Mineralogy Laboratory Methods
- Sample preparation – pretreatment and particle size separation
- Instrumental and chemical techniques used
- Differential thermal analysis (DTA)
- Chemical dissolution for minerals of short-range order
- Appendix: Reagents etc. used in mineral analyses
This document has been updated and extracted from:
Whitton, J.S.; Churchman G.J. 1987: Standard methods for mineral analysis of soil survey samples for characterisation and classification in NZ Soil Bureau. NZ Soil Bureau Scientific Report 79. 27 p.
Sample preparation – pretreatment and particle size separation
Introduction
Knowledge of the composition of the soil mineral fraction is important for understanding the origin, properties and behaviour of soils. This fraction consists of materials with a wide range of particle sizes: gravel, stones (> 2 mm), sand (2–0.02 mm), silt (0.02–0.002 mm) and clay (<0.002 mm).
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| Imogolite – an amorphous soil mineral commonly formed from volcanic parent materials. Photo: N Yoshinga | Weathering volcanic glass shard in a soil clay sample. Photo: C Hedley |
Since these fractions differ in composition and properties it is important to determine both the amount and nature of each fraction. The clay-size fraction is the most reactive and has the greatest influence on soil properties. To identify and quantify the mineral constituents in the various size fractions the soil sample is fractionated as described below.
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| Weathering volcanic glass shard. Photo: C Hedley |
Most soils in New Zealand are formed from parent materials that consist largely of crystalline silicate minerals and the secondary minerals produced by weathering during soil formation, which are also usually crystalline. However, soils derived from volcanic ash and subsoil horizons of many podzolised soils often contain significant quantities of short-range-order clay minerals, such as allophane and/or imogolite. A proportion of these minerals may be dissolved during the pre-treatments required for particle separation, e.g., up to a third of the allophane present may dissolve during the citrate-dithionite-bicarbonate (CDB) treatment (Parfitt & Childs 1987). This can lead to an underestimate of allophane and/or imogolite in analyses of clay fractions. For this and other reasons we prefer to estimate allophane and/or imogolite from whole soil analyses.
Methods
Step 1: Calcium removal
It is usual, though not essential, to remove exchangeable or carbonate calcium as a first step. This is because calcium, if present, may precipitate as calcium oxalate during the peroxide treatment in step 2. Calcium oxalate would then appear in the clay fraction and interfere with identification of the components present.
Approximately 10 g of soil are placed in a 90 ml centrifuge tube, to which about 50 ml of water and 1 drop of bromophenol blue indicator are added, followed by a dropwise addition of 1:1 hydrochloric acid (HCl) dropwise until the colour changes from blue to yellow. If the colour is difficult to see (as with dark coloured topsoils) more indicator is added as required. The addition of excess acid should be avoided, and the final pH of the solution should be 3.5. If there is much calcium carbonate a vigorous reaction will occur with frothing and it can be difficult to remove all the calcium carbonate completely. In this case, acid treatment should remove the small-particle-size material, leaving coarser less-reactive particles to be identified in the sand fraction. In general, no more than 5 ml of 1:1 HCl are required and the treatment is stopped at this stage. Centrifuging is carried out for 10 min at 1500 rpm to sediment the soil. The supernatant liquid and any floating plant debris, etc., are poured off.
Step 2: Organic matter removal
Approximately 10 ml of "100 volume" hydrogen peroxide are added to the tube, together with 10 ml of distilled water. After stirring, the tubes are left to stand overnight. Any samples that froth excessively (usually including all topsoils) are transferred to 600 ml beakers. After standing overnight, tubes and beakers are placed into a water bath and heated gradually until the water baths are boiling. They are kept at this temperature, stirring occasionally with Teflon stirring rods, until all frothing ceases. As the samples must not dry out, water is added if necessary. Samples containing much organic matter may need extra additions of hydrogen peroxide to achieve complete decomposition of the organic matter present.
