Particle size distribution of mineral species, a measure of chemical weathering and a control of chemical activity through specific surface

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1 Page 100 Chapter 3 MINERAL FRACTIONATION FOR SOILS Particle size distribution of mineral species, a measure of chemical weathering and a control of chemical activity through specific surface Contents:... Page Size limits employed (Fig. 3-1)...101,103 Fractionation aids qualitative and semi-quantitative analysis...102,104 Fractionation aids size function of species Equivalent spherical diameter SEPARATION OF MINERALS BY SIEVE AND BEAKER SEDIMENTATION Flow sheet for mineral segregation (Fig. 3-2) Fraction sizes (Table 3-1) Time for gravity sedimentation (Table 3-2); nomograph (Fig. 3-3)...114,115 Procedure Tabulation of results SEPARATION OF MINERALS BY TUBE CENTRIFUGE Time required for sedimentation (Table 3-4); nomograph (Fig. 3-6)...128,130 Procedure Angle head centrifuge SEPARATION OF MINERALS BY THE SUPERCENTRIFUGE Elutriation principle in Sharples supercentrifuge Flow rate for Sharples supercentrifuge; nomograph (Fig. 3-12)...155,158 Procedure QUESTIONS The mineralochemistry of soils is made complex by the occurrence of numerous mineral species, amorphous inorganic substances, and organic complexes mixed together in various proportions in different soils. This situation is analogous to the complexity of the organochemistry of soil, arising also because of the complex mixture of molecular species and variously altered or polymerized organic substances mixed together in soil organic matter (Chapter 12). The approach to mineralochemical analysis is to separate the mineral series or species as much as possible prior to application of various soil analytical techniques. Selective dissolution of various oxides (Chapter 2) helps with mineral dispersion and segregation into size fractions. The latter segregation of size fractions, with resultant concentration of mineral series or species, aids X-ray diffraction analysis (Chapter 4), thermal analysis (Chapter 6), specific surface interpretation (Chapter 7), examination with the electron microscope (Chapter 9), examination by means of the polarizing microscope (Chapter 10), and further selective dissolution of mineral species as well as the allocation of chemical elements to mineral species (Chapter 11).

2 Page Size limits employed. The colloid chemistry of several fine fractions of soils has been studied. * The use of a 0.2um limit being given preference to the 0.lum limit for reasons stated as follows: In the first place the determination is quick, involving rotation at 3500 revolutions for about 45 min in the centrifuge used. Secondly, this point falls into line with the other points chosen for ordinary mechanical analysis, being 0.1 of 2 um. Thirdly, there are good grounds for choosing 200 nm as the upper limit of the truly colloidal matter of clays, i.e., colloidal matter as the colloidal chemist understands the term. Lastly, as will be shown in the paper which follows, at <200 nm the figures obtained using various alkali cations to saturate the clays show no appreciable variation, whereas at 100 nm and 50 nm the lyotrope series is often in evidence. Various clay fractions finer than the 2um limit had previously been employed for elemental analysis of soil colloids for purposes of characterization, separation being effected by ion settling or supercentrifugation. # Differences in refractive indexes were noted** in minerals of the finer clay fractions. Three to 7 times greater exchange capacity was found ## in the clay less than 0.2um than in the coarse clay (2 to 0.2um in diameter). Separation at 0.2um diameter limits has been advocated. *** In our laboratory, the size fractions equivalent ### found to be most valuable for X ray diffraction analysis and other minera1ochemical procedures are the following: 1. Coarser sands um (microns) 2. Very fine sand to 50 um 3. Coarse silt to 20um 4. Medium silt to 5 um 5. Fine silt... 5 to 2 um 6. Coarse clay... 2 to 0.2 um 7. Medium clay to 0.08 um 8. Fine clay um (80 um) *Marshall, C. E. (1931) J. Soc. Chem. Ind., 50T:444. # Bradfield R. (1923) Mo. Agr. Exp. Sta. Res. Bul. 60; Brown and Byers (1932) U.S.D.A. Tech. Bul ** Marshall, C. E. (1930) Trans. Faraday Soc., 26:173. # Truog, E. et al. (1937) S.S.S.A. Proc., 1:175. *** Nagelschmidt (1939) J. Agr. Sci., 29:477. ### The size fractions are greatly affected by turbidity currents (Fig. 3 8) and density decreases by hydration (Table 3 6), and so the values given are arbitrary equivalent spherical diameters (para 3 8)

