Chapter XII: Introduction (11)

=243. Interpretation of Silt Analysis.=—The primary conceptions upon which the interpretation of the mechanical analysis is based may be briefly stated as follows:[161] The circulation of water in the soil is due to gravity, or the weight of water, acting with a constant force to pull the water downward, and also to surface tension, or the contracting power of the free surface of water (water-air surface), which tends to move the water either up or down, or in any direction, according to circumstances. There is a large amount of space between the grains in all soils in which water may be held, ranging from about thirty per cent in light sandy lands to sixty-five or seventy per cent in stiff clay soils. The relative rate of movement of water through a given depth of soil will depend upon how much space there is in the soil; upon how much this space is divided up, _i. e._, upon how many grains there are per unit volume of soil; upon the arrangement of the grains of sand and clay; and upon how this skeleton structure is filled in and modified with organic matter. It also appears that the ordinary manures and fertilizers change this surface tension, or pulling power of water; that they also change the arrangement of the grains, and consequently the texture or structure of the soil may be changed and the relation of the soil to water, through the effect of the ordinary manures and fertilizers in causing flocculation or the reverse.

=244. Number of Particles in a Given Weight of Soil.=—The approximate number of particles in the soil can be calculated from the results of the mechanical analysis by the following formula:[162]

(_a_/((π(_d_)³ω)/6)) ÷ A

Where _a_ is the weight of each group of particles, _d_ the mean diameter of the particles in the several groups in centimeters, ω is the specific gravity of the soil, and A is the total weight of soil. For the specific gravity of ordinary soils, the constant 2.65 may be used.

In using the formula the per cents are expressed as grams. Thus, if there were twenty per cent of silt, this would be taken as twenty grams, and if the results of the analysis added up ninety-seven per cent the whole weight of soil would be taken as ninety-seven grams. The diameter _d_ is taken as the mean for the extreme diameters taken for any group, for instance, for the silt this would be 0.003 centimeter, which is assumed to be the diameter of the particles in that group. This formula can only give approximate values, as the number of separations in a silt analysis must necessarily be small, amounting usually to not more than eight or ten grades, on account of the time and labor required for closer separations. There is relatively rather a wide range in the diameters of grains within any one of these grades, and absolute values could not be expected without a vast number of separations, so that all the grains in each group would be almost exactly of the same size.

The clay group has relatively the widest limits, which is unfortunate, as this is the most important of all the groups on account of the exceedingly small size of the particles. The figure 0.0001 millimeter is taken as the lowest limit of the diameter of the clay particles. These particles have been heretofore assumed to be ultra-microscopic, but by the use of a microscope of high power with oil-immersion objective and staining fluids, it has been possible to define the clay particles in a turbid liquid which has stood so long as to be only faintly opalescent.

Pending more exact measurements, the figure 0.00255 millimeter has been used as the diameter of the average sized particle in the clay group.

The following table gives the approximate number of grains per gram in the different types of subsoils calculated from the mechanical analysis of the typical soils already given:

NUMBER OF PARTICLES OF EACH CLASS IN ONE GRAM OF SOIL. ─────────────────┬─────────────────┬─────────────────┬───────────────── Silt classes. │ No. 1 │ No. 2│ No. 3 Diameter (_d_) in│ Early truck. │ Truck and small│ Tobacco. centimeters. │ │ fruit.│ ─────────────────┼─────────────────┼─────────────────┼───────────────── 0.15 │ 0│ 0│ 3 0.075 │ 85│ 34│ 102 0.0375 │ 5,511│ 4,011│ 1,900 0.0175 │ 37,230│ 54,610│ 11,890 0.0075 │ 207,500│ 199,700│ 267,900 0.003 │ 2,073,000│ 1,355,000│ 8,092,000 0.00075 │ 38,210,000│ 35,360,000│ 140,900,000 0.000255 │ 1,915,000,000│ 3,918,000,000│ 6,637,000,000 │ —————————————│ —————————————│ ————————————— │ 1,955,000,000│ 3,954,973,355│ 6,786,273,795 ─────────────────┼─────────────────┼─────────────────┼───────────────── Silt classes. │ No. 4 │ No. 5 │ No. 6 Diameter (_d_) in│ Wheat. │Grass and wheat. │ Limestone. centimeters. │ │ │ ─────────────────┼─────────────────┼─────────────────┼───────────────── 0.15 │ 0│ 0│ 12 0.075 │ 726│ 4│ 60 0.0375 │ 8,273│ 181│ 157 0.0175 │ 32,340│ 5,556│ 1,456 0.0075 │ 554,100│ 202,600│ 125,900 0.003 │ 3,962,000│ 10,670,000│ 8,231,000 0.00075 │ 73,990,000│ 154,900,000│ 199,900,000 0.000255 │ 10,150,000,000│ 14,570,000,000│ 19,430,000,000 │ ——————————————│ ——————————————│ —————————————— │ 10,228,547,439│ 14,735,778,341│ 19,638,258,585 ─────────────────┴─────────────────┴─────────────────┴─────────────────

=245. Estimation of the Surface Area of Soil Particles.=—The approximate extent of surface area of the soil grains in one gram of soil can be calculated from the foregoing by the following formula:[163]

π(_d_)²_n_

in which _d_ is the mean of the diameters of any group in centimeters, and _n_ is the number of particles in the group.

The following table gives the approximate extent of surface area of the particles in one gram of soil calculated from the preceding table:

APPROXIMATE EXTENT IN SQUARE CENTIMETERS, OF SURFACE AREA IN ONE GRAM
OF SOIL.

