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The isomorphism and thermal properties of the feldspars online

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especial precautions and could be regulated to maintain a constant
temperature at a particular point for long periods of time.



Ptw



26 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.

The coil, which was obtained from Dr. Heraeus, was of platin-
iridium wire (90 parts Pt., 10 parts Ir.), 1.5 mm - in diameter, and

required about 3000 watts to
maintain a constant temperature
of 1600 C. The furnace was
carried at times on a no- volt
direct-current street main, but
accurately constant temperatures
could not be depended on without
the storage battery.

The insulation in these furnaces
was so perfect that shutting off or
reversing the heating current at
the highest temperatures did not
produce a quiver in the galvanom-
eter to which the thermo-element
was connected, although the sen-
sitiveness of the system was such
that a leakage amounting to a
single micro-volt (corresponding to




FIG. 3. The furnace, showing ther-
mo-element and charge.



less than 0.1) at 1600 would have caused a displacement of more
than two millimeters on the scale.

STANDARDS.

The thermo-electrical potential was measured upon a potentiom-
eter (Wolff, Berlin, Reichsanstalt calibration) in terms of a standard
cadmium cell (saturated) prepared by ourselves. Two of these
cells were used interchangeably during the earlier measurements.
Toward the close of the series four fresh cells were prepared for com-
parison with the earlier ones and were found to agree with them within
o.ooo i. One of these later cells (the readings of the four were iden-
tical to the fifth significant figure) was verified by Dr. Wolff, of the
Bureau of Standards, by comparison with the standard Clark cells of
that institution and found to be 1.0195 V at 20 C., assuming the legal
value (United States) of the Clark cell, 1.434 V, at 15 C. Substitut-
ing the Reichsanstalt value, Clarki 5 = 1.4328,* our cells would give
a normal potential difference of 1.0186 at 20. The temperature
determinations which follow are, therefore, calculated in terms of this
number.

With the apparatus here described, the authors were enabled to
command any furnace temperature up to 1600 conveniently, to regu-



* Jaeger u. Kahle, Wied. Ann., 65, p. 926, 1898.



FIRST GROUP OF MINERALS INVESTIGATED. 27

late it quickly and with great exactness, or to hold it constant for long
intervals. An oxidizing or reducing atmosphere could also be easily
introduced whenever desired. It is, however, undesirable to expose
either coil or thermo-element too freely to oxygen at very high tem-
peratures on account of the considerable losses by sublimation to
which the platinum metals are subject.

With the help of the standard metals mentioned, which are readily
obtainable andean be used repeatedly, thermo-elements or resistance
pyrometers can be calibrated in any laboratory, and used for all
measurements up to the limit of the Reichsanstalt scale (i 1 50 C.) with
no greater error than that inherent in the scale itself. Above this
temperature up to 1600 the continuation of the thermo-electric scale
probably still furnishes the most convenient and trustworthy extra-
polation which has yet been perfected.

The uniformity and certainty of this extrapolation will best be
illustrated by the measurements upon anorthite (the highest melting
point we measured). The melting temperature of a mineral of very
poor conductivity for heat and relatively low specific gravity is much
more difficult to measure than that of a metal, but the agreement of
the results tabulated below (see Anorthite, p. 37) is sufficiently good
to demonstrate the accuracy of the extrapolation. The thermo-
electric potential, therefore, appears to deserve entire confidence for
consistent extrapolation through the 450 immediately above the
present Reichsanstalt scale.

FIRST GROUP OF MINERALS INVESTIGATED.

The particular group of minerals chosen for the first investigation
was the soda-lime feldspar series, and orthoclase. The reasons for
this choice will be fairly obvious. Aside from its being altogether the
most important group of rock-forming minerals, unusual interest has
been attracted to it through Tschermak's theory that these feldspars
bear a very simple relation to one another, that they are (orthoclase
excepted, of course) in fact merely isomorphous mixtures of albite
and anorthite. This hypothesis has given occasion for serious and
extended study, both from the optical and thermal sides.

A complete review of the literature of the feldspars will not be
attempted here. Although opinion is still somewhat divided,* it is
probably fair to say that the optical researches have not yet definitely
established or disestablished the isomorphism of the albite-anorthite

* Fouque et Levy, Synthesedes Mineraux et des Roches, p. 145, 1882; C. Viola,
Tschermak Min. & Petr. Mitth., 20, p. 199, 1901; Lane, Journ. Geol., XII, 2, p.
83, 1904; J. H. L. Vogt, " Die Silikatschmelzlosungen," Christiania, 1903.



