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.
[Illustration:
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.
[Illustration:
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
[Illustration:
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.
]
[Illustration:
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.
]
[Illustration:
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.