Soil number. ————— —————— —————— —————— —————— —————— Diameter, millimeters. 1 2 3 4 5 6 1.5 0.0 0.0 0.4 0.0 0.0 0.1 0.75 1.8 0.6 1.8 12.8 0.1 0.1 0.375 24.3 17.7 8.4 36.5 31.0 0.7 0.175 35.8 52.6 11.4 31.1 5.3 1.4 0.075 21.3 35.3 47.3 97.9 35.8 22.2 0.03 218.8 38.3 228.9 112.0 301.4 232.7 0.0075 67.4 62.5 248.9 130.8 273.5 353.4 0.00255 390.8 800.5 1355.0 2072.0 2976.0 3965.0 ————— —————— —————— —————— —————— —————— Total 760.2 1007.5 1902.1 2493.1 3593.1 4575.3

=246. Logarithmic Constants.=—The following logarithmic constants have been used in the calculation of the approximate number of grains per gram and of the surface area, using 2.65 in all cases as the specific gravity of the soil.

Diameter. (_d_) Approximate number of grains. Surface area.
log.(π(_d_)³_w_)/(6) log.(_d_)²π

0.15 centimeters \̅3.6703 \̅2.8493
0.075 „ \̅4.7674 \̅2.2473
0.0375 „ \̅5.8641 \̅3.6451
0.0175 „ \̅6.8711 \̅4.9831
0.0075 „ \̅7.7674 \̅4.2473
0.003 „ \̅8.5734 \̅5.4513
0.00075 „ \̅1̅0.7674 \̅6.2473
0.000255 „ \̅1̅1.3616 \̅7.3101

=247. Mineralogical Examination of the Particles of Soil Obtained by Mechanical Analysis.=—The principal object of the mechanical analysis of soils as has already been set forth is the separation of the soil into portions, the particles of which have the same hydraulic value. It is evident without illustration that particles of the same hydraulic value do not necessarily have the same size. The rate of flow of a liquid carrying certain definite particles does not imply that these particles are of the same dimensions. Of two particles of the same size and shape, that one which has the lower specific gravity, will be carried off at the lower rate of flow. At the end of the operation, therefore, the several portions of the soil obtained will be found composed of particles of sizes varying within certain limits, and of these particles the larger ones will tend to be composed of minerals of lower specific gravity, and the smaller ones of minerals of higher specific gravity. Of the same mineral substance, the particles which are most irregular, exposing for a given weight the largest surface will be found to pass over at a lower velocity than those of a more nearly spherical shape. The same law holds good for particles falling through a liquid at rest, _i. e._, the heavier and more spherical particles, weight for weight, will sooner reach the bottom of the containing vessel. To complete the value of a mechanical analysis, it becomes necessary to submit the several portions of soil obtained not only to a chemical but also to a mineralogical examination. Only the outlines of the methods of examining silt separates for mineral constituents can be given here and special works in petrography must be consulted for greater details.[164]

It is evident that the methods of separation and examination from a mineralogical point of view about to be described can only be applied to silts of the largest size. The finer silts can not be separated into portions of different specific gravities by separating liquids of varying densities on account of the slowness with which they subside, thus tending to adhere to the sides of the separating vessels and to form floccules which are not all composed of the same kind of mineral particles. While, therefore, these processes are more appropriately described in connection with the silts obtained by hydraulic elutriation, they can be applied with greater success to the fine particles passing the different sieves used in the preparation of the soil for analysis or to the finely pulverized soil as a whole.

The minerals which have contributed to soil formation, moreover, are better preserved in the larger silt particles and therefore more easily identified. While the desirability of securing like determinations in the finer silts is not to be denied, in the present state of the art the analyst must be content with the examination of the larger particles.

=248. Methods of Investigation.=—The chief points to be observed in the examination of the fine particles of soil are the following: (1) the size and shape of the particles; (2) measurement of crystal angles; (3) separation into classes of approximately the same specific gravity; (4) separation by means of the magnet; (5) determination of color and transparency; (6) determination of refractive index; (7) examination with polarized light; (8) examination after coloring; (9) chemical separation. For many of the optical studies above noted, it is first necessary to prepare thin laminae of the mineral particles and properly mount them for examination. For the purposes of this manual only those processes will be described which are essentially connected with a proper understanding of the nature of the soil particles. For the more elaborate methods of research the analyst will consult the standard works on mineralogy and petrography.

=249. Microscopical Examination.=—The direct examination of the silt particles with the microscope should attend the progress of separation. Unless the particles obtained have the same general appearance, the separation is not properly carried on. Especially is the microscope useful to determine that the value of the silt separation is not impaired by flocculation. Unless flocculation be practically prevented during the separation of the finest particles, many of these will be left as aggregates to be brought over subsequently with particles of far different properties. No special directions are necessary in the use of the microscope. The silt particles are removed with a few drops of water by means of a pipette, a drop of the liquid with the suspended particles is placed on the glass, covered and examined with a convenient magnification. A micrometer scale should be employed in order that the approximate sizes of the particles may be determined. A _camera lucida_ may also be conveniently used for the purpose of delineating the form of particles of peculiar interest.

=250. Petrographic Microscope.=—Any good microscope furnished with polarizing apparatus may be used for the examination of the silt particles and sections. For directions in manipulating microscopes the reader is referred to works on that subject. A special form of microscope for petrographic work is made by Bausch and Lomb of Rochester. The stand of this instrument is shown in Fig. 37. The base, upright pillars and arm are made of japanned iron. The stage is made in two forms, first, plain revolving, having silvered graduates at right angles and second, a mechanical stage with silvered graduations on the edge with vernier and graduations for the rectangular movements. The mirror bar is adjustable and graduated and the mirror is of large size, plane and concave. The double chambered box in the main tube carries the upper Nicol prism (analyzer). The lower Nicol prism (polarizer) is mounted in a cylindrical box beneath the stage to which it is held by a swinging arm. It is adjustable also up or down and is provided with a compound lens for securing converged polarized light. In revolving the prism a distinct click shows the position of the crossed Nicols.