28 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.

group. Investigation from the thermal point of view has been even
less satisfactory by reason of the subjective methods employed, to
which reference has already been made, though the recorded results
indicate with reasonable unanimity that the melting point of anorthite
is above that of albite and that the intermediate feldspars will prob-
ably fall between the two.* Beyond this conclusion, the great body
of evidence is more or less contradictory and sometimes contro-
versial in character.

ORTHOCLASE (PRELIMINARY).

Somewhat unluckily, our measurements began with natural ortho-
clase (microcline) from Mitchell County, North Carolina, a quantity
of which was placed at our disposal by the U. S. National Museum.
The material was powdered so as to pass readily through a loo-mesh
sieve, and placed in 100 cc. or 125 cc. platinum crucibles, sometimes
open and sometimes covered, in charges of from 100 to 150 grams.
These charges were heated slowly in the electric furnace from 600 to
above 1400 C., but, although the thermal apparatus was sufficiently
sensitive to detect an unsteadiness of a tenth of a degree with certainty
not the slightest trace of an absorption or release of heat was found.
The charge at the beginning of the heating was a dry crystalline
powder which was prodded from time to time with a stout platinum
wire to ascertain its condition as the heating progressed. At about
1 000 traces of sintering were evident; at 1075 it had formed a solid
cake which resisted the wire, at 1150 this cake had softened suffi-
ciently to yield to continued pressure, and at 1300 it had become a
viscous liquid which could be drawn out in glassy, almost opaque
threads by the wire. Under the microscope the opacity was seen to
be due to fine included bubbles, the material being entirely vitreous.
The cooling was equally uninstructive ; the vitreous mass solidified
gradually without recrystallization or the appearance of any thermal
phenomenon. Frequent repetitions with fresh charges and varied
conditions added nothing to our knowledge of the melting tempera-
ture, and the matter began to look very unpromising.

We also reheated charges of the resulting glass, which was some-
times repowdered and sometimes in the cake as it had cooled. But
except to observe that the glass powder began to sinter earlier (800),
no new facts appeared, f

* J. H. L. Vogt, loc. tit., p. 154, expresses the opinion that the soda-lime feld-
spars fall under Type III of Roozeboom's types of isomorphous series with a
minimum between anorthite and albite.

t These sintering temperatures varied within considerable limits with the fine-
ness of the material and, therefore serve only in a very rough way to define the
state of the charges.



BORAX. 29

Then we tried by various means to recrystallize the melted ortho-
clase. We mixed crystalline powder with the glass, we applied suc-
cessive quick shocks to the cooling liquid for several hours with an
electric hammer below the crucible, we varied the rate of cooling and
even tried rapid see-sawing between 800 and 1300. We circulated
air, water vapor* and carbonic dioxide through the charge throughout
the heating, and finally introduced a rapid alternating current sent
directly through the substance while cooling, but no trace of crystalli-
zation resulted. An extremely viscous, inert mass always remained,
which gradually hardened into a more or less opaque glass. It ap-
peared somewhat translucent if very high temperatures had been
reached, but was never clear.

Following orthoclase, a number of specimens of natural albite were
tried under similar conditions and with entirely similar results.

Later on, when more experience had been acquired, these minerals
were taken up again and a satisfactory explanation for their behavior
was found. But for the moment all the defining phenomena ap-
peared to be so effectively veiled by some property, presumably the
viscosity, that we were constrained to look about for some similar
compound which should give us a better insight into the behavior of
mineral glasses and their thermal relations, and to lay aside the feld-
spars until they could be more successfully handled.

This outline of our unsuccessful experiences is given here in some
detail, in order to show the actual difficulties which confront the
student in working with the feldspars, in the face of which it is cer-
tainly not surprising that uncertain and contradictory conclusions
have been reached.

BORAX.