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| Step 2: Soils in suspension with organic matter removed. Photos: C Hedley | |
The soil samples in the beakers are transferred back to centrifuge tubes when frothing has ceased and volumes are sufficiently small. When all frothing has ceased, the tubes are filled with water and the samples are stirred, making sure any soil material adhering to the sides of the tubes is dispersed. The tubes are centrifuged for 15 min at 1500 rpm and the clear (though often coloured) supernatant liquid is discarded. If the supernatant contains some suspended clay, a few drops of saturated sodium chloride (NaCl) are added and the samples centrifuged again.
Step 3: Removal of iron and aluminium oxides and oxyhydroxides
Thirty ml of citrate reagent (0.26M) and 5 ml of 1M sodium bicarbonate ("CBD" step) are added to each tube containing organic-free soil, stirred, and placed in a water bath at 90–100°C. When solutions in the tubes are hot they are stirred again and approximately 1 g solid sodium dithionite is added to each tube with further gentle stirring. (Any excessive effervescence or frothing that occurs at this stage is damped down by a squirt of cold distilled water from a squeeze bottle). Tubes are left in the water bath for a further 15 min with occasional stirring, before being centrifuged for 15 min at 1500 rpm. The clear, though often coloured, supernatant solution is discarded. If solutions are cloudy they are flocculated by adding 1 ml or more of saturated sodium chloride solution, gently stirred and centrifuged again.
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| Step 3: Removal of iron and aluminium oxides. Photos: C Hedley | |
This CDB step is repeated one or more times as required (usually until reddish or brownish colouration disappears and the liquid is clear), taking care to crush any aggregates or iron-cemented concretions, using the end of a glass rod. Finally, the soil material is rinsed by adding 30 ml of citrate reagent, stirred, then heated in the water bath. After centrifugation, the clear supernatant solution is discarded, and the remaining soil is retained in the tube.
Step 4: Clay separation
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| Step 4: Clay is separated from the soil suspension, leaving sand and silt fractions in the test tube. Photo: C Hedley |
Distilled water is added to the tubes to the 10 cm mark and the suspensions are stirred thoroughly with a motorised stirrer. The tubes are then centrifuged at 500 or 800 rpm for the appropriate time and temperature (see Table 1). The supernatant, containing suspended clay, is poured into a beaker of 600–1000 ml capacity and the separation repeated until the supernatant remains almost clear (usually 5 or 6 times). The residue, sand plus silt, is kept in the tubes for separation of sand and silt fractions (Step 7).
Note: If soils have a high clay content then the first two of these separations should be carried out at higher rpm and longer time to ensure all silt and sand have settled at the bottom of the tubes.
| Table 1: Sedimentation times for clays (<2μm) using 10cm fall in International No. 2 centrifuge (15cm rotor arm radius to liquid surface). | ||||
Temperature (°C) |
Time |
|||
500
rpm |
800
rpm |
|||
16 |
9 min 8 8 8 8 7 7 7 7 7 |
0 sec 50 35 24 10 59 47 37 25 16 |
3 min 3 3 3 3 3 3 3 3 2 |
30 sec 23 20 15 10 5 1 57 53 48 |
Step 5: Saturation with cations
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| Step 5: The soil clay suspension is saturated with magnesium ions to flocculate the clays. Photo: C Hedley |
(a) About 10 ml aliquots of clay suspension (from step 4) are transferred to 15 ml centrifuge tubes. They are then saturated with potassium ions by adding about 3 ml of 1M KCI to the clay suspension, without shaking. The clay is allowed to flocculate overnight under gravity. The clear supernatant liquid is then sucked off, and a further 10 ml of KCl is added. The tubes are shaken, and the clay is again allowed to flocculate overnight. The clear supernatant liquid is sucked off, the K+-saturated clay suspension is washed with distilled water and centrifuged at 1500 rpm for 15 min, and the clear supernatant discarded. Washing and centrifuging is repeated 3 times or until the clay begins to disperse, i.e. the suspension remains slightly cloudy following centrifugation.