FIGURE 37.
]

=251. Form and Dimensions of the Particles.=—In order to study the contour of the fine silt particles, it is well to suspend them in a liquid whose refractive index is markedly lower than that of the particles themselves, and for this purpose pure water is commonly used. Care must be taken that not too many particles are found in the drop of water which is to be placed on the object holder and protected with a thin, even glass. The tendency to flocculation in these fine particles will make the study of their form difficult if they are allowed to come too close together. The size of the particles, or linear diameter, is to be determined by means of an eye-micrometer. This consists of a glass plate on which a millimeter scale is engraved with a diamond, or photographed. The millimeter scale is the one usually employed, each millimeter being divided into tenths. On microscopes designed especially for photographic work the micrometer is fastened to the eyepiece, and so adjusted as to read from left to right, or at right angles thereto. Sometimes an eyepiece-micrometer has two scales at right angles so that dimensions may be read in two directions without change. With an eyepiece-micrometer, not the dimensions of the object, but those of its magnified image are read, and the degree of magnification being known, the actual size of the object is easily calculated. The actual measurements may also be obtained by placing in the field of vision, a stage-micrometer and determining directly the relation between that and the eyepiece-scale. If, for example, the stage-micrometer is ruled to 0.01 millimeter, and the eye-micrometer to 0.1 millimeter, and one division of the stage-rule should cover three divisions of the eye-rule, then the one division of the eye-micrometer would correspond to an actual linear distance of 0.0033 millimeter in the object. If the two lines of division in the two micrometers do not fall absolutely together, the calculation may be made as follows: suppose that six divisions, 0.6 millimeter, in the eyepiece correspond to nearly five divisions, 0.25 millimeter, in the stage piece. To get at the exact comparison, take ninety-six divisions of the eye-scale and they will be found to be somewhat longer than eighty-one and somewhat shorter than eighty-two divisions of the stage-scale. It follows therefore that

one division of the eye-scale >0.008438 millimeter, and
„ „ „ „ „ „ <0.008541 „ ;

and, hence, one division of the eye-scale corresponds almost exactly to 0.008489 linear measure.

=252. Illustrations of Silt Classes.=—In figure 38 are shown the relative sizes and usual forms of a series of silt separates made by the Osborne beaker method. The photomicrographs were made by Dr. G. L. Spencer from specimens furnished by Prof. M. Whitney.

The soil represented by the separates is from a truck farm near Norfolk, Virginia.

The particles represented in each class are not all strictly within the limits of size described. For instance, in the largest size (No. 1) are two particles at least which show a diameter of more than one millimeter. The particles in general, however, are within the limits of the class; _viz._, one-half to one millimeter, and this general observation is true of all the classes. In the case of the finer particles, especially of clay, the tendency to flocculation could not be overcome in the preparation of the slides for the photographic apparatus. The clay particles are so fine as to present but little more than a haze at 150 diameters of magnification. The particles seen are clearly, in most cases, aggregates of the finer clay particles. The larger particles show the rounded appearance due to attrition and weathering. It would have been more instructive to have had the particles of the different classes all photographed on the same scale, but this is manifestly impossible. The lowest power which shows any of the clay particles to advantage is at least 150 diameters, and with the larger particles such a magnification would have been impracticable.

=253. Measurement of Crystal Angles.=—The fine silt particles rarely retain sufficient crystalline shape to permit of the measurement of angles and the determination of crystalline form thereby. The rolling and attrition to which the silt particles have been subjected have, in most cases, given to the fragments rounded or irregular forms which render, even in the largest silts, the measurement of angles impossible. For the methods of mounting minute crystals and the measurement of microscopic angles, the analyst is referred to standard works on mineralogy and petrography.

=254. Determination of the Refractive Index.=—For a study of the theory of refraction, works on optics should be consulted. The general principles of this phenomenon which concern the determination of the refractive power of fine earth particles are as follows: if a transparent solid particle is observed in the microscope imbedded in a medium of approximately the same refractive power and color, its outlines will not be clearly defined, but the imbedded particle will show in all of its extent the highest possible translucency. If, therefore, the form or perimeter of the particle is to be studied with as much definiteness as possible, it should be held in a medium differing as widely from it as possible in refractive power. For minerals, water is usually the best immersion material. On the other hand, when the internal structure of the particles is the object of the examination, it should be imbedded in oil, resin (Canada balsam), etc., or in some of the liquids mentioned below.

If particles of different refractive powers and the same character of surface be studied in the same medium, they will not all appear equally smooth on the field of the microscope. Some of the surfaces will seem smooth and even, others will appear rough and wrinkled. Those particles whose refractive index is equal to or less than that of the liquid appear smooth, because all the emergent light therefrom can pass at once into the environing medium. On the other hand, the surfaces of those particles which have a higher refractive power than the medium will appear roughened, because, on account of the unavoidable irregularities on the surface, many of the emergent rays of light must strike at the critical angle and so suffer total reflection, and consequently those portions of the surface will be less illuminated, producing the phenomenon of apparent roughness above noted. In the case of any given particle, liquids of increasing refractive power can be successively applied until the change in the appearance of the surface of the particle is noticed. The refractive index of the liquid being known, that of the particle is in this way approximately to be determined.

The following liquids, having the indexes mentioned, are commonly employed:

Substance. Refractive index. Water 1.333 Alcohol 1.365 Glycerol 1.460 Olive oil 1.470 Canada balsam 1.540 Oil of cinnamon 1.580 Oil of bitter almonds 1.600 Oil of Cassia 1.606 Concentrated solution of potassium and mercuric iodid 1.733 Concentrated solution of barium and mercuric iodid 1.775

The solution of potassium and mercuric iodid may also be used for all refractive indexes from 1.733 to 1.334 by proper dilution with water.

The mineral particle may also be imbedded in Canada balsam and over it a drop of a liquid of known refractive power placed. By a few trials one of the liquids will be found having practically the refractive index of the particle under examination.