The substance chosen for this preliminary work was ordinary
anhydrous borax (sodium tetraborate) . We chose this merely be-
cause it was a simple glass and unlikely to undergo chemical change.
It is easily obtainable pure and its thermal phenomena are within
easy reach. The study of borax proved to be most instructive.
It gave us an effective insight into the behavior of this class of sub-
stances, and in particular served to define the phenomena of melt-
ing and solidifying in substances which undergo extreme under-
cooling and which recrystallize with difficulty, or not at all. The
results of this study of borax were, therefore, of much interest in them-
selves and were given in a paper before the National Academy of
Sciences at its spring meeting in Washington last year (April 21, 1903),
but were not printed at that time.



30 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.

The borax glass upon which our measurements were made was
prepared in the usual way by heating the crystals until the water of
crystallization had been driven off and the viscous mass was reason-
ably free from bubbles. If the borax is pure, the anhydrous product,
when cooled, is a brilliant, colorless glass, isotrophic, of conchoidal
fracture, and specific gravity 2.37. The specific gravity was deter-
mined in the fraction of kerosene boiling above 1 85 C. About 100 g.
of this glass were then broken up and placed in a platinum crucible
in the electric furnace. The thermo-element was placed in position
as indicated in fig. 3, the heating current properly regulated, and ob-
servations of the temperature made at intervals of one minute, while
the glass softened and passed gradually over into a thin liquid (800).
Then the current was reduced and the cooling curve observed in the
same way. These observations gave an unbroken curve, both for the
heating and cooling, as in the case of all the glasses,* without a definite
melting or solidifying point, although the arrangements for detecting
an absorption or release of heat were very sensitive. Prodding at in-
tervals with a platinum rod showed the change to be perfectly gradual
from a clear, hard cake through all degrees of viscosity to a fairly thin
liquid and back again. This observation is of considerable interest
as showing that the absence of bounding phenomena between the cold
glass, which fulfills the mechanical conditions for a solid very perfectly,
and the liquid, is not confined to mixtures of complicated chemical
composition, but is exhibited also by true chemical compounds of
undoubted purity. It is, therefore, not conditioned by composition,
but by the physical nature of the substance.

Having verified this behavior of anhydrous borax by several repeti-
tions of the experiment, various disturbing influences were applied to
the slowly cooling liquid in the hope that some temperature or range
of temperature would be found within which the vitreous condition
would prove unstable and crystallization be precipitated. The jar
produced by an electric hammer pounding upon the outside of the
furnace during cooling proved to be sufficient to bring down the entire
charge as a beautiful crystalline mass of radial, fibrous structure,
brilliant luster, rather high refractive index, and increased volume.
Fig. 4 will give a good idea of the appearance of the anhydrous crys-
talline borax in the crucible. Its specific gravity proved to be 2.28
as compared with 2.37 for the glass, a somewhat unusual relation, f
which may, in part, account for the quasi stability of the vitreous form
during cooling.



* See Tammann, loc. tit.; also Roozeboom, "Die heterogenen Gleichgewichte,
etc.," Braunschweig, 1901.

* Tammann, loc. cit., p. 47 et seq.




FIG. 4.

CHEMICALLY PURE BORAX (ANHYDROUS),
CRYSTALLIZED FROM THE GLASS.

Heliotype CD., Boston,



ui mourns wiiicu uiiueicuui in boiiuiiying. we next vaneQ tne experi-
ment by first cooling quietly to about 100 below the melting point



* See Tammann, loc. tit.; also Roozeboom, "Die heterogenen Gleichgewichte,
etc.," Braunschweig, 1901.

* Tammann, loc. tit., p. 47 et seq.



BORAX.



Observations were then undertaken upon the crystalline borax with
a thermo -element as before, to determine the melting temperature and
solid modifications, if such existed, but none of the latter were found.
The charge melted uniformly at 742 and the melting point was well
defined. A curve showing the minute-to-minute observations on the
crystalline borax between the temperatures 650 and 775 is shown in
fig. 5, a.

