American Mineralogist
Vol. 57, pp. 1768-1781 (7972)
THE HEAT CAPACITIES AT LOW_TEMPERATURES AND
ENTROPTES AT 298.15K OF NESQUEHONTTE,
MgCO3'3H2O,AND HYDROMAGNESITE1
Newell,Street,
Rrcnenn A. Ronrn,U.S. GeologicalSu,rueg,8001
Siluer SprinE,Marglnnd 20910
AND
Bnucn S. HpIr.rncwa:i-,Departmentof Geol,ogg,
Uniuersityof
Mi,nmesota,
Minne'atpolis,M'inruesota55455and U.S. Geologiaal,Suruey,
Siluer Spring,MaryLand 20910
Assrnecr
The heat capacity of hydromagnesite, from Hindubagh, West Pakistan, and
synthetic nesquehonite, MgCOa.SHrO, have been measured from approximately 18
to 309 and to 341 K, respectively.
The thermodynamic properties at 298.15K (25,0'C) C"n, (Hortt.tu - H"i/7,
(G"rnr.rr - H"o)/7, and (Sorea.rr- S'o) for hydromagnesite,5MgO'4COz'5IIzO,
are 125.86+ 0.37,67.35+ 0.20, -53.03 + 0.16,and 120.38+ 0.37 calmol-r(-r, and
for nesquehonite,MgCOa.SIIzO, a,re56.81 + 0.17, 26.11 + 0.08, -20.65 * 0.06, and
46.76 + 0.14 cal mol-rK-l. The heat capacity of nesquehonite exhibits a )r-type
transition between 280 and 320 K. Between 295 and 310K the heat capacity increases
veiy rapidly from 55 to 64.7 cal mol-lK-r and then drops precipitously to 57 cal
mol-lK-r with a maximum at 306.5K (33.35'C). The entropy changeassociatedwith
this transistion is about 0.6 cal mol-rK-r.
InrnooucrroN
Hydromagnesite is a common product of the low-temperature
alteration of serpentine rocks where it is found in associationlrith
artinite and brucite. It is also found, much less commonly, with
magnesite and in this association the magnesite has apparently
forrned from the hydromagnesite(Christ and Hostetler, 1970).
Nesquehonite(MgCOg'3H2O)was originally found at the base of
stalactitesand as incrustationsin an anthracitecoal mine at Nesquehoning,CarbonCounty, Pennsylvania(Palache,Berman,and Frondel,
has alsobeenfound as a fracturefilling in
1951,p.226). Nesquehonite
from
mineral springs,and has recentlybeen
serpentineand as deposits
scale
in an air scrubberof an air condidescribedfrom a carbonate
(1969)
it occurs with hydrocalcite
where
tioner by Marschner
(CaCOs.HzO).Nesquehoniteis involved in numerousequilibria connectedwith the aqueousgeochemistryof magnesiumin natural waters
(Kazakov,Tikhomirova, and Plotnikova, 1957,and Langmuir, 1965).
'Publication
authorized by the Director, U.S. Geological Survey.
1768
HEAT CAPACITIES:NESSUEHONITE,HYDROMAGNESITE1769
Several values for the standard Gibbs free energy of formation of
MgCOe'3HzOare available basedon the solubility studiesof Kline
(1929), Langmuir (1965), and F. B. Hostetler (written communication, April, 1970).Langmuir (1965)has summarizedthe aqueoussolubility data for MgCOs.SH2Oand estimatedthe entropy as 46.9t 2.1
cal mol-1K-1.
In his analysis of the equilibria in the system MgO-COrHzO
Langmuir (1965)suggested
that the value of Stout and Robie (1962),
(seealso Robie; 1965) for the standard Gibbs free energy of formation of MgCOs (magnesite)was incorrectby 4 kcal. If true this would
severelymodify the calculated stable phase relations in the MgOCO2-H2O system. It therefore seemedworthwhile to determine the
standard Gibbs free enersr of formation of MgCOe'3HzO and
SMgO' CO'5H2O by calorimetricmethodsas an independentcheck
on Langmuir's conclusions.While this work was in progress,Christ
and Hostetler (1970) confirmedthe value for the free energyof formation calculatedby Stout and Robie (1963) on the basis of careful
solubility measurementsof MgCOa in HzO at fixed CO2 pressuresat
g0'c.
Table
1,
Chemical
aElyaes
of hydromagnesite.