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| Step 5. Photo: C Hedley |
(b) 10 ml of saturated MgCl2 is added to the remaining bulk of the clay suspension, followed by 1 drop of bromophenol blue indicator. If necessary, 1:1 HCl is added dropwise, until the colour turns yellow. The beaker is filled with distilled water and the clay suspension allowed to flocculate overnight. The clear supernatant liquid is then sucked off, the beaker refilled with distilled water and the clay again allowed to flocculate overnight. The clear supernatant liquid is sucked off and the washed Mg++-saturated clay retained for X-ray slide preparation, etc.
(c) If the clays contain smectites and the nature of the smectite (i.e. montmorillonite, beidellite, etc.) needs to be determined the Greene-Kelly test (Greene-Kelly 1953), is applied. The clay is saturated with Li+ ions. Step 5(a) (above) is followed for a separate 10 ml portion, using LiCl in place of KCl. Moessbauer spectroscopy (Cardile et al. 1987) suggests that results can be misleading if the test is carried out on samples that have been chemically pre-treated. This is because such pretreatment appears to bias the test towards the detection of montmorillonite.
Step 6: Preparation of X-ray slides
A clean dry glass slide (approximately 25 mm × 25 mm) is covered with as much clay suspension (1–2 ml) as can be held on the slide when resting level. The suspension is allowed to dry in air. Two or three slides are prepared in this manner – one each of: Mg2+-saturated clay, K+-saturated clay, and, if necessary Li+-saturated clay. A fourth slide is prepared if kaolin minerals are thought to be present and their nature needs to be assessed using the "Formamide test" (Churchman et al. 1984a). A porous ceramic tile is used instead of glass in this case.
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| Step 6: Clay slides prepared for X-ray analysis. Photos: C Hedley | |
The air-dry samples are examined by X-ray diffraction (XRD). Further treatments are carried out on these slides as follows: (a) Mg2+-saturated slides are sprayed with a 10% glycerol in water solution, using a convenient aerosol spray bottle. Each slide is allowed to dry, giving time for the glycerol to react. At least 2 h are needed and, if convenient, each slide is left overnight. This glyceration step may need to be repeated to achieve complete expansion of some smectites. If there is incomplete or no expansion as indicated by the absence of a peak at 1.4–1.5 nm, the glyceration step is repeated and the glycerated Mg2+ slide re-examined by XRD. (b) After recording the diffractogram for the Mg2+-glycerol sample, the slide is heated at 550°C for 2 h, cooled and another diffractogram obtained. While K+-saturated slides can also be heated to 550°C and used (instead of Mg2+-glycerol slides), K+-saturated clay films are often fragile and susceptible to fritting and flaking, giving rise to poorer XRD patterns.
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| Step 6. Photo: C Hedley |
Step 7: Silt and sand separation
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Silt is separated from the residue remaining in the centrifuge tube after Step 4. This is done by dispersing with sufficient distilled water to fill the tube to the 10 cm mark, stirring, and allowing it to stand for the appropriate time (about 5 min, see Table 2). The suspension, containing only silt at the end of the time, is poured into a 600 ml beaker if silt analysis is to be carried out, or more commonly, is discarded. This step is repeated until no further silt remains in suspension (usually 4–5 times).
The silt in suspension in the 600 ml beaker is allowed to settle out overnight and the clear supernatant liquid is sucked off. The silt is washed with distilled water, air-dried, and stored in vials for XRD and optical examination.
The sand fraction, that is, the residue remaining in the tubes after silt separation, is dried by heating the tubes in an oven at 110°C overnight and stored in vials.
| Table 2: Sedimentation times for silts, using 10cm fall under gravity. | ||
Temperature (°C) |
Time |
|
15 |
5 min 5 5 5 4 4 4 4 4 4 |
27 sec 19 10 3 55 48 41 34 28 22 |
Step 8: Sand fraction density separation for heavy minerals and volcanic glass
The sand fraction can be further separated into a light and heavy fraction (depending on the density of the sand particles) using a heavy liquid of appropriate density. Sodium polytungstate (SPT) of density 2.85–2.90 g cm-3 is normally used for heavy minerals, while SPT of density 2.45 g cm-3 is used for volcanic glass. SPT is much less toxic than bromoform, the heavy liquid that has traditionally been used for most density separations.
