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9


39-49


95-23


4-77


4-598


4-585


0-2181


10


40-30


93-96


6-04


4-533


4-520


0-2212



f Conditions of formation,

1. From pyrite, melted in HjS, kept a little above m. p. for 1 h. in nitro-
gen and then cooled in nitrogen.

2. From sulphnr and iron, otherwise like 1.

3. From pjnrite, heated to equilibrium in HaS at 1800*', then quickly
cooled.

4. From pyrite, heated in HsS to 900*", then cooled in nitrogen.
6. From pyrite, melted in HaS and cooled rather slowly in same.

6. From marcasite, melted in HaS and cooled rather sickly in the same.

7. From pyrite, heated to 800° 6 h. in HaS, then cooled in nitrogen.

8. From pyrite, heated to 700*" 2i h. in HaS, then cooled in nitrogen.

9. From pyrite, heated to 600** 3h. in HaS, then cooled in nitrogen.

10. From pyrite, heated to 600** 15 h. in HaS, then quickly cooled in the
same.

♦Day and Allen, Publication No. 31, p. 55, Carnegie Institution of Wash-
ington.



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198



AUen^ Creiisha/w^ Johnston^ and Larsen —



this the density (at 4°) and the specific volume was calculated.
Table IV contains these data. Column 1 contains the total
percentage of sulphur, Columns 2 and 3 the quantities of
FeS and S calculated on the hypothesis that pyrrhotite is a
solid solution of sulphur in ferrous sulphide. In fig. 7 are
plotted as abscissas the quantities of dissolved sulphur (Column



Fio. 7.



0-2200



0-2175

a

^ 0-2150

(d



I



0-2126



0-2100























































































































































,'


'


































4




































/










































































,''










»


























7








^


y



































y




























X






y




























/


/






>




























/







y


^'

































_>


^


%

































y


y
































4


y
































*

r.,


/


X
































1^


r
































_-i


i:A


^


































i^














c.

























12 3 4 5 6

Percentage of dissolved solphnr.

Fio. 7. The yariation of specific volume with the dissolved sulphur in
pyrrhotite.



3, Table IV), and as ordinates the specific volumes (Column 6,
Table lY). It will be noticed that the scale of the plot is very
lar^
as



rge and the locus of the points is not only a continuous curve
1 the theory of solid solutions demands, but it is also a straight



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Mineral Sulphides of Iron. 199

line within the limits of the errors.* If we compare this line
with the dotted line joining the specific volumes of sulphur
and ferrous sulphide, we see that a considerable contraction has
taken place in the process of solution.f

Equilibrium between solid pyrrhotite and the partial
pressure of sulphur in hydrogen sulphide. — The composition
of a variable pnase of two components, iron and sulphur, as
pjrrhotite is snown to be, would of course be fixed when both
temperature and pressure are fixed. By heating in hydrogen
sulpnide the pressure is fixed, though not independently of the
temperature. To obtain equilibrium, the products were heated
for about three hours, at the measured temperature, and then
by the device shown in fig. 6, the crucible and its contents were
quickly lowered to the bottom of the enclosing tube. The
process of heating and quick cooling in hydrogen sulphide was
repeated until the density of the product was constant. The
densities of the products thus successively prepared usuallv
agreed exactly in the third decimal place. The rate at which
the sulphur is absorbed by pyrrhotite in the cooling is too slow
to affect these results except possibly in the determinations
made at the highest temperatures (1100^-1300®), where a small
(juantity of sulphur may perhaps be taken up. As this point
is important, some data on tne rate at which the cooling
proceeded are here given.

Initial temperature 1300° 1 100° 800° 600°



Temperature after 1 m:
« « 2

« " 6 *'



680°
680° 480°

400°

365°

166°



420°

..I- 305°



In Table V are collected the quantities of sulphur dissolved
in pyrrhotite at different temperatures in hydrogen sulphide
gas. The sulphur in Nos. 1 and 6 was determined by analysis ;
in the rest it was calculated from the specific gravity. The
results in Table YI, showing the dependence of composition
on temperature when the products are cooled in nitrogen, are
given by way of control. In fig. 8, the curve in space shows
how the composition of pyrrhotite varies with both tempera-

* Three of the points are beyond the errors of the determination of
snlphar, and specific gravity, but if we allow h small error in homogeneity
in the process of preparation, probably occasioned by the splinters of por-
celain, the statement holds for these points also.

t The density of rhombic sulphur is very nearly 2'075, and its specific
volume is therefore 0*4819. The specific volume of the ferrous sulphide is
estimated by extrapolation to be 0*2093.