1
Mgo
43.10
43.76
43.37
44.02
44.01
43,29
44,14
coz
37.64
3?.33
37.11
36.84
35.?0
36.10
36.14
Hzo
tg.26
19.63
1 9 .3 8
19.60
19.53
20.25
!9.72
0.16
FeO
Fero,
0.04
Ins o]uble
I 0 0 .0 0
1 0 0 .? 2
99.86
100.76
sample.
99.24
99.84
100.00
sMgO.4CO2.5H2O
,
3.
West Pakistu,
Ilindubagh,
Survey (W-169044).
(Heat
of solution
Eindubagh,
West Pakistm.
Suryey (W-169045).
(Heat
capacity
4 . Coalinga,
Hindubagh,
Rockville,
California.
West
Analyst
Pakistu.
Maryland.
7 . aMeO.3CO;aHrO
EUen
(Shams,
AnalystJ.J.
cray,
Eample.
) Amlyst
) Anatyst
U. S, ceological
Ellen
Ellen
Survey
Gray,
cray,
U. S. Geological
U. S. Ceological
(W-169043).
1965).
Fahey,
U.S.
ceologicalSuryey
(Lurabee,
1969),
1770
R, A. ROBIE AND B. S. HEMINGWAY
A second reason for a calorimetric determination of AGoy of
MgCO3'8HzOand 5MgO'4COz'5HzO (hydromagrresite)is to obtain
an independentvalue of the standardfree energyof formation of Mg'by combiningthe calorimetricdata with precisesolubility results.
In this paper we report measurementsof the heat capacitiesof
hydromagnesite
and nesquehonite
and calculatedvaluesof the entropy
- Soo,for thesephases.
change,Sozse.rr
Maronrels
Hgd,roma4nesite
The sample of hydromagnesite used for our low-temperature heat capacity
measurements was kindly supplied by Dr. S. A. Bilgrami who collected it from
the Zhob Valley, Hindubagh, West Pakistan, (Bilgrami, 1964), and who supplied
the following information: "the mineral occurs as veins in fractured serpentine
on the eastern slope o{ Jungtoghar about 900 yards east of mine 88. The veins
vary in thickness from a fraction of a centimeter to L5 centimeters, are several
meters long and follow no definite direction" (S. A. Bilgrami, written communication to B. S. Ifemingway, December 8, 1966). The material was in the
form of small twinned crystals of between 1 and 6 mm on a side. Chemical
analyses of this sample of hydromagnesite together with one from Coalinga,
Table
2.
Chemical
amlyses
of nesquehonite,
MgCO3'3HZO,
723456
MgO
+28.40
29. 96
cQz
30.44
HroI
-
* {
Hzo '
39.21
21.1
33.40
36.68
29.13
30,12
3 0 .4
32,.45
34.44
3 1 ,8 1
38.62
( 24.3
{
34.39
28,L2
39.06
100.00
( rs.z
sio2
0, 26
A12O3
1,80
K2o
MgO
0. 01
0.42
99.31
100.08
LOO.24
99,24
21
63
3L2
665
28.40
By difference
98.11
Elapsed time (days)
after synthesis
I
x
Residue after ignition.
1. Sample NBH-4
2.
SampleNBH-2
Arely€t Ellen cray, U. S. Geological Suryey (W-171159),
AnalystsD,B.
Heclq J.R. Schleicher, IUinoisGeologicalSurey(R-6534).
3. Sample NBH-4
Amlyst Leomrd Shapiro, U. S. ceological Survey (W-1?1159).
4.
AnalystEllencray,
Sample NBH-3
5. Sample NBH-I
6.
MgCO3.3II2O
U.S. GeologicalSuryey(W-169042).
Amlyst Ellen cray, U,S. ceological Suney (W-169041).
HEAT CAPACITIES: NES]AEHONITE, HYDilOMAGNESITE
rZZt
200
400
600
ELAPSEDTIME, DAYS
Fro. 1. change of the chemicar composition of Mgcoa.Blrro
elapsed time after preparation.
as a function of
california (which we used-for some,preliminary heat of
solution measurements),
and two recent analyses from the literature are given
in Table l.
The crystal structure of hydromagnesite has not been
determined and the
basic geometry of the unit
is still questionable <rvr,r.ao"r.,ls5a).-^cooru-cell
quently, the correct chemical formula for hydromagnesite
is in doubt. parache,
Berman, and Frondel 0951, p. 2ZZ) accepte.dthe
iormula gMeCO".MgiOUl,.