3 Page a. The coarser sands may be further subdivided by sieving at the U.S.D.A.# or International limits (Fig. 3-1). Although narrower and more numerous size fractions have been obtained** (Fig. 3-1), seldom is additional minera1ochemical information obtained by the separation of more fractions than the 8 fractions listed above. Types of fractionation other than size segregation generally are best applied to the size fractions rather than the whole soil. Thus the separation of minerals of heavy specific gravity for study with the polarizing light microscope (Chapter 10) is most frequent-1y applied to the very fine sand fraction (100 to 50 microns) Studies made in this laboratory of subdivisions of the clay fraction at 0.1, 0.08, 0.06, 0.04, and 0.02u indicate that a separation at the 0.08u limit achieves as much segregation of minerals as generally can be obtained by more size-segregation in this range. Little increase in mineralochemical information is gained by determination of particle size distribution within the fraction less than 0.08u in diameter. The 0.08u separation is relatively fast and convenient with either a supercentrifuge or high speed angle head centrifuge. Departure from a logarithmic interval in the choice of the 0.08u limit is fully justified by the distribution of minerals found by experimentation. A relatively small part of the clay fraction occurs below 0.02u. The thinnest montmorillonite particles seen in the electron microscope were estimated ## to be 0.002u thick, but the width of the thin platy particles gives a fairly large equivalent diameter (para 3-8) Use of the three fractions in the silt size range and of the three fractions in the clay size range is consistent with the abundance and importance in soils of these fractions, especially in comparison to sands which are often fractionated into more than three fractions. The use of these three subfractions of silt and clay is strongly supported as optimum by many workers reporting on minera1ochemical and weathering interpretations. The silts are very important quantitatively, frequently making up 60 to 85% of a silt loam, The 20u point divides the silt fraction of soils developed on loess and some tills about equally. The 50u point is important as the approximate upper limit of soil material of loessial origin. The 50-20u silt subfraction is excellent for Zr measurements. *** 3-5. Fractionation aids qualitative analysis. Separation of mineral species out into as simple as possible mixtures provides for as few sets of diffraction peaks as possible in each diffraction pattern, thus facilitating qualitative analysis (para 4-136), The diffraction intensity from each component, relative # Knight, S.S.S.A. Proc., 2:592 (1938). ** Steele and Bradfield, Soil Surv. Assoc. Bul., 15:88 (1934); White-side and Marshall, S.S.S.A. Proc., 4:100 (1940). ## Shaw, J. Phys. Chem., 46:1032 (1942). *** Fanning, D. S., and Jackson, M, L. (in manuscript).