0.5 g of sand is placed in a 10 ml narrow-stem centrifuge tube, and 5–6 ml of SPT are added. The content is stirred gently to disperse sand grains and break up any aggregates, and allowed to settle. If there is much fine sand it is necessary to centrifuge at 1000–1500 rpm for 10–15 min to obtain complete separation.
A glass rod with a button on one end is pushed down through the light, floating fraction (taking care none of the light material is carried down with the button) until the button seals the narrow stem that contains the heavy fraction1. The SPT above the seal, together with the light fraction, is poured into a filter paper in a glass separating funnel. The tube is washed to remove any sand adhering to the glass rod or tube, using a small plastic squeeze bottle filled with SPT of the same density. Care is taken during this washing step to avoid breaking the seal and thus contaminating the separate. The SPT that passes the filter can be returned to stock. After washing the light fraction from the tube, the glass rod-button seal is removed and the heavy fraction washed with SPT into a second filter paper. Each filter paper is labeled with sample number and heavy or light fraction etc. for future identification. The two filter papers are then washed with water and allowed to air-dry in a fume cupboard. They are further dried in an oven at 110°C (also in a fume cupboard), weighed, and stored in vials for X-ray and optical examination.
Step 9: Preparation of dry clay
When slides of Mg2+ saturated clays have been prepared and analysed, and any formamide tests completed, the remaining clay suspension is air-dried in Petri dishes, ground and stored in vials for differential thermal analysis (DTA).
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1In the case of volcanic glass separations the volcanic glass is the light fraction floating at the surface.
TopInstrumental and chemical techniques used
X-ray diffraction (XRD)
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| Philips PW1729 X-ray diffractometer. Photo: C Hedley |
X-ray diffraction (XRD) is the single most important method for identifying the variety of crystalline mineral species present in the clay fraction of soils. (Brown, 1961; Brindley & Brown, 1980; & Dixon & Weed, 1989; Bergaya, Theng & Lagaly, 2005).
The basic principle of XRD is that each crystalline substance has a characteristic arrangement of atoms which diffracts X-rays in a unique pattern. X -ray reflection takes place from lattice planes according to Bragg's Law:
nλ = 2d sin θ |
|
| where | d is the lattice spacing |
| λ is the wavelength of the X-rays | |
| θ is the glancing angle of reflection | |
| n is the order of the reflection, which can be any whole number. | |
Experimentally, λ is determined by the type of X-ray tube and thus is a known constant. Scanning a range of angles of reflection with a detector therefore gives a pattern of peaks at certain spacings and intensities that are characteristic of the minerals present. As d is proportional to 1/sin θ the diffraction angles (2θ) can be converted to d spacings by means of standard tables.
The XRD method is non-destructive and requires only small amounts of material.
Procedure
X-ray slides prepared as detailed in Step 6 are placed in sample holders, stacked in the sample magazine, loaded into the sample changer, and an X-ray diffractogram obtained. Our X-ray diffractometer is a Philips PW 1729 equipped with a broad focus cobalt X-ray tube. It operates with a PW 1775 sample changer that can accommodate up to 42 samples.
Non-orientated powder samples of sand, silt, clay or whole soil can also be run. These are finely ground with an agate pestle and mortar and packed into sample holders using the Philips PW 1781/80 back-loading powder sample holder and the Philips PW 1770 holder mounting clamp.
After packing , the powder holders are stacked in the magazine, loaded into the sample changer, and an X-ray diffractogram is obtained.
For heavy mineral samples or small samples of sand fraction, a few milligrams of the sample are finely ground with an agate pestle and mortar and mixed to a slurry on a glass slide with acetone, evenly distributed over the whole slide, and the acetone allowed to evaporate.
Interpretation
The main peaks in XRD patterns of oriented clay mineral specimens are usually attributable to the basal (001) rejection of minerals. The crystalline clay minerals belong to two distinct classes: 1:1, and 2:1 layer silicates. (See Table 3).