=255. Examination with Polarized Light.=—The internal structure of a mineral particle can often be determined by its deportment with polarized light. The theory of polarization is fully set forth in works on optics and will not be discussed here. The principle on which the utility of polarized light in the examination of soil particles rests is found in the information it may give in respect of crystalline structure. The structure of mineral particles which make up the bulk of an ordinary soil is, as a rule, so thoroughly disintegrated that all trace of its original form is lost. Some particles may exist, however, in which there is no determinable element of shape and which yet possess an internal crystalline structure which the microscope with polarized light may be able to reveal.

=256. Staining Silt Particles.=—The finer silts and clays before microscopic examination should be colored or stained. The methods used in staining bacteria may be employed for the clay particles.

Evaporation to dryness with a solution of magenta will often impart a color to the clay particles which is not removed by subsequent suspension in water. The harder and larger silt particles are not easily stained, especially if they be firm and undecomposed. On the other hand, if the particles be broken and seamed, and well decomposed, the stain will be taken up and held firmly in the capillary fissures. Valuable indications are thus obtained respecting the nature of the silt particles. Particles of mica, chlorite and talc are easily distinguished in this way from the firmer and less decomposed quartz grains.

The staining of the particles after ignition and treatment with acids gives better results than the direct treatment. Particles of carbonate which are stained with difficulty before ignition take the stain easily afterwards on account of the decomposition produced by the loss of carbon dioxid. This is the case also with particles containing water of composition or crystallization.

=257. Cleavage of Soil Particles.=—A microscopic examination of the cleavage of soil particles may be useful in determining their mineral origin. The course followed by cleavage lines and their mutual position is dependent on the direction in which the separation of the mineral fragment takes place. The character of the microscopic fragments produced by crushing a soil particle is determined primarily by the system of crystallization to which it belongs. Perhaps the most distinguishing cleavage marks in soil particles will be found in fragments of mica and orthoclase. These characteristic forms are shown in Figs. 39 and 40. The first (Fig. 39) shows the pinacoidal cleavage in a fragment of mica. Fig. 40 illustrates the appearance of the cleavage lines in a fragment of orthoclase. Figs. 41 and 42 show the characteristic cleavage lines in fragments of epidote and titanite.

=258. Microchemical Examination of Silt.=—The methods of quantitative chemical examination of silts will be given in another part of this manual. Certain qualitative and microchemical tests, however, are useful in identifying silt particles. For instance, any soluble iron mineral will be detected, even in minute quantity, by the blue coloration of the solution produced by the addition of potassium ferrocyanid. Manganese will be revealed by fusion with soda and saltpeter on platinum foil, in the oxidizing flame, producing the well-known green coloration due to the sodium manganate formed.

More valuable indications of the character of the fragments examined are obtained by microchemical processes. The best method of decomposing the silt particles for this purpose is by treatment with hydrofluosilicic acid. When the particles are composed of silicates, pure hydrofluoric acid is to be preferred.

The method of treatment is essentially that of Boricky.[165] The slide used is protected by a film of Canada balsam, and a few of the silt particles are placed thereon, and fixed in place by slightly warming the balsam. Each particle is then treated with a drop of hydrofluosilicic acid, care being taken not to let the drops flow together. The acid must be pure, leaving no residue on evaporation. The acid should be prepared by the analyst from a mixture of barium fluorid, sulfuric acid and quartz powder, or the commercial article should be purified by distillation before using. The acid should be kept in ceresin or gutta-percha bottles and must be applied with a ceresin or gutta-percha rod. Each particle should be as completely dissolved as possible by the acid, and the rate of solution may be hastened by gentle warming, provided the heat is not great enough to remove the balsam and allow the acid to attack the glass. The bases present in the silt particles crystallize on drying as fluosilicates. In case of a too rapid crystallization, the mass may be dissolved in a drop of water or of very dilute hydrofluosilicic acid, and allowed to evaporate more slowly. Some fragments need more than one treatment with acid to secure complete solution, and particles of mica may even resist repeated applications. In such a case the decomposition may be made in a platinum crucible with hydrofluoric acid, adding afterwards an excess of hydrofluosilicic acid and evaporating to dryness. The crystals may then be dissolved in a little water and a drop of the solution allowed to crystallize on the slide.

=259. Special Reactions.=—The number of microchemical reactions is very great, but there will be given here only some of the more important for silt identification.

_Sodium._—Sodium mineral fragments dissolved in hydrofluosilicic acid and dried give the combinations shown in Fig. 43. With sodium and aluminum the forms shown in Figs. 44 and 45 are obtained. With an increasing amount of lime in the mineral, the crystals tend to become longer. For microscopic work it is not advisable to try to produce the tetrahedral crystals of the double uranium sodium acetate because the commercial uranium acetate often contains sodium and even the pure article will often take up sodium from the bottles.

_Potassium._—Fragments containing potash give isotropic clear cubes, or octahedra of low refracting power, or combinations of these forms with each other and with rhombic dodecahedra. These crystals have the composition K₂SiF₆. Their forms are shown[166] in Figs. 46 and 47. In case much sodium be present, the first crystals obtained may be strongly double refractive rhombohedra, but on dissolving in water and allowing to recrystallize, the normal forms will be obtained. If the crystals be dissolved in hydrochloric or sulfuric acids, and treated with platinum chlorid, the characteristic yellow octahedral crystals of K₂PtCl₆ will be obtained. Ammonium and cesium compounds also give this reaction.

_Lithium._—When fragments containing lithium are treated with the solvent mentioned, monoclinic crystals are produced on drying. These crystals dissolved in sulfuric acid and freed from calcium sulfate by treatment with potassium carbonate give aggregates of lithium carbonate resembling a snowflake. At a high temperature lithium solutions treated with sodium phosphate give spindle-shaped crystals of lithium phosphate. The double lithium aluminum silicofluorid is shown in Fig. 48. The ease with which traces of lithium may be detected by the spectroscope renders unnecessary any further description of its microchemical reactions.