4 Page 103 Thinnest Montmorillonite plates Shaw (1942)

5 Page 104 to the total, is greater and each peak is as specific as possible, For example, feldspars can be determined better in the coarse clay (2 0.2 micron fraction) than in the whole clay fraction below 2 microns. X-radiation of fractions of wide size distribution has often resulted in the erroneous attribution of the diffraction analysis of the coarser size particles to the entire fraction including the finer fractions. Some interstratified layer silicates of the 1:1, 2:1, and 2:2 families have low (00L) diffraction intensity, and this phenomenon can best be recognized and the minerals identified and estimated if they are concentrated into fractions exclusive of more perfectly crystallized minerals such as quarts, kaolinite, and mica, which give strong diffraction patterns and mask the diffraction of the interstratified layer silicates (para 4 152). Also, montmorillonite series minerals often (though not always) occur abundantly in the fraction less than 0.08 microns in diameter, but have often been overlooked in diffraction patterns of the whole fraction of soil particles less than 2 microns in diameter. Studies of Bray* and coworkers...showed the inadequateness of the usual clay examination where fractionated material is not used. Minerals were found which were not originally indicated by ordinary methods, because they were masked by the larger amounts of the dominant minerals present. Neither the petrographic nor x ray studies of the whole clay (C 2u) indicates, for example, the presence of beidellite (series) when either the sericite like clay mica (illite series) or kaolinite is the dominant mineral. The nondetection of significant amounts of montmorillonite in 2u clay, which was abundant in illite, was noted also in soils by the author and co workers #. Large amounts (up to 40% of the clay fraction**) of hydrous micas of poorly resolved basal spacings may not be detected in large amounts of montmori1lonite or kaolinite Fractionation aids quantitative analysis. The criterion of quantitative x ray diffraction analysis of soil minerals is the measured intensity of diffraction at angles which are characteristic for each mineral. The intensity of diffraction is a function of the mineral concentration in a mixture, but is a function also of the particle size and degree of crystal perfection (freedom from distortion) of each individual mineral. Finer particles have weaker diffraction intensities which cannot be evaluated accurately in the presence of stronger diffraction intensities of larger particles of the same mineral species. The only way to obtain a weighted average analysis from diffraction intensity is to x radiate each relatively narrow size fraction separately. If each individual mineral series could be segregated into a single discrete physical fraction, each could be identified by x-ray diffraction, regardless of variation in diffraction intensity per unit weight as influenced by particle size and crystal perfection of sample and standard ##. The amount present in a monomineral fraction could be determined quantitatively by weighing. Segregation of amorphous material (Chapter 11) along with one of the separated crystalline series would have to be detected. In practice, segregation of minerals is not complete enough to produce monomineral fractions. Obviously, segregation of mineral series and species as much as possible into separate fractions is a great advantage to quantitative a1location of elemental analysis (Chapter 11). For example, having two layer * Bray J., Am. Ceram. Soc., 20:257 (1937), who is summarizing work also of Grim and Bray, J. Am. Ceram. Soc., 19:307 (1936), and Bray, Grim, and Kerr, Bul. Geol. Soc. Am., 46:1909 (1935) # Jackson and Hellman, S.S.S.A. Proc., 6:133 (1942): Pennington and Jackson, Ibid., 12:452 (1948); Jackson, Natl. Acad. Sci. Natl. Res. Council Pub. 327, p. 218 (1954). ** Hellman,, et al., S.S.S.A. Proc., 7:194 (1943).

6 Page 105 si1icate fractions, one higher in K content (often obtained in the coarse clay) and the other lower in K content (often obtained in the fine clay) is a distinct analytical advantage over having a single fraction with an intermediate content of K. Similar advantages of mineral segregation and concentration into different fractions accrue to CEC, thermal, surface, and microscopic analysis Fractionation aids study of size function of mineral species. The size distribution of each mineral species gives information about the extent and processes of geochemical weathering* of minerals in soils and rocks. Soils which are only slightly weathered may have significant amounts of primary minerals such as albite in the fine clay fraction. In soils of somewhat greater weathering, montmori1lonite and (or) mica may be the most abundant minerals in the fine fraction. In more highly weathered soils, oxides and hydroxides of Fe, Al, Mn, and Ti are concentrated in the fine fraction. Amorphous aluminosilicates and oxides often separate into the fine fraction and this increases the precision of diffraction analysis of the coarser fractions. In the medium clay fraction, kaolin, mica, and vermiculite are concentrated together with lesser amounts of quartz, chlorite, and interstratified layer silicates. Concentration of kaolin in the medium clay of montmorillonitic soils has been reported # Quartz, kaolin, and mica minerals (giving a 10A diffraction peak in the presence of glycerol) typically occur in the coarse clay fraction. Little separation of minerals species has been found to occur through the separation at l um; also, the long interval from 2 to 0.2 um includes a satisfactory percentage of the clay. Quartz, feldspars, vermiculite, chlorite and mica occur in the fine silt. Highly weathered soils, such as Ferruginous Humic Latosols, have crystals of geochemical secondary minerals such as hematite, gibbsite, and anatase which have grown sufficiently large to fall in the silt size range. The particle size limit below which the feldspars and quartz disappear, the size extinction function **, is characteristic of the degree of weathering. In Gray Brown Podzolic soils of the North Central Region of United States, the feldspars tend to become extinct at a particle diameter of 1 or 2 um, while quartz tends to become extinct at a particle diameter of 0.1 um. In Latosols, wherein weathering has been more extensive, quartz may occur as particles of 10 to 50 um, but becomes extinct at particle diameters of 5 or 2 um. * Jackson and Sherman (1953) Adv. Agron., 5:219- # Coleman and Jackson (1946) S.S.S.A. Proc., 10:381- ** Jackson et al. (1948) J. Phys. Coll. Chem., 52:1237-

