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.
[Illustration:
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.
[Illustration:
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.
[Illustration:
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.
[Illustration:
FIG. 60 a. FIG. 60 b. FIG. 60 c.
BRÖGGER’S APPARATUS.
]
=271. Apparatus of Brögger.=—All silt separations in narrow tubes are
open to the objection of permitting more or less flocculation. Some of
the lighter particles are thus carried down by the heavier, and, on the
other hand, some of the heavier float with the lighter. This disturbing
action Brögger[177] seeks to avoid by the following device, Fig. 60, a,
b, c. The length of the apparatus is forty-six centimeters, and its
greatest diameter 3.5 centimeters. The opening in the large stop-cock A
is the same diameter as that of the apparatus at that point. The cubical
content of the apparatus with A open and B closed is about seventy-five
cubic centimeters. In conducting the separation the cock B is closed,
the separating liquid and silt introduced, A being open, the stopper K
inserted and the whole well-shaken. In the first separation, the silt S,
lying over B is contaminated with some of the lighter particles S′₂,
while the lighter particles above A, S₂, are mixed with some of the
heavier particles, S′₁. After closing A the apparatus is again
well-shaken and inverted as in Fig. 60 b. The two parts of the silt will
now undergo another separation as indicated. The apparatus is now
carefully inclined as in c, when the various grades of silt will flow in
the directions indicated by the arrows, but without mixing, passing each
other on opposite sides of the apparatus. When the movement is complete,
A is carefully opened, the apparatus still being held as in c, and the
light silt formerly between A and B will flow above A, while the heavy
silt above A will flow down and join the silt collected over B. This
operation may be repeated until a perfect separation is effected.
Finally B is opened and the heavy silt collected in a beaker, and the
lighter silt then removed from the upper part of the apparatus.
[Illustration:
FIGURE 61.
APPARATUS OF WÜLFING.
]