-Dooo"y
3'go, whereas the strukturbericht (1940, Band
v, rgBZ, p. sil ud
et al. (196J) adopted the formula +MgCb".Mg(OE),.4II,,O.
White GgZl) has
studied^ the infrared spectra of hydromagnesile
and interpreted his data as
indicating distinct kinds
carbonate gro-ups with oo
^of
bonding between the CO"*
"oii"oru-ro"-rrJi.oguo
groups and Jatei molecules. He
zuggestua tlrni oror,
of the rrso occurred as oH- rather than lro.
The chemicli
in Table I are inadequate to resolve this discrepanc5r
"oJ"."""ri"tua
between chemical
formulag.
magnesite range from 2.14 to ZB e ml-t.
ve 2,286 g ml-' as the most likely value.
-r
for the material of analysis 6, table f .
data are those of Murdock (lg54) who
c- e.06,
r = 1858,
c- 8.42.r
-oooui#lltt;0"T,
ilf"?,t"#ffi::tffiJ,il
the specific gravity of the hydromagnesite sampre
f.om west pakictan using the
1772
R. A. ROBIE AND B. S.,IIEMINGWAY'
Tab.|e3.Experimentalheatcapacitymeagurementsfor46T.63S
g r a m s o f h y d r o m a g n e s i t ef r o m H i n d u b a g h ,l , J e s tP a k i s t a n '
AT
TEMP.
HEAT
CAPACITY
KELVIN
C A L / M O L - KK E L V I N S
HEAT
C A P AICT Y
KELVIN
C A L / M O L - KK E L V I N S
S E RE
I SIII
S E R I E -SI
53.06
55.38
59.35
6 4 .3 3
69.47
74.53
79.79
8 5. 0 3
90.30
95.79
17 . 3 3
18.77
2l .33
24.58
2 7. 9 7
3'l.31
34.77
3 8 . 19
41.56
45.00
1.20?
3.443
4.5.l0
4.836
5.278
5.248
48.38
5l .55
54.60
57.63
60.64
63.57
66,46
69.30
72 . 0 7
74 . 7 5
77.39
79.87
5.344
E, 29.6
E
(l4
5.528
5.722
5.tt+
5.622
5.743
5.592
III
SERIES
163.77
169.24
I 7 : 45
.6
I 7 9 ;B 3
1 8 5 .12
19 0. 4 7
19 5 . 9 0
201.32
2 0 6. 7 2
2 1 2. 1 6
80.50
82.87
8 5 .I I
87.27
8 9. 2 7
9l .45
93.60
95.62
07
qa
9 9 .5 0
217.72
223.4'l
2 2 9. 0 7
234.71
240.32
245.92
251;52
257.17
262.93
5.694
S E RE
I SII
I 0t .34
I' I0' t 6 . 6 5
I .90
1 1 7. 2 7
1?2.78
12 8 . 3 0
I 33.9i
1 3 9. 6 2
14 5. 3 2
I 50.97
156.65
16a.31
^T
TEMP.
5.403
5.282
5.322
5.408
5.469
5.448
5. 4 4 6
5.705
5.690
5.654
5.643
5.602
5.6.|5
5.623
5.705
5.853
I01.4
|03.5
10 5 . 4
1 0 7. 2
109..l
' 1I .l l02. .B5
.l.l4.3
' : 1. l 6 . 0
S E R I E SI V
262.35
2 6 7. 9 1
2 73 . 2 7
279.09
285.23
2 9 1. 2 8
297.23
303..l4
308.98
I |.5l . B
I 7.5
l l 9. l
1
' 12202. .64
124.0
125.6
127.4
129.9
,
5.828
5.334
5 . 4 81
6.192
6.114
6.042
5.967
5.900
5.835
S E R I E SV
20.48
23.42
26.29
28.39
30.49
32.64
35.78
3B.Bl
42.6'l
4 7. 2 1
5t .85
56.37
61.07
2.030
2.793
3.674
4.478
5.618
6.382
7.536
9.092
ll.0l
13 . 6 8
16.52
19.40
22.41
3 . 13 8
2
. l..264583
i.968
1.84.l
2.054
2.270
3.338
4.473
4.722
4.572
4.472
4.940
HEAT
CAPACITIES:
NESIUEHON'TE,
H'DROMAGNESITE
IzTg
The 'X-ray density corresponding to the
formula bMgO.4CO,.itr O is 2.21dg cm*.
The best fit'to the ohemical-,specific gta"iiy,
andrX'ray data indioate that
the correct formula for hydromas".uii"lirl.iMgOr4COn.5lLO
and we .have
assumed this: formula in .tabulaiing
our, heat rcapaeity,measureryre4ts..