7 Page Equivalent spherical diameter. Soil particles are often plate-shaped. According to convention, the particle diameter referred to herein is the equivalent spherical diameter of the particles. For diameters smaller than 20 um this refers to spheres having the same settling velocities, according to Stokes law for streamline flow (para 3-13), as the actual particles. For particle sizes of 50 um and above, the equivalent spherical diameter refers to the nominal sieve opening size. Because the particle densities vary from one mineral species to another, only an average density can be employed for a given size separation. The density is taken as 2.65 for all diameters down to 0.2 um. For finer sizes the density, given in tables, is based on the mineral density in a heavy liquid such as tetrabromoethane, which of course is greater than the mineral density when the mineral is highly solvated in a water suspension. The diameters referred to therefore are equivalent spherical diameters of particles having the stated density. Particles of greater density, such as of hematite, will be separated at smaller actual diameters than stated. Particles of lower than the assumed density will be separated at diameters greater than the stated diameter. Each fraction has an upper and a lower "limiting diameter." The upper size limit in sedimentation is determined by the maximum size of particle which, starting at the top of the suspension, can remain in suspension during the sedimentation time. SEPARATION OF MINERALS BY SIEVE AND BEAKER SEDIMENTATION The sieve type of separation of cleaned mineral particles can be employed down to a particle diameter of about 50 um (Fig. 3-2). Wet sieving involves some decrease in nominal sieve opening (Table 3 1) of the sieve, and the final sizing is therefore done by dry-sieving The settling times (Table 3-2) have been calculated by means of Stokes law for streamline or viscous flow about a falling spherical body, wherein the resistance to fall is 3 πdnv, in which D is the particle diameter in cm, v is the settling velocity in cm/sec, and n is the viscosity in poises. This law is combined with the differential specific gravity, delta-s = sp-s1 (particle relative to liquid), and the gravitational constant, g, the value of which is 980 cm/sec 2, thus giving the working form of Stokes law, 2 g( s p s1 ) D 2 ν = = k1d (3-1) 18n Since v = h/t, in which h is the depth of fall in cm and t is the time in seconds, 18nh ( (3-2) 2 t = ) = 2 2 g sp s1 D D The nomograph (Fig. 3-3) gives in more detail the time for sedimentation (Table 3-2) for various sizes of particles. k

8 Page 111 Fig. 3-2.

9 Page 113 Table 3-1. Sand, silt, and c1ay fractions obtained for analysis by diffraction thermal, microscope and other methods Fraction Name Size range, microns Technique of obtaining fraction Very coarse sand Sieve round-hole, on 1- mm, through 2-mm Coarse sand Sieve, round-hole, on 0.5mm Sand Medium sand Sieve, screen, on 0.25 mm (60 meshes per inch) Fine sand Sieve screen, on 0.1 mm (140 meshes per inch) Very fine sand l00-50 Sieve, screen, on 0.05 mm (300 meshes inch) Coarse silt Sieve, decantation Silt Medium silt 20-5 Decantation, centrifuge Fine silt 5-2 Decantation, centrifuge Coarse clay Decantation, centrifuge Clay Medium c1ay Decantation, supercentrifuge Fine clay Decantation, supercentrifuge

10 Page 114 Table 3-2. Time required # at various temperatures for gravity sedimentation of soil particles, a specific gravity of 2.65 being assumed and a 5-cm suspension depth being employed. Particle Time required for sedimentation at a suspension temperature of limiting diameter 20 C 25 C 30 C 35 C Microns Hr Min Sec Hr Min Sec Hr Min Sec Hr Min Sec *Sedimentation times for a more extensive range of partic1e sizes and temperatures are given in the accompanying nomograph.

11 Page 115 Fig Nomograph of particle settling time under gravity acceleration. From Tanner and Jackson, S.S.S.A. Proc. (1947) 12:60 (1948).