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200 Allen, Crenahaio, Johnstoii, and Laraen —

ture and pressure. The discontinuity in the curve between
1165° and 1200° which is conditioned by the change of state
will be discussed later (p. 207).

Maximum percentage qf sulphur in pyrrhotite, — The most
concentrated solution of sulphur in ferrous sulphide obtained

Fig. 8.



Fio. 8. Cnrve in space showing the dependence of the composition of
pjrrhotite on temperature and pressure.

C (composition) = percentage of dissolved sulphur.
P = pressure in millimeters of mercury.
T = temperature.

synthetically contains 6 per cent of sulphur and 94 per cent of
FeS. This solution was obtained at 600°, where the absorption
of sulphur from hydrogen sulphide is slow. At 575 the
reaction was so slow that the attempt to get a saturated product
was discontinued. At 550°, as we will show farther on,
pyrite is formed. The curve in fig. 9 shows an extrapola-



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Mineral Sulphides of Iron.

Tabus V.

Solphnr diasolyed by FeS in an atmosphere of HiS
at measnred temperatores.



201





TemperatnTe


Time of heating


Percentage of
dissolTcd
snlphnr




Time in which
eqailibrinm
was reached


Total


1


600°


12 h.


16 h.


6-04


2


800°


3 "


7 "


4-41


3


1000°


2 «


4 "


3-6


4


1100°


2 «


4 «


3-3


5


1165°


1 "


2 "


3-2


6


1200°


20 min.


li"


2-6


n


1300°


20 "


1 «


1-96



Table VI.

Sulphur in pyrrhotite cooled in nitrogen from
Tarious temperatures.





Temperature


Time of heating
in H,S


Percentage of
disBolyed sulphur


1


1210°


Few minutes


•41


2


About 1200°


lib.


•63


3


1000°


6 h.


2^70*


4


900°


6 h.


3-11


5


800°


6 h.


3-74


6


700°


2ih.


4-14


7


600°


3 b.


4-77



* The sulphur in No. 8 was calculated from the density.

tion from which we judge the maximum quantity of sulphur
in pyrrhotite obtained by heating in hydrogen sulphide must be
about 6*5 per cent. If we compute the analyses of natural
pyrrhotite in terras of FeS and S, we find that the limit of solu-
bility agrees well with this. The highest value calculated from
Lindstrom'sf figures is 6*08 per cent. From Rose'sJ analyses
we derive the v^ue of 6*76 per cent. The maximum percentage
of sulphur in the pyrrhotite analyses quoted by Dana§ is 40*46
per cent. This particular occurrence, however, contained about
0-5 per cent of copper and cannot, therefore, be satisfactorily

fLoc. cit.

\ Gmelin Kraut Handbuch der Ch., 6th Ed., Vol. Ill, pt. 1, 882.

g System of Mineralogy, 6th Ed., p. 74.

Am. Jour. Sci.— Fourth Skriks, Vol. XXXIII, No. 195.— March, 1913.
14



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AUen^ Crenshaw^ Johnston^ and Zarsen —



used for a calculation of this sort. Dana gives also an analysis
of pyrrhotite by Funaro, which contained 40*27 per cent total
sulphur, equivalent to 6 per cent of dissolved sulphur. This
pyrrhotite contained also 3*16 per cent of nickel, which, accord-
ing to Penfield, is mechanically intermixed with pyrrhotite in
the form of pentlandite. If this be true, the ratio of sulphur to



Fig. 9.





•a



§4

■•3



\




s


^^


^^




"^^


1


1


"^











500° eOO* 700°
Temperatore.



800" 900' 1000° 1100° 1200° 1300°



Fio. 9. Carve showing the percentage of snlphnr dissolved by ferrous
sulphide in hydrogen sulphide gas as the temperature varies.

iron in the pyrrhotite would be raised a little, since pentlandite
belongs to the type of sulphides MS, and the equivalent
quantities of iron and nickel are almost the same.

Some allowance for errors in the analyses of the natural
mineral should be made; still, the agreement between the
maximum quantities of sulphur in the natural and synthetic
pyrrhotite is striking.

Relation between pyrrhotite and pyrite. — The diagram in
fig. 10 shows the relation between pyrrhotite and pyrite. The
curve 1, 1 shows the partial pressures of sulphur vapor in one
atmosphere of hydrogen sulphide, as they vary with tempera-
ture. These results are taken from Preunner and Schupp,* and
♦Zs. phys. Chem., Ixviii, 161, 1909.



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Mineral Sulphides of Iron.