_Calcium._—Nearly all mineral particles, save quartz grains, contain calcium. When these particles are dissolved by treatment with hydrofluosilicic acid, they form on drying hydrated monoclinic crystals of calcium silicofluorid (CaSiF₆ + 2H₂O). These crystals assume many forms, some of which are shown in Figs. 49 and 50. These crystals are easily decomposed by sulfuric acid, the well-known long prismatic crystals of gypsum taking their place. On treatment of silt particles containing lime with hydrofluoric and sulfuric acids, only a part of the lime passes into solution if the content thereof be large. Where but little lime is present and the sulfuric acid is in large excess, all the lime passes into solution and the characteristic gypsum crystals appear as in Fig. 51.

_Magnesium._—Rhombohedral crystals of magnesium silicofluorid separate from the solution of particles containing magnesium in hydrofluosilicic acid. They have the composition MgSiF₆6H₂O and their common forms are shown in Fig. 52. Quite characteristic also are the crystals of struvite (NH₄MgPO₄ + 6H₂O), which are produced in a very dilute solution of the magnesium compound first obtained by carefully adding ammonium hydroxid and chlorid until a faint alkaline reaction is produced, and then placing a drop of dilute sodium phosphate at the edge of the solution. The crystals should be allowed to form slowly in the cold. Their form is shown in Fig. 54.

FIGURE 38. PHOTOMICROGRAPHS OF SILT PARTICLES.
]

No. Diameter in mm. Name. Magnification. Diameters.
1 1.0–0.5 coarse sand ×10
2 0.5–0.25 medium sand ×10
3 0.25–0.1 fine sand ×10
4 0.1–0.05 very fine sand ×30
5 0.05–0.01 silt ×30
6 0.01–0.005 fine silt ×150
7 0.005–0.0001 clay ×150

Figures 39–42, show examples of the various degrees of perfection and
relative positions of cleavage lines.
Figure 39, illustrates pinacoidal cleavage in mica from granite.
Magnified thirty diameters.
Figure 40. A cleavage of orthoclase from augite syenite magnified
twenty-seven diameters.
Figure 41. Cleavage of epidote magnified sixty diameters.
Figure 42. Cleavage of titanite magnified seventy-five diameters.
Figure 43. Sodium fluosilicate crystals magnified seventy-two
diameters.
Figure 44. The same with aluminum fluosilicate magnified twenty-seven
diameters.

Taken from Rosenbusch, Mikroskopische Physiographie.
]

Figure 45. Sodium and aluminum silicofluorid crystals magnified 100,
140 and 160 diameters.
Figure 46. Potassium silicofluorid crystals magnified 130 diameters.
Figure 47. Another preparation of the same magnified 140 diameters.
Figure 48. Lithium and aluminum silicofluorid crystals magnified 100
diameters.
Figure 49. Calcium silicofluorid crystals magnified 45 diameters.
Figure 50. Another preparation of the same magnified 42 diameters.
]

Figure 51. Calcium sulfate crystals magnified twenty diameters.
Figure 52. Magnesium silicofluorid crystals magnified thirty
diameters.
Figure 53. Cesium aluminum sulfate crystals magnified twenty
diameters.
Figure 54. Ammonium magnesium phosphate crystals magnified ten
diameters.
Figure 55. The same crystallized from dilute solution magnified thirty
diameters.
Figure 56. Ammonium phosphomolybdate crystals magnified 140 diameters.
]

_Barium._—From solution of barium bearing minerals in hydrofluosilicic acid fragments, no characteristic crystals, are obtained. Treated with hydrofluoric and sulfuric acids the barium is left as sulfate. If this salt be dissolved in boiling oil of vitriol and a drop of the solution placed on the slide, a mixture of rectangular tablets and St. Andrew’s cross-shaped growths will be separated before any crystals of gypsum which may be present appear. When strontium is present, the barium sulfate residue obtained by treatment with hydrofluoric and sulfuric acids should be fused with sodium and potassium carbonate, washed with water until the sulfuric acid is removed, the residue dissolved in hydrochloric or nitric acids, and the solution treated with potassium chromate. Pale yellow crystals of barium chromate are thus obtained, which resemble in form those secured by dissolving the barium sulfate in oil of vitriol. Strontium is not precipitated by this treatment. If potassium ferrocyanid be used instead of barium chromate with the hydrochloric acid solution, crystals of barium potassium ferrocyanid are formed of a bright yellow color and rhombohedric shape.

_Strontium._—From a hydrofluosilicic acid solution, strontium crystallizes in columns or tablets of the monoclinic system as strontium silicofluorid, SrSiF₆. On treating these with sulfuric acid, rhombic plates of strontium sulfate are formed, which serve to distinguish this element from calcium. On treatment of the particles of the original mineral with hydrofluoric and sulfuric acids, the strontium remains in the insoluble residue. When this residue is treated with boiling oil of vitriol, rhombic plates of celestine are separated. If the residues above mentioned be dissolved by fusion with the alkaline carbonates, washed with water, dissolved in hydrochloric acid and treated with oxalic acid, octahedral crystals of strontium oxalate are formed.

_Iron._—Mineral particles containing iron give crystals, when treated as is first described above, which are fully isomorphous with those obtained from magnesium. By moistening the crystalline mass with potassium ferrocyanid, the presence of iron is at once revealed by the blue coloration produced.

_Aluminum._—No crystals containing aluminum are formed from the mineral particles containing this substance when dissolved in the solvent already mentioned. If, however, the gelatinous mass be dissolved in a little sulfuric acid and a fragment of a cesium salt added, beautiful crystals of cesium alum are obtained, illustrated in Fig. 53.

_Phosphorus._—When a mineral fragment containing phosphorus is treated according to the usual analytical methods for securing the ammonium magnesium phosphate, crystals are obtained of the form shown in Figs. 54 and 55. A phosphatic fragment of silt may be identified when soluble by treatment with nitric acid and ammonium molybdate. On slowly drying, rhombohedral crystals are produced, yellow by reflected, and green by transmitted light. Their form is shown in Fig. 56.