1:1 layer silicates comprise layers consisting of one sheet of silicon-oxygen tetrahedra condensed with one sheet of aluminium-oxygen octahedra. The oxygens of the octahedral sheet are either linked to Si atoms in the adjacent tetrahedral sheet or are present as hydroxyls. The 1:1 layers are stacked together so that the repeat distance (basal spacing) is ≥ 0.72nm. The basal spacing of kaolinite is about 0.72 nm and that of hydrated halloysite is 1.01 nm. Kaolinite consists of relatively large platey particles, often with a hexagonal shape, whereas halloysite particles are often tubular, although spheriodal and lath-like forms may also occur. It is possible to expand some halloysites to about 1.1 nm by treatment with glycerol (Table 3).
The unit layer of 2:1 layer silicates is based on that of mica. It consists of one sheet of aluminium ions in octahedral co-ordination with oxygen (octahedral sheet), sandwiched between two sheets of silicon-oxygen tetrahedra (tetrahedral sheets). In most micas, e.g. muscovite and biotite, the 2:1 layers are stacked so the repeat distance is 1.0 nm. The negative layer charge (~1 per formula unit) arises from substitution of Al3+ for Si4+ in the tetrahedral sheet. This charge is balanced by K+ ions occupying interlayer sites.
Illites may be considered clay-size micas with some replacement of interlayer potassium by other cations such as calcium, sufficient to cause a slight disorder in the layer stacking. The basal peak is, centred at 1 nm, is broader than those of muscovite and biotite. In practice, however, there is little or no clear distinction between the 1 nm peaks illite and mica and no differentiation is made between these two minerals. Mica is used as the term for both.
Vermiculites are 2:1 layer silicates with a mica-like structure, in which all of the potassium in the interlayer spaces has been replaced by other cations, that are usually hydrated. Consequently, the basal spacing is greater than 1 nm and its value can vary depending on the nature of the interlayer (exchangeable) cations. Vermiculites exhibit a characteristic basal spacing of 1.4 nm when the exchangeable cation is magnesium. Some vermiculites will collapse to 1.0 nm on heating. This collapse generally takes place at temperatures below 550°C (Table 3).
Vermiculites that collapse to 1.0 nm on potassium saturation are simply referred to as ‘vermiculite’, whereas vermiculites that only collapse to 1 nm on heating are termed ‘hydroxy’ interlayered vermiculite, (HIV) (Barnhisel 1977).
Smectites also have a 2:1 layer structure but substitutions in both the tetrahedral and octahedral sheets give rise to a negative layer charge that is smaller than that of mica or vermiculite. As a result, the interlayer forces are very much weaker than in vermiculites, and some smectites can expand in water. Other polar liquids such as ethylene glycol and glycerol can also enter the interlayer space and cause expansion. In the presence of glycerol, magnesium-saturated smectites give a basal spacing of about 1.8 nm, while potassium-saturated specimens show a basal spacing of about 1.0 nm (Table 3) at normal atmospheric humidity. The most common smectites in soils are montmorillonite, which is Mg-rich; beidellite, which is Al-rich; and nontronite, which is Fe-rich.
The structure is similar to that of vermiculites, except that the space between the layers is completely occupied by magnesium hydroxide (the "brucite layer") or aluminium hydroxide (the "gibbsite layer"). Chlorites give a basal spacing of 1.4 nm that does not collapse on heating. The sequence of layers in chorites, especially those with "gibbsite" layers, is similar to that of kaolin minerals. As a result it is often difficult to distinguish chlorites from kaolin by X-ray diffraction. The brucite or gibbsite layer can, however, be dissolved by dilute acid, while kaolin is resistant to this treatment.
