Soil Mineralogy
CLASSIFICATION OF THE CLAY MINERALS The manner in which atoms are assembled into
3.23 OTHER METHODS FOR COMPOSITIONAL ANALYSIS
Thermal Analysis
Principle Differential thermal analysis (DTA) con-sists of simultaneously heating a test sample and a thermally inert substance at constant rate (usually
OTHER METHODS FOR COMPOSITIONAL ANALYSIS 75 Table 3.7 X-ray Diffraction Data for Clay Minerals and Common Nonclay Minerals
d (A˚ ) Minerala d (A˚ ) Minerala
14 Mont. (VS) Chl. Verm. (VS)b 2.93–3.00 Felds.
12 Sepiolite, heated corrensite 2.89–2.90 Carb.
10 Illite, Mica (S), Halloysite 2.86 Felds.
9.23 Heated Verm. 2.84 Carb. Chl.
7 Kaol. (S). Chl. 2.84–2.87 Chl.
6.90 Chl. 2.73 Carb.
6.44 Attapulgite 2.61 Attapulgite
6.39 Felds. 2.60 Verm., Sepiol.
4.90–5.00 Illite, Mica, Halloysite 2.56 Illite (VS), Kaol.
4.70–4.79 Chlor. (S) 2.53–2.56 Chlor., Felds., Mont.
4.60 Verm. (S) 2.49 Kaol. (VS)
4.45–4.50 Illite (VS), Sepiolite 2.46 Quartz, heated Verm.
4.46 Kaol. 2.43–2.46 Chlorite
4.36 Kaol. 2.39 Verm., Illite
4.26 Quartz (S) 2.38 Kaol.
4.18 Kaol. 2.34 Kaol. (VS)
4.02–4.04 Felds. (S) 2.29 Kaol. (VS)
3.85–3.90 Felds. 2.28 Quartz, Sepiol.
3.82 Sepiol. 2.23 Illite, Chl.
3.78 Felds. 2.13 Quartz, Mica
3.67 Felds. 2.05–2.06 Kaol. (WK)
3.58 Carbonate, Chl. 1.99–2.00 Mica, Illite (S), Kaol. Chl.
3.57 Kaol. (VS), Chl. 1.90 Kaol.
3.54–3.56 Verm. 1.83 Carb.
3.50 Felds., Chlor. 1.82 Quartz
3.40 Carb. 1.79 Kaol.
3.34 Quartz (VS) 1.68 Quartz
3.32–3.35 Illite (VS) 1.66 Kaolin
3.30 Carb. 1.62 Kaolin
3.23 Attapulgite 1.54B Verm. (S), Quartz
3.21 Felds. 1.55 Quartz
3.20 Mica 1.58 Chl.
3.19 Felds. (VS) 1.53 Verm., Illite
3.05 Mont. 1.50 Ill. (S), Kaol.
3.04 Carb. (VS) 1.48–1.50 Kaol. (VS), Mont.
3.02 Felds. 1.45B Kaol.
3.00 Heated Verm. 1.38 Quartz, Chl.
2.98 Mica (S) 1.31, 1.34, 1.36 Kaol. (B)
a(B)⫽broad; (S)⫽strong; (VS) ⫽very strong; (WK)⫽weak; Mont.⫽montmorillonite; Ch1.⫽chlorite; Verm.⫽ vermiculite; Kaol.⫽kaolinite; Carb.⫽carbonate; Felds.⫽feldspar; Sepiol.⫽sepiolite.
bItalics indicates (001) spacing.
about 10C / min) to over 1000C and continuously measuring differences in temperature between the sam-ple and the inert material. Differences in temperature between the sample and the inert substance reflect re-actions in the sample brought about by the heating.
Thermogravimetric analyses, based on changes in weight caused by loss of water or CO2 or gain in
ox-ygen, are also used to some extent. Thermal analysis techniques are described in detail by Tan et al. (1986).