=260. Petrographic Examination of Silt Particles.=—The larger silt particles and the minute fragments of minerals in the soil can best be studied in thin sections. For this purpose the following plan, proposed by Thoulet, may be used. Mix the soil minerals in considerable proportion—Thoulet recommends ten per cent, but a greater percentage is often better—with zinc oxid and make into a paste with sodium silicate. The paste should be worked to the consistence of putty and then rolled into little tablets about one-eighth of an inch thick and an inch in diameter. After drying a day or two without heating, the tablets become hard enough to mount and grind like rock sections. These tablets are mounted in Canada balsam on glass slides and ground as thin as possible with fine emery on the turn-table or glass plate, as rock sections are treated. As these tablets are not as strong as rock sections usually are, they require care in this treatment. Some of the grains also are apt to be torn out in the process of grinding and to compensate for this loss a number of slides should be prepared with each lot of soil minerals. When this operation has been successful, the optical properties of the various minerals can be studied as in rock sections.

As the iron oxid contained in the soils obscures the transparency of the minerals, it is well to treat a portion of the material under examination with hot hydrochloric acid for a short time to remove this oxid and then prepare slides with the cleansed material and compare results with the untreated. As the acid will dissolve phosphates and carbonates, and will partly or wholly decompose some other minerals, the operator must be guided by his judgment in its use.

=261. Machine for Making Mineral Sections.=—A convenient apparatus for this purpose has been described by Williams[167] and is represented in Fig. 57. It is supported on a substantial table provided underneath with electric batteries and a motor for driving the cutting disks seen on the top. The table is three feet six inches square and two feet nine inches high.

FIGURE 57.

MACHINE FOR MAKING MINERAL SECTIONS.
]

The grinding apparatus consists of two circular disks of solid copper, nine inches in diameter, and three-eighths inch thick, which may be used alternately as different grades of emery are required. They are attached either by a screw or square socket to a vertical iron spindle which revolves smoothly in a conical bearing. The grinding disk is surrounded when in use by a large cylindrical pan of tin, which is not shown in the cut, which has an opening in its center to allow of the passage of the spindle.

The sawing apparatus consists of a horizontal countershaft placed on a different part of the table and connected with the motor by a separate belt. It carries at one end a vertical wheel of solid emery, and at the other an attachment, level-table and guide for the diamond-saw. A small water-can with spout, not shown in the cut, is suspended over the edge of the table to keep the saw wet when it is in use.

The machine is very conveniently driven by a storage battery when street circuits cannot be drawn on.

For the details of making mineral sections, the works on petrography may be consulted.

=262. Separation of Silt Particles by Specific Gravity Solutions.=—In silt separates the specific gravity of the different mineral particles present may vary from graphite (1.9–2.3) to hematite (5.2–5–3).

The following list gives the specific gravities of some of the more common minerals which may be met with in soils:

Gypsum 2.31
Albite 2.56–2.63
Quartz 2.65
Talc 2.74
Chlorite 2.78
Muscovite 2.85
Calcite 2.5–2.78
Dolomite 2.90
Tourmaline 2.94–3.3
Biotite 3.01
Apatite 3.16
Pyroxenes 3.22–3.5
Epidote 3.39
Titanium Minerals 3.48–4.75
Iron oxids 5.2–5.3

The finest particles of silt are separated by gravity with great difficulty, inasmuch as they tend to remain suspended in the solutions for an indefinite period. With the coarser silts, however, useful data are often obtained by this method. The separation is preceded by extraction of the particles with hydrochloric acid to remove encrusted soluble matter, and by ignition to destroy any traces of organic matter. Those mineral matters which are soluble in acid or are changed by ignition must, of course, be sought for in separate portions of the silt,

=263. Thoulet’s Solution.=[168]—The standard solution is of such a density that particles of 2.65 specific gravity-will just float thereon, using for this purpose a solution of about 2.7 specific gravity. The solution from which the above standard is prepared is made as follows:

One part of potassium iodid is weighed and placed in a beaker and one and one-quarter part of mercuric iodid is placed on top of it. Then water is added in the proportion of ten cubic centimeters to 100 grams of the mixture, and after some time (twelve to twenty-four hours), with occasional stirring, the salts will nearly completely dissolve. Filter from the undissolved residue and evaporate in a porcelain dish until crystals form on the surface of the liquid. Allow to cool, pour off the liquid from the crystals and evaporate the liquid for another crop. The first solution, after cooling, has a specific gravity between 3.10 and 3.20, the second a specific gravity of 3.28, practically the limit of density of the solution. The solution of 2.7 specific gravity and other densities are made by cautiously adding a few drops of water at a time and ascertaining the specific gravity by the Westphal balance or other convenient method.

The strong solution, according to Goldschmidt,[169] may be prepared directly by using potassium iodid and mercuric iodid in the ratio of 1 : 1.24. Twenty-five cubic centimeters of water, 210 grams of potassium iodid, and 280 grams of mercuric iodid afford a solution of 3.196 specific gravity at 15°, on which fluorspar fragments will float.

=264. Klein’s Separating Liquid.=—A solution of cadmium borotungstate, of the composition 2H₂O,2CdO,B₂O₃,9WO₃ + 16H₂O, has been proposed by Klein[170] for separating silt particles. This salt is obtained by dissolving pure sodium tungstate in five times its weight of water, adding one and a half parts of boric acid and boiling until, complete solution takes place. On cooling; the borax is separated in crystalline form. The mother-liquor after the removal of the crystals is carefully concentrated by boiling. By stirring the cold solution, there is a further separation of sodium borate and polyborate. This operation is continued until glass will float on the mother-liquor. The salt in solution then has the following composition: 4Na₂O,12WO₃,B₂O₃. To this boiling concentrated solution, is added a boiling saturated solution of barium chlorid, in the proportion of one part of the chlorid to three parts of the original double tungstate. An abundant pulverulent precipitate is formed, making the whole mass mushy. The mass is filtered under pressure and well-washed with hot water. The residue is then suspended in hot water containing one part in ten of hydrochloric acid of 1.18 specific gravity. It is then evaporated to dryness in the presence of an excess of hydrochloric acid and decomposed, by which process hydrated tungstic acid is separated. The boiling mass is taken up with water and the boiling continued for two hours with occasional addition of water to take the place of that evaporated, and the tungstic acid separated by filtration.

