 21 Clay Minerals
• Mixed Mineralogy

20
Clay Structure
21
11 Clay Minerals

• Like an open face sandwich
• One silica tetrahedron (bread)
• One aluminum octahedron (filling)
• The most common 11 minerals is Kaolinite

22
11 Clay Mineral
23
11 Clay MineralsCEC 7 meq/100 gnon-expanding
24
21 Clay Minerals

• Like a sandwich with two slices of bread
• Two silica tetrahedrons (bread)
• One aluminum octahedron (filling)
• The 21 clays can be broken into 2 groups
• Expansive
• Non expansive

25
21 Clay Mineral
26
21 Clay MineralsCEC 40 meq/100 gnon-expanding
27
Non-expansive 21 Clays

• the sheets or layers are held together strongly
• neither water nor a change in the interlayer
  cations causes them to swell
• Illites are one group of non-expandable clays

28
Expansive 21 Clays

• Bound together by very weak hydrogen bounds
  (easily broken)
• Will swell upon wetting
• Smectites (montmorillonite) are one group of
  expandable clays

29
21 Clay Minerals (expanding)CEC 100 200
meq/100 g (vermiculite) 70 120 meq/100 g
(smectite)
30
(No Transcript)
31
Properties

• Surface Area
• Cation Exchange Capacity
• Expansion

32
Surface Area

• Smectite group minerals have higher CEC
• Smectite group minerals have a higher surface
  area
• Isomorphic substitutions plays a role in the
  above and in the expansive nature of this group

33
Selected Properties
34
Why water causes the mineral to expand

• Water is dipolar- which simply means it can be
  attracted to a net negative charge or a net
  positive charge
• Water carries many different ions in soil
  solution
• Water has a physical size

35
11 Nonexpansive
Strong H-bonds
36
21 Non-expanding
K interlayer
37
Hydroxy-interlayered Vermiculite
Al-hydroxy- interlayer
38
21 expandable
Exchangeable Cations
Weak H-bonds
39
21 expandable
Hydration of Cations
40
Soil mineralogy and consistence

• Clay mineralogy
• Water movement
• Management
• Consistence
• Field method to relate soil properties to clay
  mineralogy

41
Identification

• Layer spacing
• X-ray diffraction
• Expansive properties

42
Layer Spacing
43
Bragg's Law
44
(No Transcript)
45
X-Ray Diffraction
Quartz
Kaolinite
HIV
46
X-Ray Diffraction
Quartz
Illite
Kaolinite
Smectite
47
Formation

• Parent material
• Weathering

48
Typical Weathering Sequence

• Poorly drained conditions (slow water movement)
• H4SiO40 dissolves from primary minerals
• Accumulates is soil
• Favors 21 clay formation (smectite)
• Well drained conditions (high rainfall)
• H4SiO40 leaches through profile
• Bases leach through profile (more acid)
• Favors 21 illite/vermiculite to 11 kaolinite
  to gibbsite

49
(No Transcript)
50
Jackson-Sherman Weathering Stages
51
Selected Mineral Transformations
- Ca2
H4SiO40
Ca2
- K
52
Weathering Pathways
Noncrystaline hydrous oxides of Si, Al, Fe and Ti
Olivine Pyroxene Amphibole
Goethite Hematite Gibbsite
Chlorite
Na
H
OH
Biotite
Trioctahedral Vermiculite
Na
Smectite
Na
Dioctahedral Illite
Muscovite
K
- Si
Si
Gibbsite
Noncrystaline aliminosilicates
Feldspar
Kaolinite Halloysite
- Si
- Si
Al
Quartz
Silicic Acid
Chalcedony Opal
53
Parent Material

• The 11 minerals are usually weathered from
  acidic or felsic parent materials
• The expanding 21 minerals are usually weathered
  from basic or mafic parent material
• Parent material plays the biggest part of whether
  the soil will be expansive

54
Clay Minerals

• Layered silicates
• Properties relate to surface area and charge
• Weathering is predictable
• Parent material of primary importance
• Drainage also may relate to clay mineralogy