203



are extrapolated above 1130° and below 750°. Curve 2, 2,
represents so far as may be with partial data the dissociation
pressures of pyrite at various temperatures. Here it is assumed
that the vapor pressure at 665° is one atmosphere (see p. 205).
At 550° pyrrhotite was found to pass over into pyrite when



Fig. 10.






s

I 100

1 ^

.a 80

S 70

I ^
50

40

80

20

10



























































































































































































































































1


1/










<M
















s

/


f
























. >


/


























/












1














/














1
/












/
















/










/


r
















/








y


/


















/
/






y


/






















J


Y


/^




















/


^^*


y






















•2.-'


'*"

























500 600
Temperatnre.



700



800



900. 1000 1100 1200



Fio. 10. Carre 1^1, shows the partial pressures of sulphur in hydrogen
snlphifie at varions temperatures (Prennner and Schnpp). Carve 2, S, shows
appro^ximately the dissociation pressare of pyrite.

hea^d in hydrogen sulphide. This was proved by the fact
that the color changed to the yellower color of pyrite and the
density increased, whereas pyrrhotite decreases in density with
inclrease of sulphur. At 575° pyrrhotite showed no change in



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204 AUen^ Crenshaw^ Johnston^ and Larsen —

Fig. 11.



n •




I












1




I


8


I


8


i




^


m






t




5


IT

4_


V


IL


5


%.


V


X- -


x


i H


^


^ i J




tit




4 4-^ o




-t- - Z '




jl ,r




it-. - 2




it-.-X




3 ...^ ^




i .-/. n




I .^t ^




L C^ '^


_J


vE ^


7


tt


t


Ji-


^


tl


1/


t


it


ML


V


t


-i4


T^


tJL




■Juu




ttt




-.'iJj -,




° f


) in ^



s



a



%



I

-CI



I



'iC^isadQ



Digitized by



Gooile



Mineral Sulphides of Iron. 205

color but continued to decrease in weight the longer it was
heated. These facts are graphically shown in fig. 11. Pyrite
under the same conditions gave inside of a few hours a per-
ceptible quantity of pyrrhotite, which was proved by testing
with warm hydrochloric acid. At 565° the pyrite formed in
several hours only a doubtful trace of pyrrhotite, if any.
Between 550° and 575°, therefore, the two curves 1, 1 and
2, 2 cross, and, at that point, about 565°, at a pressure of
about 5"° of sulphur, pyrite should be in equilibrium with a
pyrrhotite containing about 6*5 per cent of dissolved sul-
phur. How this quantity would vary with conditions we
do not yet know, though, as we have just seen, the solu-
tion of sulphur in ferrous sulphide of maximum concen-
tration found in nature, formed presumably from water solu-
tions, does not vary much from it.

The change from pyrite to pyrrhotite is, then, a reversible
reaction, FeS,^Z± FeS(S)x-l-(l— aj)S. Since the system
contains a gaseous phase, the temperature at which the change
occurs is manifestly dependent on pressure.

Dissociation point — It has been previously stated that
pyrite undergoes dissociation into pyrrhotite and sulphur and
that this dissociation is detectable at 575° after the lapse of
several hours, when the heating is done in hydrogen sulphide.
If the heating is continued at a moderate rate (2° per min.)
a strong absorption of heat manifests itself at about 665°.
Here, under these conditions of heating, the dissociation there-
fore becomes suddenly accelerated, ana it is probable that the
pressure of the escaping sulphur reaches one atmosphere.
The solid phases pyrite and pyrrhotite, i. a, the saturatea solu-
tion of sulphur in ferrous sulphide, should at a fixed tempera-
ture maintain a fixed |^r«ssure. As a matter of fact, the point
is not sharp ; the tenqperature gradually rises through an inter-
val of about 20°. Tra ig. probably due to the formation of a
coating of pyrrhotite on the pyrite grains, which retards the
dissociation, so that the system requires a gradually rising tem-
perature to maintain the pressure. The fact that undecom-
posed pyrite persists so tenaciously in the product seems to
support this view. A typical thermal curve for pyrite in this
region is shown in fig. 12. It is seen to have the same general
form as a melting point curve and the end of the heat absorp-
tion is more sharply marked than the beginning, though for
the reasons stated the latter has the greater significance. Th6
following data show that the absorption ends pretty uniformly
at about 685°:



El. V


6050 mv. = 686°


ii


6037 mv. = 684°


El. S


6083 mv. = 686°


ii


6060 mv. = 684°



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206



AUen^ CrenshaWj Johneton^ cmd Laraen —



Inversimi in ferrous sulphide. — Le Chatelier and Zierfer *
discovered a transformatioQ in commercial ferrous sulphide at
about 130°. This has been confirmed by Treitschke and
Tammannf and by Rinne and Boeke.J The last named

Fig. 12.




authors were unable to find any heat absorption in approxi-
mately pure ferrous sulphide, though the addition of iron (or
carbon), which they believe goes into solid solution, makes the
heat change manifest. The dissolved iron is supposed by

• Bull. Soc. d'Encouragement de I'lndustrie, Sept. 1902, 368.

fLoc. cit.