Having determined the melting point of crystalline anhydrous
borax satisfactorily, we examined more closely into the conditions



7000



6900

(765")



6800
6700



6600

(MO*)

6500



6400
6300
6200
6100
6000
5900



-\



5800

(660*)



5700



Time - I divisior



10 minutes



FlG. 5.-



i, Melting-point curve; b, c, d, curves showing undercooling and
crystallization at different temperatures.



under which it solidified. As has been said, if the melted charge was
allowed to cool slowly, undisturbed, no return to the crystalline state
occurred. It merely thickened gradually into a transparent glass
without releasing the "latent" heat which it had taken on in melting
(fig. 7, 6). If it was subjected to the jarring produced by the electric
hammer on the furnace wall, it cooled down a few degrees below the
melting point and then began to crystallize, the heat of fusion was set
free, and a rise in temperature immediately appeared, represented by a
hump upon the cooling curve, as shown in the figure (fig. 5, b, c, d) . Up
to this point the phenomenon differs but little from the usual behavior
of liquids which undercool in solidifying. We next varied the experi-
ment by first cooling quietly to about 100 below the melting point



ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.



7000

(775)



6500.



and then introducing a few crystal fragments or starting th f e pound-
ing. Crystallization and release of the latent heat followed at once.
In fact over a range of some 250 immediately below the melting point
it proved to be within our power to precipitate the crystallization of
the undercooled mass entirely
at will. It was even possible
to cool the melted charge
quietly down to the tempera-
ture of the room and remove
it from the furnace as a clear
glass, then, on a subsequent
day, to reheat to some point in
this sensitive zone and pound
judiciously, when crystalliza-
tion would at once begin,
marked by the release of the
latent heat of the previous
fusion as before (fig. 6, a, b).
The accompanying curves
show the situation clearly.
Curves aa f and bb', fig. 7, were
obtained from charges of crys-
talline and vitreous borax, re-
spectively, of exactly equal
weight, which were cooled and
reheated in the same electric
furnace under like conditions.
The radiation from the furnace
for like temperature conditions
is practically the same, so
that the more rapid rate of
cooling and of reheating in the
crystalline charge indicates a
much smaller specific heat than
for the vitreous form.

From the point of view of the
usual definition of the solidify-
ing point of a substance, a dirfi-



6000
(680*)



8

u

E

c 5500



Q.

E

9
H

5000



4500



4000
(490)



\



Time - I division = 10 minutes



FIG. 6. Curves showing the release of the
heat of fusion at widely different tem-
peratures.



culty confronts us here: (i) We were able to vary the beginning of
solidification (crystallization) at will over a range of 250, and (2) the
temperature to which the charge rose after the undercooled liquid
had begun to crystallize did not reach the melting point, although



BORAX.



33



once crystallization was induced only 10 below it in a furnace of

constant temperature. The
rapidity with which the crys-
tallization and the accompany-
ing release of the latent heat go
on depends in part upon the
rate of cooling and the char-
acter of the disturbance which
has been applied, i. e., upon
accidental rather than char-
acteristic conditions. It thus
happens that the amount of
the heat of fusion and its slow
rate of liberation in the case of
liquids which can be greatly
undercooled and become very
viscous may be such as to
deprive it of its usual signifi-
cance as defining a solidifying
point. It is, of course, a con-
sequence of the phase rule that
the solidifying temperature of
an undercooled liquid is estab-
lished, it only equilibrium be-
tween solid and liquid (and
vapor) is reached before com-
plete solidification is accom-
plished, but equilibrium is not
necessarily attained during so-
lidification, and the devices
usually employed (sowing with
crystals, agitating) are often
totally inadequate to effect it.
The temperature to which a
crystallizing liquid rises after
undercooling is not necessarily
constant or in any way related
to the melting point and is,
therefore, not, in general, enti-
tled to be regarded as a physi-
cal constant.
We then endeavored to ascertain whether the unstable domain had

a lower limit also. For this purpose we mixed a quantity of the




20



40 60 80

Time (minutes)



100



FIG. 7. Curves showing difference in spe-
cific heat between crystalline (aa') and
vitreous (&&') borax under like condi-
tions of cooling and reheating.