From the solution, beautiful quadratic crystals separate having the composition 9WO₃,B₂O₃,2BaO₂H₂ + 18H₂O. These are purified by several recrystallizations and freed from any scales of boric acid by washing with alcohol. Any reducing action, revealed by a violet coloration of the crystals, can be avoided by adding a few drops of nitric acid. From a boiling solution of these crystals, the cadmium salt desired is obtained by treatment with the proper amount of cadmium sulfate solution to precipitate the barium. The barium sulfate is separated by filtration. The cadmium borotungstate is soluble in less than ten parts by weight of water. From this solution it is obtained in pure form by evaporation under a vacuum, or by carefully concentrating on a water-bath and cooling. A saturated solution of these crystals at 15° has a bright yellow color and a specific gravity of 3.28.

If a dilute solution of the above salt be carefully evaporated on a water-bath, any violet color which may be present disappears when the specific gravity reaches 2.7. If the evaporation be continued until a crystal of augite will float on the hot liquid, crystals may be obtained on cooling which, dissolved in as little water as possible, make a solution which will almost support olivine. If the two solutions be united, the specific gravity of the mixture is 3.30–3.36. The highest attainable specific gravity; _viz._, 3.6, is produced by continuing the evaporation on a water-bath until the liquid will support olivine, and then allowing to stand in a closed place for twenty-four hours. The crystals of cadmium borotungstate thus obtained are freed as much as possible from the mother-liquor by drainage and then melted at about 75° in their own water of crystallization. A liquid is thus obtained on which spinel will float. The same concentration may also be obtained by careful heating on a water-bath. At its highest specific gravity this solution has an oily consistence and this renders its practical use in the separation of fine particles somewhat restricted. By filtering the liquor when a crystalline crust begins to form during evaporation, a cold solution of 3.360–3.365 specific gravity is obtained which is found practically useful. It has a higher specific gravity than Thoulet’s mixture, is not injurious to any of the mineral particles, not even of iron with which it is brought into contact, but the trouble of preparing it is far greater than that of the mixture of mercuric and potassium iodids.

=265. Rohrbach’s Solution.=—The solution of barium mercuric iodid recommended by Rohrbach[171] for this purpose was originally prepared by Suchsin. The solution must be rapidly prepared on account of the tendency of the barium salt to decomposition. The solution is prepared by weighing rapidly 100 grams of barium and 130 grams of mercuric iodid, mixing the two salts well in a dry flask and adding twenty cubic centimeters of water. The mixture is raised to a temperature of 150°–200° on an oil-bath. The formation and solution of the double salt are promoted by constant stirring.

After solution, the liquor is boiled for a few minutes and then evaporated on a water-bath until it will bear a crystal of epidote. On cooling, a small quantity of a yellow double salt is separated by crystallization and the resulting mother-liquor is dense enough to carry a fragment of topaz. Inasmuch as the liquor is filtered with difficulty, the clear mother-liquor should be separated by decantation after standing for several days. This solution has the disadvantage of not being dilutable with water, the addition of which causes a separation of red mercuric iodid. Were this solution not so easily decomposed, it would prove of high value in silt separation.

=266. Braun’s Separating Liquid.=—In many respects the separatory solution proposed by Braun[172] is superior to those already mentioned. It is the commercial methylene iodid, CH₂I₂, which has at 16° a specific gravity of 3.32, at 5° of 3.35, and at 25° of 3.31. It is a strongly refractive liquid having a refractive index of 1.7466 for the yellow ray.

As a separating medium the liquid is open to two objections; _viz._, first, it cannot be diluted with water and, second, it turns brown on heating or on long exposure to the sunlight.

When dilution is necessary, it should be accomplished with benzene or xylene. To bring the diluted liquor again to its maximum density, the benzene must be removed by evaporation, which causes a considerable loss in the liquid. When this substance becomes opaque, the transparency may be restored by removing the separated iodin by shaking with potash lye, washing with pure water, drying by the addition of pieces of calcium chlorid and filtering. The same result may also be reached by freezing and separating the liquid portion. The frozen portion on melting will have the density of the original liquid.

=267. Method of Bréon.=—Instead of a solution of a salt, Bréon[173] has proposed to use salts in a fused state for separating mineral particles. Lead and zinc chlorids may be used for this purpose in a melted state, having the specific gravities of 5.0 and 2.4, respectively. By mixing the molten salts in different proportions, any desired specific gravity between the extremes mentioned may be secured. The fusion is accomplished at 400° in a test-tube. The silt is added gradually with constant stirring until a sharp separation is secured between the sinking and floating particles. After cooling, the tube is broken, the two parts separated, and the silt recovered by dissolving the mixed salts in hot water containing a little nitric acid. Only the coarser silts can be separated by this method. Fused silver nitrate, melting point 198°, specific gravity 4.1, has also been used for separation.

=268. The Separation.=—Forty cubic centimeters of the solution in the Thoulet process are placed in the separatory tube A, Fig. 58, together with from one to two grams of the silt and the stopper F inserted. The tube G is connected with a vacuum apparatus by means of which any air particles adhering to the mineral fragments are removed. The silt which sinks in the solution is removed after G has been disconnected by opening the cock C and sucking through B at I. The cock C is closed and the separated particles washed into a beaker at H after opening D. Water is next added to the materials left in A in quantities previously determined to secure a given specific gravity and thus a second, a third, etc., separation secured. An intimate mixture of the solutions in A can be effected by closing D, opening C, and blowing through B in such a way that no liquid is allowed to pass through C.