The results of differential thermal analysis are pre-sented as a plot of the difference in temperature between sample and inert material ( T ) versus tem-perature (T ) as indicated in Fig. 3.39. Endothermic re-actions are those wherein the sample takes up heat,
Table 3.8 X-ray Identification of the Principal Clay Minerals (⬍2m) in an Oriented Mount of a Clay Fraction Separated from Sedimentary Material
Mineral Basal d Spacings (001)
Glycolation Effect
(1 h, 60C) Heating Effect (1 h) Kaolinite 7.15 A˚ (001); 3.75 A˚ (002) No change Becomes amorphous 550–
600C Kaolinite, disordered 7.15 A˚ (001) broad; 3.75 A˚
broad
No change Becomes amorphous at lower temperatures than kaolinite Halloysite, 4H2O
(hydrated)
10 A˚ (001) broad No change Dehydrates to 2H2O at 110C Halloysite, 2H2O
(dehydrated)
7.2 A˚ (001) broad No change Dehydrates at 125–150C;
becomes amorphous 560–
590C
Mica 10 A˚ (002); 5 A˚ (004)
generally referred to as (001) and (002)
No change (001) becomes more intense on heating but structure is maintained to 700C Illite 10 A˚ (002), broad, other
basal spacings present but small
No change (001) noticeably more intense on heating as water layers are removed; at higher temperatures like mica Montmorillonite group 15 A˚ (001) and integral
series of basal spacings
(001) expands to 17 A˚ with rational sequence of higher orders
At 300C (001) becomes 9 A˚
Vermiculite 14 A˚ (001) and integral series of basal spacings
No change Dehydrates in steps Chlorite, Mg-form 14 A˚ (001) and integral
series of basal spacings
No change (001) increases in intensity;
⬍800C shows weight loss but no structural change Chlorite, Fe-form 14 A˚ (001) less intense
than in Mg-form;
integral series of basal spacings
No change (001) scarcely increases;
structure collapses below 800C
Mixed-layer minerals Regular, one (001) and integral series of basal spacings
No change unless an expandable component is present
Various, see descriptions of individual minerals
Random, (001) is addition of individual minerals and depends on amount of those present
Expands if
montmorillonite is a constituent
Depends on minerals present in interlayered mineral
Attapulgite (palygorskite)
High intensity d reflections at 10.5, 4.5, 3.23, and 2.62 A˚
No change Dehydrates stepwise (see description)
Sepiolite High intensity reflections at 12.6, 4.31, and 2.61 A˚
No change Dehydrates stepwise (see description)
Amorphous clay, allophane
No d reflections No change Dehydrates and loses weight
Compiled by Carroll (1970).
OTHER METHODS FOR COMPOSITIONAL ANALYSIS 77
Figure 3.38 Pictorial representation of response of phyllosilicates to differentiating treat-ments. Approximate spacings in nm (1 nm ⫽ 10 A˚ ) (from Whittig and Allardice, 1986).
Reproduced with permission from The American Society of Agronomy, Inc., Madison, WI.
and in exothermic reactions, heat is liberated. Analysis of test results consists of comparing the sample curve with those for known materials so that each deflection can be accounted for.
Apparatus Apparatus for DTA consists of a sample holder, usually ceramic, nickel, or platinum; a furnace;
a temperature controller to provide a constant rate of heating; thermocouples for measurement of tempera-ture and the difference in temperatempera-ture between the sample and inert reference material; and a recorder for the thermocouple output. The amount of sample
re-quired is about 1 g. Although the temperatures at which thermal reactions take place are a function only of the sample, the size and shape of the reaction peaks depend also on the thermal characteristics of the ap-paratus and the heating rate.
Reactions Producing Thermal Peaks The impor-tant thermal reactions that generate peaks on the ther-mogram are:
1. Dehydration Water in a soil may be present in three forms in addition to free pore water: (1)
Figure 3.39 Thermogram of a sandy clay soil.
adsorbed water or water of hydration, which is driven off at 100 to 300C, (2) interlayer water such as in halloysite and expanded smectite, and (3) crystal lattice water in the form of (OH) ions, the removal of which is termed dehydroxylation.
Dehydroxylation destroys mineral structures. The temperature at which the major amount of crystal lattice water is lost is the most indicative property for identification of minerals. Dehydration reac-tions are endothermic and occur in the range of 500 to 1000C.