34 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.

crystals with the glass and powdered them together to about the fine-
ness represented by a i5o-mesh sieve and heated them very slowly.
In this condition the glass proved to be very unstable and crystallized
readily with a rapid release of its latent heat at about 490. Very
slow heating (10 minutes per i degree) gave a temperature a few de-
grees lower, but such variations as could be applied within the period
of a working day did not suffice, under the most favorable conditions,
to change this temperature materially. The first evidence of molec-
ular mobility in borax glass, shown in the sticking together of the
finest particles (sintering), and the first traces of crystallization and
release of latent heat, appeared consistently at about 490 to 500.
Still a third phenomenon attracted our attention to this temperature.
On every occasion when borax glass was heated rapidly, either pow-
dered or in the solid block, a slight but persistent absorption of heat
appeared in this same region and continued over some 20, after which
the original rate of heating returned. We were entirely unable to
explain an absorption of heat in an amorphous substance under these
conditions except by assuming an actual change of state to exist
between amorphous glass and its melt, in which case the absorbed
heat would reappear somewhere upon the corresponding cooling curve,
which it failed to do. We then reasoned that any assumed change
in the molecular structure which would account for an absorption of
heat would also be likely to cause an interruption in the continuity
of the curve of electrical conductivity, and the relative conductivity
was determined throughout this region, but no such interruption
appeared.

Finally the matter was abandoned. The evidence did not appear
sufficient to establish any discontinuity in the cooling curve of the
glass, so long as no crystallization took place.

When these relations had been clearly established, we turned again
to the feldspars.

It became clear very early in the investigation that only artificially
prepared and chemically pure specimens would be adequate for our
purpose. Each of the end members of the series, anorthite and albite,
as found in nature, is always intermixed with some quantity of the
other, while the intermediate members generally contain iron and
potash, and all are liable to inclusions.

There was nothing new in this plan. Fouque and Levy* had
demonstrated the possibility of making pure feldspars by chemical
synthesis and had studied their optical properties some years ago. We
undertook to prepare much larger quantities than they (200 grams)



Synthese des Mineraux et des Roches.



ARTIFICIAL FELDSPARS.



35



and to make a careful study of their heating and cooling curves under
atmospheric pressure the conditions under which anorthite and the
plagioclases crystallize, the relations between the amorphous and crys-
talline forms, the sintering of crystalline and vitreous powders, in
short, their entire thermal behavior, as we had done with the borax.
At the same time it was our purpose to make careful determina-
tions of the specific gravities of both the vitreous and the crystalline
products, analyses of such portions as might be of special interest,
and also to prepare microscopic sections wherever they were likely
to throw light on the relations involved. The latter, after preliminary
examination, were very thoroughly studied by Prof. J. P. Iddings of
the University of Chicago, whose large petrographic experience with
mineral crystallites makes his judgment of very exceptional value.
His analyses (see Part II) of the slides form an important part of this
discussion. We are also indebted to Mr. W. L/indgren of the United
States Geological Survey for valuable assistance in the microscopical
study of our products.

ANALYSES OF ARTIFICIAL FELDSPARS.





An. AbiAn s .


A Dl An 2 .


AbaAn^


Found.


Calcu-
lated.


Found.


Calcu-
lated.


Found.


Calcu-
lated.


Found


Calcu-
lated.


SiO,


43-33
36.21
.29
20.06
. ii


43.28
36.63

20.09


47.10
34- 2 3
15
17.00

1.74


47.18
34-oo

16.93
1.87


51.06
3I-50
. 22
I3-65

3-68


51-30
31.21

'ia'es

3-79


60. 01

24-95
.29
7.09
7-79


59.81

25-47

6^98

7-73


AloO,


Fe.,O,


CaO


Na 3 O






100.00





100.22




IOO. II





100.13





The constituents used in our syntheses were precipitated calcium
carbonate, anhydrous sodium carbonate, powdered quartz (selected
crystals), and alumina prepared by the decomposition of ammonium
alum. None of these contained more than traces of impurities, if we
except the quartz, in which 0.25 per cent of residue, chiefly oxide of
iron, was found after treatment with hydrofluoric and sulphuric acids.
All but the calcium carbonate were carefully calcined and cooled in a
desiccator before weighing. To obtain a homogeneous product, the
weighed constituents were mixed as thoroughly as possible mechani-
cally and heated in large covered platinum crucibles (100 cc. capacity)
in a Fletcher gas furnace.* After some hours' heating, during which
the temperature usually reached 1500 or more, the product was
removed from the furnace, cracked out of the crucibles, powdered,



* Buffalo Dental Company, No. 41 A. A Fletcher furnace of this type, with
ordinary city gas pressure and a small blast motor, will melt all of the feldspars.



36 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.

passed through a "loo-mesh" sieve, and then melted again. This
process probably gives a fairly homogeneous mixture, though a third
fusion in the resistance furnace was generally made before determining
the constants.


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