12 Page 123 Apparatus Needed apparatus includes a large funnel supporting a 50 micron (300 meshes per inch) sieve (figure 3 4); 1-liter, 600-ml, and tall form 100-mi beakers; water washing bottle equipped with a coarse tip; sieve set (table 3-1) for separation of sand fractions; 8 cm evaporating dishes; glazed paper; and sample vials. Reagents Reagents needed for the segregation procedure include distilled water; an 18-liter stock of dilute Na2CO3 solution of ph 9.5 in distilled water (about 2 gm of Na2CO3 in 18 liters of water); and acetone in a wash bottle. Procedure * The flow-sheet for the mineral segregation procedure should be consulted (figure 3-2). The soil has been thoroughly dispersed by H2O2 treatment, removal of reduction soluble iron oxides, and by saturation with Na (para 2-65g; 2-72) Separation of sands from silts and clays (at 50u). To facilitate the separations of the various finer size fractions by centrifugation later, the total sand fraction (grains greater than 50 microns in diameter) are removed from the suspension of dispersed soil by wet sieving. First, the soil in the centrifuge tube or beaker is thoroughly dispersed by shaking and tituration until all visible aggregates are broken up and the individual particles seem discrete. A 50 micron sieve is supported on a glass funnel, so as to deliver into a 600-ml beaker (figure 3-4). The thoroughly dispersed and freshly stirred soil suspension is allowed to stand (exactly) 40 seconds for each 10 cm (4 inches) of its depth, and is then decanted through the sieve. The purpose of this action is to permit particles greater than 50 microns to have settled out so as not to obstruct the sieve as particles less than 50u are decanted through the sieve. This procedure is remarkably effective compared to use of either a longer or shorter settling time. When most of the suspension has been poured through, the sands are vigorously stirred in more distilled water, another 4 second per cm settling is permitted, and the particles less than 50u are poured through the sieve as before. The sands are then swirled in more distilled water, and the mixture quickly poured into one side of the sieve. All grains are then washed onto the sieve, by means of a coarse jet of water and with the aid of a policeman. The remaining silt and clay are washed from the sand on the sieve by means of a coarse jet of water, as the sand is displaced from one side of the sieve to another. Finally, the bottom of the sieve is washed clean with a jet of water After the sands are washed clean of silt and clay, as indicated by a clear washing solution coming through the sieve and funnel (and complete absence of aggregates on the screen), the sieve and sands are washed with two or three small portions of acetone delivered from a wash bottle. A few silt particles less than 50 microns lodge in the sieve and will not * Portions of this procedure have been published elsewhere, Jackson et al S.S.S.A. Proc. 14:77 (1950).

13 Page micron sieve openings (300 meshes per inch) Suspension of particles less than 50 microns in diameter (silt and clay) Fig Apparatus for wet-sieving of dispersed soil suspension to effect the 50-micron separation,

14 Page 125 pass in the wet condition because of the decreased effective diameter of the sieve openings due to its being wet. The sieve therefore is dried in an oven for a few minutes and then tapped over the same funnel to pass the remaining silt particles. The silt and c1ay particles are washed from the funnel into the beaker and the funnel is removed The sands (greater than 50u) remaining on the sieve are carefully transferred to a glazed paper and then to a weighed bottle, are dried and weighed, and set aside for x ray diffraction, light microscope analysis (Chapter 10), and selective dissolution analysis for quartz, feldspars and micas (Chapter 11). The sands dry to constant weight at l00oc in a few minutes. The weight of the sand fractions is recorded (table 3-3) and the percentage is calculated later on the basis of the total recovered mineral portion of the sample (particles less than 2 mm). In preparation for petrographic or diffraction analysis, the sands are further subdivided into coarse and fine sub fractions by means of well cleaned sieves (outlined in table 3-1) and then into heavy, medium and light fractions by the heavy liquid specific gravity technique (Chapter 10) Separation of coarse silt (50 to 20u). The suspension of silt and clay (particles less than 50 microns) contained in the 600-ml beaker, is next separated at the 20-micron particle diameter limit. To do this, the suspension is stirred and then allowed to stand quietly for at least five minutes (but not over 10 minutes) for each 10 cm (4 inches) of depth. The supernatant liquid is then decanted into a 1-liter beaker, care being exercised to prevent the decantation of any of the settled particles. This and the combined subsequent decantates are labeled less than 20u. The sedimented portion is brought into suspension again by the addition of dilute Na2CO3 solution of ph 9.5 (solution of ph 8 to 10.3 NaOH are used if needed to disperse Latosols, rich in kaolinite, halloysite, gibbsite, and iron oxide; for soils high in allophane, HCl dispersion is employed; para 2-78). This suspension is then quantitatively transferred to a tall-form l00-ml beaker having a mark encircling it at a height of 5 cm from the bottom, inside measurement. The suspension is diluted to a depth of exactly 5 cm. This suspension is again allowed to settle, this time for 3 minutes, and the supernatant liquid decanted as before. The sediment is again dispersed in Na2CO3 solution, to a 5 cm depth, and the suspension is allowed to settle for about two minutes, the exact time for the 20 micron limiting diameter at the existing temperature being obtained from table 3-2 or more precisely by means of a nomograph (figure 3 3) Sedimentation and decantation are continued until the finer silt and clay are separated from the coarse silt in the l00-ml beaker, usually requiring five or more decantations. For the last two times, the resuspension is made in pure distilled water, so as to rinse all of the Na2CO3 (or HCl in allophanic soils) from the sediment consisting of coarse silt. The coarse silt, remaining in the tall-form beaker, is washed into an evaporating dish. The particles are allowed to settle, the excess liquid decanted, and the coarse silt fraction dried and weighed. The weight of coarse silt is entered in table 3-3