Nesquehonite
ite) was :prepared by a method sug_
Department of Geblogy, University of
solution of IGCO" was prepar'ed ,froni
ium carbonate. A 1.g molar solution ,of
ied Reagent MgCL.6H"O (M_BB). Two
;ton was poured into a 500 ml beakei
lolution and the beaker covered with a
diately upon mixing df the two solutions
while the crystals of nesquehonite grew
nically separated from the residual gel
vely in distilled water, 0.1 N HCI (this
lel), distilled water, and finally in me-
J
o
j
O
*-^
F
z
0.
IEI/IPERATURE.K
Fra' 2' observed low-temperature heat capacity
of hydromagnesite from Eindubagh, west Pakistan' solid rine is the leasi
nf to trrul*p..ir*oilr
no;otu.
"quu."r
1174
R. A. ROBIE AND B. S' HEMINGWAY
At,room temPerature and ambient C
unstable and after three to four weeks it
The samples were examined Periodi
determine whether any change had occt
alteration detected by diffractometer :
d, = IOS A which may appear as earlY
and 2'21 A were noted on one
tional reflections with a )'+.A5,4.41,i.g1,2.86,
measurements were made
capacity
heat
sample after four weeks. All of the
date the meazurements were
the
of
weeks
two
within
prepared
oo lrateriut
--_no.,,samplesofsyntheticnesquehonitewereanalyzedchemicallyatvaryrng
made.
are given in Table 2' T'he quanelapsed times after preparation and the results
3 are suspect' Spectroscopic
analr{s
in
tities of SiO,, IGO, t"d;to;*ported
prepared from the same source of K'CO"
were
z
uoa-i*ilitft
samples
of
analysis
the amount of SiO" and no Al'Ou
and MgCl''6HrO showed Ltt ttt"o oo"-tenth
as compared to analYsis 3'
CO" and H"O a9 a'function
The change in ttre we'ight percentage of {s^O'
are shown in Figure 1'
artJ.!,"p"t*ti""if
time
of the elapsed
YccOi'3ILO and Coa as a function of
HO
both
loses
It is readily seen that il*'c'o;;Eo
time after preparation under ambient conditions'
p"piotttv (1950) have determined-the unit
Kinsolving, Maccill,,ary, u.lra
the infrared spectra of Mgco"'
tr*'"*r-ined
wrrit"*"(isni
aod
cell parameters
crystal
and MacGillvary (lVIz) determined the
38"O. Very recentlv, St;;h*
stnrcture of nesquehonite.
Rrsrn rs
Thecalorimetefusedinthisinvestigationhasbeendescribedby
reported on the. basis
Robie and Hemingway (Ig72)' The data are
of 1968 (Comit6
Scale
of the International i'ractical Temperature
to
calorie^eQual
one
with
International des Poids et Mesures,1969),
=
MgCOs-'-3H'O
of
+.fS+Ojoules, and using the formula--yeight
= 467'638.grams
iei.soo'g*ms and tor +MgC'Og'Mg(OII)z'4HzO
b a s e d o n t h e l g 6 g t a b l e o f " a t o m i c w e i g h t s ( C o m m i s s-theiononAtomic
calori'Weights,
1970)- Fo" ih" -"usurements on hydromagnesite
Hindubagh''West
the
of
*;. fiIled with 118.781grams'tn uacw,
;;#
53 percent of
Pakistan, crystals. The empt! calorimeter contributed
at 300 K' The
percent
lfru totuf heai capa.ity measuredat 40 K and 30
weighed
studies
of nesquehoniteused for the heat capacity
;;;pl;
t In this connection we report the followilg observation which bears upon
heat'-capacity' sample-was removed
the stability of MgCO"'SE'O"The a33-sram
flask which was tightly
erlenmeyer
in a 250 ml
from the calorimeter Jtt"tta
later' the flask was
months
four
Approximately
sealed with a rubber tiopptt'
of the stopper was
removal
The
op.o"a to obtain u ,n-pi" for x-ray analysis,
much like that
sample'
some
of
eas- 1nd
accompanied by a violent expulsion
opening' The
before
shaken
which had been
of a bottle of warm
"h;;;;;"
later with similar but
months
several
again
flask was restoppered
""Jip"""a
slightly less violent expulsi'on of gas'
HEAT CAPACITIES: NESSUEH}NITE, HYDR1MAGNESITE
TABLE 4. EXPERIMENTAL HEAT CAPACITY
MEASUR,EMENTSFOR 133.360
GRAMS OF SYNTEETIC MgCOg.SEO.