Interstratified Minerals. Regularly interstratified minerals can occur in which one layer is of one type, e.g. 1.0 nm, and the adjacent layer of another type, e.g. 1.4 nm. These regularly interstratified minerals have peaks with particularly high spacings that derive from the sum of the (001) spacings of their constituents. The second order basal spacing of such minerals in the 1.0–1.6 nm region is generally half the sum of that of the two constituents. Examples are: mica-vermiculite (2.4 nm) consisting of alternate mica- and vermiculite-type layers; and chlorite-vermiculite (2.8 nm) consisting of alternate chlorite- and vermiculite-type layers.
| Table 3 Clay minerals basal (001) spacings (nm) after various treatments | ||||||
| I Discrete minerals | ||||||
| Mineral | Mg2+ air |
Mg2+ glycerol |
Treatments K+ |
K+ Heated 550°C |
Formamide |
|
| (a) | Mica* | 1.0 |
1.0 |
1.0 |
1.0 |
- |
| (b) | Chlorite | 1.4 |
1.4 |
1.4 |
1.4 |
- |
| (c) | Vermiculite | 1.4-1.0 |
1.4 |
1.0 |
1.0 |
- |
| (d) | HIV** | 1.4 |
1.4 |
1.4 |
1.0 |
- |
| (e) | Smectites | 1.5-1.0 |
1.8 |
1.0 |
1.0 |
- |
| Kaolinite | 0.72 |
0.72 |
0.72 |
No peak |
0.72 |
|
| Halloysite | 0.72 |
1.0-1.1 (not at all) |
0.72 |
No peak |
1.04 |
|
| II Regularly interstratified minerals. Note: the key to their identification is the presence of high spacing (i.e. 2.4, 2.8, and 3.2 nm) 001 peaks and 002 spacings. | ||||||
| Mineral | Mg2+ air |
Treatments |
K+ |
K+ Heated 550°C |
||
| (f) | Mica -vermiculite | 1.2 + 2.4 |
1.2 + 2.4 |
1.0 |
1.0 |
|
| (g) | Mica-HIV | 1.2 + 2.4 |
1.2 + 2.4 |
1.2 |
1.0 |
|
| (h) | Mica-chlorite | 1.2 + 2.4 |
1.2 + 2.4 |
1.2 |
1.2 |
|
| (i) | Chlorite-vermiculite | 1.4 + 2.8 |
1.4 + 2.8 |
1.2 |
1.2 |
|
| (j) | Mica-smectite | 1.2 + 2.4 |
1.4 + 2.8 |
1.0 |
1.0 |
|
| (k) | Chlorite-smectite | 1.4 + 2.8 |
3.2 |
1.2 |
1.2 |
|
| III Irregularly interstratified minerals | ||||||
| Mineral | Mg2+ air |
Treatments Mg2+ glycerol |
K+ |
Heated 550°C |
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| Interlayered hydrous mica | 1.0-1.4 |
1.0-1.4 |
1.0-1.4 |
1.0-1.2 |
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| *Mica is generally indistinguishable from illite and the two terms are considered synonymous, with mica being the preferred term | ||||||
| **HIV = Hydroxy interlayered vermiculite | ||||||
(note: 1 nm = 0.1 Å)
Irregularly interstratified minerals can also occur. They do not have spacings in the 2.4–3.2 nm range, but instead give peaks in the 1.0–1.6 nm range with spacings reflecting the relative contributions of the two or more constituents. They may be named after these constituents, if known, or else termed "interlayered hydrous micas" (see Table 3).
Differential thermal analysis (DTA)
Introduction
Differential Thermal Analysis (DTA) measures the exothermic and endothermic effects that occur when a sample is either heated or cooled, e.g., water loss, oxidation of organic matter, recrystallisations, sheet inversion, etc.
Quantitative estimates of percent gibbsite and kaolin in a sample can be obtained using DTA. Compared with values from XRD, those from DTA are more precise and subject to less variation from differences in crystallinity and particle size of the clay minerals.
Instrument
A TA Instruments Q600 Simultaneous Differential Thermal Analyser (SDT) is used for differential thermal analysis (DTA) of clays and soils.
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| TA Instruments Q600 Simultaneous Differential
Thermal Analyser. Photo: C Hedley |
Interpretation
Once the curve has been obtained it is compared with curves of standard pure mineral specimens and those reported in the reference literature (e.g. Mackenzie 1970; Dixon & Weed 1989; Bergaya, Theng & Lagaly 2005). A qualitative analysis of the minerals present may then be established.