15 Page 126 Table 3 3. Recovery of mineral fractions, expressed in grams and calculated as percentage of mineral portion, of mineral plus oxide portion, aid of original sample weight of soil and the percentage based on the mineral portion of soil less than 2 mm is calculated.

16 Page 127 * J. Am. Chem.Soc., 45 : 2910 ( 1923) ; also given by D Steel and Bradfield, Am. Soil Survey Assoc. Bul., 15:88 (1934).

17 Page 128 Table 3-4. Time required* at various centrifuge speeds and temperatures for sedimentation of particles, an International No. 2 centrifuge with No. 240 head being employed with l00ml tubes, a 10-cm suspension depth, a 1-cm of sediment, and a 9-cm net depth of fall.

18 Page 129 With 1 cm of sediment, the International No. 2 centrifuge, R = 25 cm and. = 16 cm. At 25 C n = One of the values in Table 3 4 may be recalculated. as an exercise in the use of equation 3 7. More detailed information on sedimentation times is given in the nomograph (figure 3 6). Equations 3 6 and 3 7 hold for the regular tube centrifuge (para 3 49), for the angle-head centrifuge (para 371), for the supercentrifuge (para 3 80), and also for the 2 layer method (para 3 73). The procedure given (para-3 49) is the single layer method. of separation by centrifugation (figure 3 5). The principles of limiting diameter (para 3 8) applicable to this method are the same as for beaker decantation with gravity sedimentation (para 3 35). The suspension density increases with depth owing to partial sedimentation of particles and this stabilizes the column against mechanical mixing The two layer method of separation (para 3 73) carries differential density a step further. A number of alternative formulas for the supercentrifuge have been developed. (para 3 99) The centrifuge nonograph (figure 3 6) is prepared for a temperature of 20 C and. a particle specific gravity of 2.5. A specific gravity of 2.5 is found to be about right for siliceous soil in the particle size range of approximately 0.2 microns, as judged by the specific gravity found for particles of about this diameter by means of a heavy liquid. In the finer fraction, the observed specific gravity in heavy liquids falls to about 2.2. In water dispersion the colloidal particle specific gravity would necessarily be even lower. The picnometer specific gravity of dried layer silicate colloids is 2.6 to 2.8, but this is not applicable to highly hydrated colloidal particles which sediment with interlameller water attached. In the coarser sizes, the specific gravity of 2.65 is appropriate. Corrections, C T to various particle specific gravities for use at varied temperatures may be made by means of Table 3 5, as follows : Centrifugation time (at wanted specific gravity, temperature) Apparatus = Centrifugation time from nomograph C T (3-8) Apparatus needed for the size segregation procedure includes an International No. 2 centrifuge (International Equipment, Boston, Mass.) with No. 240 head, speed indicator, time clock; and a rack of eight l00-ml centrifuge tubes fitted with rubber stoppers (a shaker* for the rack of centrifuge tubes is a great convenience); glass rod with rubber ball plunger (figure 2 1, page 34); glass rod with policeman; and a 6-liter flask. Reagents Needed reagents include distilled water, Na 2C0 3 solution of ph 9.5.