TEMP.
HEAT
CAPAC
I TY
(ELVIN
c A L / I ' 1 O L - KK E L V I N s
AT
TEI,IP.
HEAT
CAPACITY
KELVIN
C A L / I . 1 O L - KK E I V I N S
S E R I E SI
54.02
55.76
58,93
63.38
68.21
73.26
78.68
84.64
90.87
97.20
103.42
r09.4t
il5.21
7.869
8.285
9.033
10.i0
'1t 21..4209
13.61
i4.93
16.32
17.61
18.88
20,05
21.It
AT
S E R I E SI I I
1.220
2.251
4.108
4.7s1
4.884
s-254
5,588
6.210
6-291
6.408
6.044
5.944
s.686
S E R I E SI I
235.17
240.43
237.64
243.13
248.51
253.t6
257.50
262.56
?68.16
273.88
279.48
285.19
290.99
296.61
301.99
307.t3
40.55
41,18
40.64
41.58
42.55
43.29
43.96
45.16
45.51
47.O9
48.32
50.02
52.06
54,79
59.55
61.08
5.325
5.263
5.455
s.375
5.29s
3.999
4.698
5.430
5.8t3
5,694
5.600
s.934
5.786
5.608
s.338
5.270
S E R I E SI Y
221.47
226.98
232.41
38,32
39.)5
40.00
5.576
5.489
5.404
17.14
18.72
20,37
21.92
22.62
24.26
25.90
27.38
28,6t
29.86
31,23
33.12
35,56
40.33
4 3 . 7t
47.46
sl. l3
54.36
58.04
62.13
S E R I E SV
30t.17
302.75
304.31
305.84
307.54
309.43
3ll.'t8
312,77
58.65
60.57
62.34
63.59
60.21
58.57
58.32
58.t4
2 73 . 7 I
276.79
279.71
282,69
?85.40
63.35
62.16
59.2t
s8.55
58.2t
58.05
57.88
5 7, 8 9
57.9i
58.04
58.03
58.20
58.36
58.71
59.35
59.96
60.68
S E R I E SV I I
4 7. 5 6
48.39
49.20
50.10
50.83
1.681
1.568
1.319
1.345
1.176
t.l3B
0.95t
0.846
1.130
t.l16
l.4t O
2.136
2.288
2.620
3.557
3.934
3.421
3.068
4.322
3,878
S E R I E SV I I
l-606
t.575
1
. 1. 5
. 54288
1.891
1.921
r.603
t.605
2B7.AA
289.8',t
291.35
292.74
294.12
295.48
2 9 6. 8 3
298.17
S E R I E SV I
305.33
306.87
308.43
310,02
3il,5r
313.21
3t4.78
3 16 , 3 8
317.91
3',t9.56
321.27
323,17
325.0'
328.19
332.58
335.93
3 4 1. 2 4
0.60?
0.781
0.902
1.055
1.278
1.50t
1.753
1.849
2.268
2.425
2.697
3.134
3,728
4.665
5.470
6.287
7 . 16 3
7.937
8.819
9.793
sI.90
52.58
53.07
53.88
54.48
55.17
5s.97
56.96
2.138
1.766
1.403
1.39i
'I ,, t3. 83 17 1
',t
.359
I .345
S E R IE S V ' II I
t.530
1-547
1.591
1.600
t.604
t.606
1.605
I .604
t.603
t.600
t.9t8
] 914
t.909
4.432
4.400
4.366
4.331
3 . 0 63
3.03t
2.998
2,965
2.886
298.97
300.26
301.54
302.24
302.57
3 02 . 8 9
303.22
303.54
303.81
304.14
304.47
304.79
305.11
305.43
s05.68
3 0 6 .0 0
306.32
306.64
306.97
5 7. 2 5
s8.35
59.59
59.94
60.72
6 0. 9 2
6 1. 5 2
61.82
61.92
62.5A
63.t0
63.33
63.93
64.19
64.t4
6 4. 5 3
64.63
64.27
63.40
't
.332
1.3t7
I.300
0,360
0.3s7
0.356
0,354
0.352
0.3s2
0.349
0.347
0.347
0.344
0.344
0.343
0 ,342
0.341
0.342
0.345
1775
R. A, RABIE AND'8, S' HEMI'NGWAY
J
9
J
o
F
o
gzc
I
Frc. 3. Observed low-temperature heat capacity of MgCO"'3HzO'-(synthetic
line
nesquehonite). Solid line is ihe least squares fit to the data' The dashed
the two
represents our estimate for the normal heat capacity' The area between
,.r*"a ,o.r.rponds to an enthalpy of transition of 176 cal mol-t'
43.396grams rm u,acun.The empty calorimeter was approximately 60
pu"..nt of the observedtotal heat capacity between40 and 300 K' In
correct'ingthe weights of the calorimetric samplesfor buoyancy' we
used the densities2.24 and,1.85 grams crn:s for hydromagnesiteand
respectively.