A quantitative estimate of gibbsite and kaolin is made by measuring peak heights or areas under characteristic endothermic peaks at temperatures of 280–330°C for gibbsite and 520–560°C for kaolin and by obtaining the concentration of the minerals present from standard graphs.
Chemical dissolution for minerals of short range order
Introduction
Several physio-chemical techniques (e.g., DTA, IR) have been used for the qualitative and quantitative analyses of allophane and imogolite in soils. The results, however, have not be completely satisfactory. For this reason, we have developed a chemical dissolution method based on Tamm's oxalate extraction (Tamm 1922) to estimate allophane and/or imogolite concentration in a soil sample. The acid-oxalate extraction, detailed in Blakemore et al. (1987) also provides a method of estimating ferrihydrite concentrations.
Allophane and imogolite
The method for allophane and/or imogolite estimation uses acid-oxalate-extractable Al and Si (Alo and Sio) values and pyrophosphate-extractable Al (Alp) values for whole soil samples (Parfitt and Wilson 1985). The Al/Si ratio is calculated from (Alo-Alp)/Sio and then the allophane and/or imogolite concentration of the whole soil is calculated by multiplying Sio values by the appropriate factor for the Al/Si ratio. A few key values are:
Al/Si ratio |
Multiplying factor |
||
1.0 1.5 2.0 2.5 3.0 |
5.0 6.0 7.5 9.0 11.5 |
||
Ferrihydrite
The ferrihydrite concentration of a whole soil sample can be estimated by multiplying the acid-oxalate-extractable Fe values (Feo) by 1.7 (Childs 1985), i.e. % ferrihydrite in whole soil = Feo × 1.7.
Conversion of whole soil values to clay fraction values
The amount of allophane and/or imogolite and ferrihydrite in the clay fraction of soils can be calculated from the content of these minerals in the whole soil and the clay content.
References
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Appendix: Reagents etc. used in mineral analyses
Bromophenol blue indicator: 1% in ethyl alcohol.
Ceramic tiles: Purchased or made to suit instrument. A possible supplier is Analytron Pty Ltd., P.O. Box 521, ACT Australia.
Citrate Reagent: 75 g C3H4(OH).(COONa)3.2H2O dissolved in 1 L of distilled water and the pH adjusted to 7.3 by addition of a few drops of saturated citric acid solution.
Clove oil: Used as supplied (light straw-coloured variety is preferable to darker varieties).
Formamide: HCONH2. Used as supplied. Sprayed from any convenient aerosol spray bottle.
Glycerol: CH2OH.CHOH.CH2OH. 10% solution in distilled water sprayed from any convenient aerosol spray bottle.
Hydrochloric acid 1:1: Concentrated HCl diluted with an equal volume of distilled water.
Hydrogen peroxide: 100 vol. Used as supplied.
Lithium chloride: 100 g LiCl dissolved in 1 L of distilled water.
Magnesium chloride: 100 g MgCl2 dissolved in 1 L of distilled water.
Potassium bromide: KBr. Used as supplied. However, the KBr used must give satisfactory blanks. Heating above 500°C is often sufficient to ensure satisfactory results.
Potassium chloride: 100 g KCl dissolved in 1 L of distilled water.
Sodium bicarbonate: 70 g NaHCO3 dissolved in 1 L of distilled water.
Sodium chloride: 350 g NaCl dissolved in 1 L of distilled water.
Sodium dithionite: Na2S2O4.H2O. Used as solid, as supplied.
Sodium polytungstate (SPT): 3Na2WO4.9WO3.H2O powder dissolved in water and density adjusted to 2.80 g cm-3 for heavy minerals, and to 2.45 g cm-3 for volcanic glass. To increase density, either add more SPT powder to the solution or evaporate water from the solution. To decrease density add water.
X-ray glass slides: (25 × 75 × 0.1 mm) microscope slides cut into 3 equal portions, i.e. 25 × 25 × 0.1 mm.