19 Page 130

20 Page 131

21 Page 139 Procedure* The centrifuge separation procedure to be followed removes the clay particles less than 2u from the medium and fine silt, then the medium silt is separated from the fine silt. The flow sheet (figure 3 2) may profitably be reviewed in this connection Separation of silt from clay at 2u. The suspension of silt and clay consisting of the combined decantates in the l liter beaker, resulting from the 20u separation (para 3 36), is allowed to stand undisturbed for more than eight hours for each 10 cm (4 inches) of depth. An excess of 50% over the limiting time is employed for the first sedimentation. Or, to avoid the necessity of waiting for eight hours for each 10 cm, the entire suspension may be centrifuged as in the following paragraph. After the sedimentation period, the clay fraction (particles less than 2u in diameter) is decanted or siphoned into a 6 liter flask. This decanted suspension is labeled less than 2u The sedimented solids remaining after decantation of the clay from the 1-liter beaker are brought into suspension again in the ph 9.5 Na2CO3 solution and transferred to centrifuge tubes, to a suspension depth of 10 cm. The suspension is centrifuged for a time 50% in excess of 2.7 minutes at 750 rpm (International No. 2 Centrifuge with No. 240 head). The supernatant suspension, remaining after centrifuging, is decanted into the 6 liter flask, being combined with the first decantate (para 3-50 The sediment in the bottom of each tube is suspended into a small portion of the ph 9.5 Na2CO3 solution, by means of shaking or with the aid of a rubber plunger on a glass rod. The plunger is lowered in the suspension carefully and the sediment tapped gently with short strokes to avoid splashing the suspension over the top of the tube. The suspensions of one sample in the several tubes are combined by transfer to fewer centrifuge tubes or if possible to one tube as the successive centrifugation and decantations progress. The particles adhering to the stopper and plunger are washed into the suspension and the suspension depth is brought exactly to the 10 cm mark. Next, the tube is centrifuged at exactly 750 rpm for 2.7 minutes (table 3 4). Centrifuge speeds corresponding to various temperatures and other speeds can be found on the accompanying nomograph or by direct calculation (para 3 42) The processes of resuspension, centrifugation, and decantation are continued until the supernatant suspension is fairly clear after the centrifuging periods. Usually five to eight times are given in all. It has been found experimentally in unpublished studies by Dr. N. N. Hiellman and the author that over 90% of the clay yield is obtained in the first 3 centrifugings and that 97 to 99% of the clay yield is obtained in the first 5 centrifugings. Certainly never over 10 centrifugings should be required. Because of the gradual breaking up of the finer silt particles, particularly of mica and kaolinite during the process of resuspension, the suspension does not become entirely clear, even with a very large number of centrifugings. Segregation of *Portions of this procedure have been published, Jackson, et al., S.S. S. A. Proc., 14:77 (1950).