JZs. anorg. Chem., liii, 338, 1907.



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Mineral Sulphides of Iran.



207



Einne and Boeke to facilitate the transformation, first by
raising the temperature at which it takes place, and second by
loosening the crystal structure. The transformation point in
the presence of 7 per cent dissolved iron is found to be 138°,
and further addition of iron does not change it. At lower
temperatures a temperature interval is found, and the inver-
sion is lowered, but the temperature cannot be followed in
mixtures containing less than 5 per cent dissolved iron. Our
results confirm the above in so far as the experiments were
carried. The change could not be detected in pyrrhotite or
pure troilite. It is a natural inference that the change does
take place in these alsOy but that presamably the heat absorp-
tion in them is very gradual and is therefore overlooked.

Melting temperature of pyrrhotite. — Pyrrhotite melts in
hydrogen sulphide at 1183°. (Table VII.) Of course, a solid
solution melts through an interval and not at a point, but we
have as yet no quantitative methods which enable us to deter-
mine the length of such an interval at high temperatures. The
maximum of the heat absorption falls at the above point. The
melts seem to be quite thin,* and there is usually little or no
undercooling, if the temperature is not too rapidly lowered.

Table VII.



Prepara-


Thermoele-
ment


Melting temperatures


Freezing temperatures


tion


mv.


degrees


mv.


degrees


1


V


11580


1181


11590


1181


2


•(


11685


1181






3


u






11585


1181


4


A


11683


1183






5


it


11087


1183






6


U


11675


1182






6


ii


11700


1184






7


ii


11705


1184






8








11690


1183



Examples of freezing and cooling curves of pyrrhotite in
hydrogen sulphide are plotted in fig. 13. By reference to fiff.
9 we notice that the liquid pyrrhotite contains still a consid-
erable excess of sulphur above the ferrous sulphide ratio, and
that the quantity is less than it is in the solid. From Beck-
mann's formulaf for the change of melting temperature with

♦ See Friedrich, Metallurgie, vii, 257, 1910.

t Ostwald's Lehrbuch der Chemie, vol. ii, pfc. 3, p. 38.



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208



AUertj OrenshaWj Jbh/nstan, and Larsen —



compoBition in a solid solution A =•



•02 T'



(C, - C,), where A



is the change in the meltiog point, T is the melting point
(absolute) of the solvent, I is the heat of fusion of the same, and
C, and C, the concentrations of the solute in the liquid and solid
respectively, we find that the melting temperature is raised if



Fig. 13.



1220"
1210"
1200*
1190'
1180'



H 1170°



1160*
1150'
1140*
1180*





t


t


1


T ^


X- \ ^


X -^ /


A ^iX -T


K ^2 ^^7




z >^


^ ^^ %


^^ ^-^T- ^


t y ^


4 -/


t Z^ it lu


/






'






Time in Minntes.
Fig. 18. Melting and freezing corves of pjrrhotite in hydrogen sulphide.

theZij'wtW which first forms contains less of the solute than the
original solution. We do not, to be sure, know the composi-
tion of the^V*^ liquid which forms, but as ^^ final liquid prod-
uct contains less sulphur than the original solid, it is impossi-
ble to believe that this is not the case with the first liquid also.
Melting temperature cf pyrrhotite in sulphur vapor, — I f
this conclusion is correct, it follows that by raising the pressu: re



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Mineral &aiphide% of Iron. 209

of the sulphur vapor in which the pyrrhotite is melted, the
quantity oi sulphur in the product should also be raised, and
tne melting point with it. In accord with expectation, the
maximum heat absorption in melting was raised about 5^ when
the change of state took place in an atmosphere of sulphur
vapor. The apparatus used was the same as that shown in ng. 6,
except that a second furnace below the one shown in the figure
was used for boiling the sulphur. At the bottom of the tube
were placed 100 grams of sulphur, and the temperature of the
lower furnace (measured outside the tube) was raised gradually
to a little above the boiling point of the sulphur, and only
after the sulphur burned freely at the top of the tube was the
temperature of the crucible raised through the melting inter-
val. The experimental data follow :

El. A 11740 m. V. 1188°

11726 " 1187°

11720 " 1186°

The maximum heat absorption is only about 5® higher than
it is when the pyrrhotite melts in hydrogen sulphide. A care-
ful comparison with the latter on a succeeding aay in the same
apparatus left no doubt as to the reality of rise in the temper-
ature and as to its order of magnitude.