2. Crystallization New crystals form from amor-phous materials or from old crystals destroyed at a lower temperature. Crystallization reactions usually are accompanied by an energy loss and, thus, are exothermic, occurring between 800 and 1000C.
3. Phase Changes Some crystal structures change from one form to another at a specific tempera-ture, and the energy of transformation shows up as a peak on the thermogram. For example, quartz changes from the to form reversibly at 573C. The peak for the quartz phase change is sharp, and its amplitude is nearly in direct pro-portion to the amount of quartz present. The quartz peak is frequently masked within the peak for some other reacting material, but may be readily identified by determining the thermogram during cooling of the sample or by letting it cool first and then rerunning it. The other minerals are destroyed during the initial run while the quartz reaction is reversible.
4. Oxidation Exothermic oxidation reactions in-clude the combustion of organic matter and the oxidation of Fe2⫹ to Fe3⫹. Organic matter oxi-dizes in the 250 to 450C temperature range.
Beside quartz, the only common nonclay minerals in soils that give thermal reactions with large peaks are carbonates and free oxides such as gibbsite, brucite, and goethite. The carbonates give very large endother-mic peaks between about 800 and 1000C, and the ox-ides have an endothermic peak between about 250 and 450C. Thermograms for many clay and nonclay min-erals are presented by Lambe (1952).
Quantitative Analysis Theoretically, the area of the reaction peak is a measure of the amount of mineral present in the sample. For sharp, large amplitude peaks such as the quartz inversion at 573C and the kaolinite endotherm at 650C, the amplitude can be used for quantitative analysis. In either case, calibration of the apparatus is necessary, and the overall accuracy is of the order of plus or minus 5 percent.
Optical Microscope
Both binocular and petrographic microscopes can be used to study the identity, size, shape, texture, and con-dition of single grains and aggregates in the silt and sand size range; for study in the thin section of the fabric, that is, the spatial distribution and interrelation-ships of the constituents; and for study of the orien-tations of groups of clay particles. Because the in-focus depth of field decreases sharply as magnification in-creases, study of soil thin sections is impractical at magnifications greater than a few hundred. Thus, in-dividual clay particles cannot usually be distinguished using an optical microscope.
Useful information about the shape, texture, size, and size distribution of silt and sand grains may be obtained directly without formal previous training in petrographic techniques. Some background is needed to identify the various minerals; however, relatively simple diagnostic criteria that can be used for identi-fication of over 80 percent of the coarse grains in most soils are given by Cady et al. (1986). These criteria are based on such factors as color, refractive index, birefringence, cleavage, and particle morphology. The nature of surface textures, the presence of coatings, layers of decomposition, and so on are useful both for interpretation of the history of a soil and as a guide to the soundness and durability of the particles.
Electron Microscope
With modern electron microscopes it is possible to re-solve distances to less than 100 A˚ , thus making study of small clay particles feasible. Electron diffraction study of single particles may also be useful. Electron diffraction is similar to X-ray diffraction except an electron beam instead of an X-ray beam is used.
QUANTITATIVE ESTIMATION OF SOIL COMPONENTS 79 Magnetic lenses that refract an electron beam form
the basis of the transmission electron microscope (TEM) optical system. An electron beam is focused on the specimen, which is usually a replica of the surface structure of the material under study. Some of the elec-trons are scattered from the specimen, and different parts of the specimen appear light or dark in proportion to the amount of scattering. After passing through a series of lenses, the image is displayed on a fluorescent screen for viewing. Probably the most critical aspect of successful transmission electron microscopy is spec-imen preparation.
In the scanning electron microscope (SEM), second-ary electrons emitted from a sample surface form what appear to be three-dimensional images. The SEM has a ⫻20 to ⫻150,000 magnification range and a depth of field some 300 times greater than that of the light microscope. These characteristics, coupled with the fact that clay particles themselves and fracture surfaces through soil masses may be viewed directly, have led to extensive use of the SEM for study of clays. Ex-amples of electron photomicrographs of clays and soils are given earlier in this chapter and in Chapter 5. Prin-ciples of electron microscopy techniques and addi-tional examples are presented in McCrone and Delly (1973) and Sudo et al. (1981).
3.24 QUANTITATIVE ESTIMATION OF SOIL