22 Page various minerals into separate fractions is probably more complete if the number of repetitions is limited to 5 to 8 so as to keep the physical fracture of the particles at a minimum The silt fraction (particles 20 to 2u in diameter) makes up the sediment. The medium silt is now to be segregated (flow sheet, figure 3-2) from the fine silt according to para 3-54; further separation of the clay fraction in the 6 liter flask is to be made according to para Separation of the medium from fine silt at 5u via centrifuge. The solids medium and fine silt particles 20 to 2u in diameter) remaining in the centrifuge tube are suspended in the ph 9.5 Na2CO3 solution as before, to a suspension depth of 10 cm. The separation of 5u may be accomplished either by centrifugation or by gravity sedimentation, but usually is effective by the centrifuge because of the shorter sedimentation time. (Under gravity, a 5u particle falls 5 cm in about 30 minutes, table 3 2.) The suspension is promptly centrifuged at 300 rpm for the appropriate time, approximately 2.7 minutes (the exact time is given in table 3-4 or the accompanying nomograph). The supernatant liquid is immediately decanted into a 600 ml beaker. Resuspension, centrifugation, and decantation are repeated until the separation is completed, usually requiring five centrifugation Water is substituted for Na 2CO 3 solution for the last two centrifugations. The medium silt (particles 20 to 5u in diameter) remains in the centrifuge tube, and is washed into a weighed bottle, dried, weighed, and kept for x-ray diffraction analysis. The yield is calculated as percentage of the minerals particles less than 2 um in diameter (table 3 3) The suspension of fine silt (particles 5 to 2u in diameter) is allowed to stand in the 600-ml beaker for 8 hours per 10 cm (4 inches) suspension depth, and the supernatant 1iquid is then decanted and combined with the clay fraction in the 6 liter conical flask (or discarded if perfectly clear). The fine silt is washed into a 125-m1 conical flask and kept in the moist condition for x ray diffraction analysis. A small aliquot of the fine silt is washed in 0.05 N HCl, dried, and weighed. The percentage of fine silt is calculated on the basis of the mineral particles less than 2 min in diameter (table 3 3) Separation of the coarse clay (2 to 0.2u) via centrifuge. The suspended 2u clay fraction (sometimes known as the total clay fraction) has been decanted into a 6- liter flask in the previous two separations. Two alternatives are available: (a) To proceed with the separation at 0.2 with the entire suspension volume (para 3 59) (most efficacious if the suspension volume of 2u clay is only 1 or 2 liters). Because eight 100 ml tubes hold a total working volume of only about 500 ml, the first separation at 0.2u requires two 35 minute runs per liter of suspension. (b) To reduce the suspension volume by means of a preliminary separation at 0.08u (para 3-58) prior to beginning the separation at 0.2u (most efficacious when the total volume exceeds 2 liters).

23 Page (c) Reduction of the volume can also be accomplished by flocculation of the clay suspension and decantation, but washing out the electrolyte used to produce flocculation is more time consuming than the preliminary separation by the supercentrifuge The Preliminary separation of the fraction less than 2u, is made at 0.08u by means of the supercentrifuge to reduce the suspension volume (in accordance with para 3 85, broken line in flow sheet, figure 3 2). The resulting effluent containing the fine clay particles less than 0.08u is then set aside while the separation at 0.2u (para 3 59) is completed on the sediment on the supercentrifuge bowl liner. The preliminary separation is at 0.08u instead of 0.2u because the accumulated large suspension volume would otherwise have to be passed through the supercentrifuge a second time for the 0.08u separation To begin the separation at 0.2u, the suspension of clay less- than 2u (from 6 liter flask, para 3 53, or bowl liner, para 3-90) is transferred to l00 ml centrifuge tubes with suitable policing and washing to effect complete recovery of the clay. The suspension, brought to a 10 cm depth in the tubes, is centrifuged in the international No. 2 centrifuge for the appropriate time at 2400 rpm, the time being about 30 minutes (table 3 4). The time and temperature relations for various centrifuge speeds may be obtained from the accompanying nomograph and further adjusted, if desired, for particle specific gravities (table 3 5) The supernatant liquid is decanted into a 3 liter flask and labeled less than 0.2u. The sediment in the bottom of the tubes is dispersed in ph 9.5 Na2CO3 (ph 8 to 10.3 NaOH in the case of kaolinitic clays or Latosols), and the suspensions are then combined into fewer or one centrifuge tube. (Recentrifuging of more than one tube as before may be necessary to reduce the volume of suspensions so as to go into one tube.) The particles adhering to the stopper or plunger are washed into the tube and the suspension depth again brought to exactly the 10 cm mark. The tube is centrifuged as before. The processes of resuspension, centrifugation, and decantation are continued until the supernatant liquid is fairly clear, after centrifugation Usually five centrifugations suffice. This process is essentially identical to that of separation of the clay fraction at 2u (para 3-51) except for higher centrifugation speed and longer time. The degree of completeness of separation ion as centrifuging progresses is about the same as for total clay (para 3 52) The coarse clay (particles 2 to 0.2u), remaining in the bottom of the centrifuge tube, is washed into a 125 ml conical flask and labeled; it is now ready for x ray diffraction analysis. A small aliquot of clay is washed in 0.05 N HCl and dried to calculate the percentage of coarse clay (table 3 3). The decanted suspension of the medium and fine c1ay is further separated 4 or more times at 0.08u (para 3 85).