Melting temp, of pyrrhotite in H,S on the
succeeding day, with the same element

and otherwise same conditions 1168dm. v. 1183°

11686 m.v. 1188°

Melting point of ferrous sulphide^ FeS. — Since the melt-
ing temperature of pyrrhotite is raised by increasing the pres-
sure of sulphur vapor above it, pure ferrous sulphide must melt
at a temperature lower than any of the sulphur solutions. In
the effort to obtain this point the crucible containing the pyr-
rhotite was heated in a vacuum furnace. The dissolved sulphur
volatilized in vacuo, as expected, but there was a further loss,
though a slow one, which was found to be due to a dissoci-
ation of the ferrous sulphide into the elements, a circumstance
which naturally prevented the exact location of the melting
point.

The apparatus employed is shown in fig. 14. ^ is a glazed
porcelain tube, closed at one end, 50^" in length. The open
end is closed by the perforated bra^s plug 2>, through which
passes the porcelain tube B which protects the thermoelement
m! from the action of the fused sulphide. This plug has a side
tube also of brass which connects with the pump. An air-
tight joint between the tube and plug is made with Kotinski
cement, CC, It was found necessary also to close the tube B by



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210 Allen, Crenshaw, Johnston, and Lareen —

cementing in a small
glass plug, because
the former invariably
leaked through the

1 To pump ^iQggj gjj J ^ g^^ll

hole is drilled through
this tube at O, so that
all the air in the appa-
^ be removed by the pump. The
\i unglazed porcelain has a capacity
f pyrrhotite. It rests on a ring of
1 clay, and is heated as usual by a
\ furnace after the apparatus has
lusted by the oil pump,
ed determinations of the melting
ire of ferrous sulphide under these
B, showed in general that each snc-
etermination was a few degrees
in the one which preceded it, and
Df the products as well as determi-
f their density showed that they
less sulphur than ferrous sulphide,
wing data make it evident that
sulphide slowly dissociates into
nd iron, in the vicinity of its melt-
, and that the successively lower
temperatures obtained are caused
adual accumulation of free iron in
About 15 g. of a sulphide previ-
Ited in the vacuum furnace, and
contain less sulphur than ferrous
was introduced again into the vac-
iratus, which was then completely
I by the pump. The pump was
iped and the lieating was begun,
the melting temperature the pres-
increased to 9-5™". The product
melted and frozen several times,
the apparatus was cooled to room
ire. Next morning the manometer
showing that the pressure was not
eakage but doubtless to the evolu-
ases occluded by the crucible and
ize of the porcelain.* When the
^^as removed from the vacuum tube

El on this point, see Holborn and Day, On
lermometer at High Temperatures, this

„„ ,.„ viii, 178, 185, 1899 ; Guichard, The Gases

FiQ. 14. Vacnum Disengaged from the Walls of Tubes of Glass, Porce-
furnace. lain, and Silica,C. R., clii, 876, 1911.



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Mineral Sulphides of Iron.



211



the contents were found to possess a bright metallic luster, while
about 10*^ above the cover of the resistance furnace a ring of
sulphur was condensed on the inside of the tube. This exper-
iment was repeated with similar results. A sulphide contain-
ing 36*02 per cent of sulphur (FeS contains 86*45 per cent)
was heated as before. This time the manometer read 5-5""
at the melting temperature. When the apparatus was cold the
reading was 5*0°*". This product was also perfectly bright and
a ring of sulphur was again visible* on the cool part of the
tube. The product was analyzed again and found to have
lost still more sulphur. The following table (Table VIII)
shows that the percentage of sulphur, the specific gravity, and
the last melting point of three different products prepared in
vacuo are in accord, i. e., the density increases witli the per-
centage of free iron and the melting "point" falls. The melt-
ing temperature of No. 3 should of course be lower than No.
2, out these temperatures are not easy to locate exactly ; a part
of the diflSculty is perhaps due to the fact that we are dealing
with a mixture, in which the heat absorption is not sharp.

Table VIII.
Properties of pyrrhotite after melting in vacao.



No.


Melting
temperature

in
microvolts


Melting

temperature

in

degrees


4-816
4-861

4-883


Density
(4-)


Sp. vol.


Percent
sulphur


Free iron
calcu-
lated


1
2
3


11508
11346
11416


1165
1156



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