FIG. 58.

THOULET’S SEPARATING
APPARATUS.
]

The quantity of water to be added in each case to secure a given specific gravity is determined by the formula _v_₁ = (_v_(D − _d_))/(_d_ − 1), in which _v_ is the volume of the solution, D its specific gravity, and _d_ and _v_₁ the specific gravity desired and volume of the water to be added.

_Example._—Let the specific gravity of the original solution be 3.2, its volume thirty cubic centimeters, and the desired specific gravity of the new solution 2.85.

Then _v_₁ = (30(3.2 − 2.85))/(2.85 − 1) = 5.68.

The desired specific gravity is therefore secured by adding 5.68 cubic centimeters of water, which is easily accomplished by means of the graduations on the tube.

According to Rosenbusch,[174] the calculated specific gravity as made above is not wholly reliable on account of the contraction which takes place. An empirical process is rather to be commended which consists in introducing a fragment of mineral of known or desired specific gravity and then adding water drop by drop until the fragment remains suspended in the mixture. Should too much water be added, the necessary increase in density can be secured by adding a little of the strong solution.

=269. Method of Packard.=—A separatory funnel, according to Packard,[175] may be safely used to hold the solution while separation is going on. As the lighter minerals form the bulk of soils, the heavier constituting only a small percentage, it is well to use a wide funnel holding as much as one-half liter for quantitative separations, because a large quantity of soil, say 100 grams, is necessary from which to recover the small quantity of heavy particles satisfactorily. The soil is introduced into the solution contained in the funnel, agitated, stirred with a glass rod, and allowed to stand some time. This operation may be repeated as often as desired. Separation is not absolute by this operation, the heavy and light particles being sometimes so united that they sink or float together according as one or the other preponderates. There are also particles having so nearly the same specific gravity as the solution that they remain indifferent to its action in any position. After separation has been effected, the heavy portion is drawn off through the stop-cock of the funnel and the lighter is skimmed off the top. Both must be thoroughly washed from the adhering heavy solution for further examination with the microscope, and by chemical, microchemical, and blow-pipe tests. One who has familiarized himself with the appearance of minerals in minute fragments under the microscope, in ordinary and polarized light, will be able to determine some minerals in that way. But for certain identification it is necessary to ascertain their optical properties as is done in the case of the minerals in thin sections of rocks.

_Illustration._—The following example from the work of Packard will serve to illustrate the results of separating a soil by the specific gravity method:

One hundred grams of soil, residual clay from the Trenton limestone, were placed in the Thoulet’s solution contained in the large separatory funnel. The heavy portion, after washing and drying, weighed 0.6886 gram, or 0.69 per cent. Of this, the magnet removed 0.1635 gram, or 0.16 per cent. This heavy material consisted of rounded yellowish and brown grains up to twenty-five millimeters in diameter, mingled with lustrous angular black grains which were seen under the microscope to be cubes with striated faces, cubes penetrating each other and aggregations of cubes. Combinations of cubes with octahedra and instances of the pentagonal dodecahedron were also observed. These forms, characteristic of pyrites, were also seen in the fine sand obtained as a residue on elutriating the same soil. As these crystals dissolved in hydrochloric acid, giving a strong iron solution, they were regarded as pseudomorphs of iron oxid after pyrites. The yellowish grains on treatment with acid left a grayish residue which contained some grains of quartz but was not wholly quartz. The lighter portion of the soil, over ninety-nine per cent, which floated in the Thoulet’s solution of 2.8 was next examined. It was colored red by the iron oxid which coated and adhered to the other minerals. It contained all the quartz, the feldspars if present, and the other minerals whose specific gravity is less than 2.8. It was examined by the microscope and found to consist largely of irregular grains of a mineral which acted on polarized light, obscured somewhat by the iron oxid, and was apparently quartz; and another mineral which was yellowish-brown in color and seemed to be dull and not transparent. Besides there was a large quantity of indistinguishable amorphous material. To clean these minerals the material was treated with hydrochloric acid to remove the iron oxid and other matter soluble in acid, when the quartz grains appeared transparent and gave interference colors in polarized light. But mingled with these were grains of the other mineral which now appeared grayish, dull, and without action on polarized light. The character of this mineral substance could only be determined by chemical analysis.

=270. Harada’s Apparatus.=—Although it has been affirmed by some analysts that in the subsidence of small particles it is advisable that the containing vessels have parallel sides, yet in the method just given, and in those about to be described, good results are obtained in a funnel or pear-shaped holder.

FIGURE 59.

HARADA’S APPARATUS.
]

In the apparatus of Harada,[176] Fig. 59, the separating vessel _a_ is made of thick glass furnished with a glass stopper above and a glass stop-cock _h_ below. The separating liquid and silt are placed in the pear-shaped vessel _a_, the stopper inserted, and the whole well-shaken. As soon as a ring of clear liquid is seen between the sinking and floating silt, the lower end of the apparatus is brought near the bottom of a conical glass _b_, the cock _h_ opened and the heavy silt allowed to fall out. Very little of the liquor will flow out because of the air pressure. Should an air bubble enter the apparatus and be held at the stop-cock, it should be made to ascend by gently tapping. When all the heavy silt has passed into the conical glass, the cock _h_ is closed and some water poured over the solution and silt in _b_. The separatory apparatus is now raised until the beveled end of it is in the water layer, when the water at once rises to _h_ and thus washes all the silt particles adhering to the glass into _b_. The liquid in _a_ may then be diluted by inverting the apparatus, adding the required amount of water through _h_, again shaken after closing _h_, and another separation secured as before.

This apparatus is somewhat easier to manipulate than Thoulet’s but does not admit of the same quantitative dilution of the separating liquid.

FIG. 60 a. FIG. 60 b. FIG. 60 c.

BRÖGGER’S APPARATUS.
]