nesquehonitel
Our heat .apu"ity daia for hydromagnesiteare listed in Table 3'
The data are shown graphiassuminga formula SMgO'4CO2'5}JzOcally in Figure2.
The experimentalheat capacity data for MgCO3'3HzO are listed
in chronologicalorder of measurementin Table 4 and are shown in
Figure 3. The solid line is the least squalesfit to the data. After the
data of seriesIV had beencompletedthe calorimeterwas disassembled
and the data were reduced.It was .then found that there was an
uno*uly in the heat capacity of MgCOB'3H2O'{ ""y sample of
of seriesV through
MgCOr'aHzO was prepareduna tn" measurements
VI]I were made in orier to examinethe nature of the heat capacity
anomaly in detail.
Betweenabout 280 and 320 K the heat capacity'of Mgcoa'3HzO
HEAT CAPACITIES:NESQUEHONITE,
IIYDROMAGNESITE 1777
increasesrapidly from approximately 58 cal mol-1K-1to a maximum
of.64.7cal mol-'K-l at 306.5K (,t-point) and then decreases
precipitously to about 57 cal mol.1K-1aL 320 K. Three sets of measurements
of the heat capacityof MgCOs.SH2Oweremade between280 and 320
K with temperatureincrementsas small as 0.3 K. The results are
shownin Figure 4.
Apparently no thermal hysteresisis associatedwith the transition.
We do not know the struetural causefor the transition but it may,be
related to partial rotation of the HrO groups.The enthalpy of transition
is 176 * I cal mol-'. The entropy changeassociatedwith the transition
is 0.58 + 0.02cal mol-'K-'.
The experimentalheat capacitieswere extrapolatedto 0 K by means
of a Co/T2versus? plot, and smoothed,bycomputer.From the smoothed
heat capacities,tables of the thermodlmamic functions Co,, (Ho; H")/7, Sor - 5o6,and (G", - H";)/T were generatedand are listed
for 5MgO.4CO,.5H,O and MgCO;.3H2O,in Tables 5 and 6 respectively.
For the reasonsrnentioned by Hemingway and Robie (1972) we
:<
I
-J
o
J
LRA
o
L
F
U
-3U
29O
3OO 'K
TEMPERATURE.
3sO
Frc. 4. The tr-type transition in the heat capacity o{ 'MgCOa.3ILO. The
circles with the vertical bar are the data of seri.esV. Table 4. The souares
represent the results of series VI. The triangles are the da.ta of series VIi and
the solid circles are those of series VIII.
r778
Table 5.
N. A. NOBIE AND B. S. HEMINGWAY
T h e r m o d y n a m i pc r o p e r t i e s o f h y d r o m a g n e s i t e ,
5MS0.4C0r.5Hr0
TEMPERATURE
TCo
HIIAT
CAPACITY
P
KELVINS
ENTROPY
(r; - r ; )
ENTHAI.PT
FUNCTION
(H; - H;)/r
GIBBS ENERGY
FUNCTION
-(c; - H;)/r
CALORIES,/MOL.KELVIN
5. 00
10 . 0 0
15.00
20.00
25.00
0 . 03 5
0.275
0.8?7
1.894
3.300
0.009
0 . 08 7
0.296
o.677
1. 2 4 4
0.00q
0.061
0.215
0.495
0.906
0.005
0.026
0.081
0. 182
0. 338
30.00
35.00
40.00
45.00
50.00
s.085
?.234
9.672
12 . 4 4 9
15.392
1.997
2.938
4.061
5 . 3 61
6.823
1. 4 4 5
2.112
2.901
3.808
4.818
0.552
o.826
1. 1 6 0
1. 5 5 3
2.006
60.00
70.00
80.00
90.00
10 0 . 0 0
2 1. 7 1 9
28.324
31
31r.9
41.375
47.575
1 0 .1 8 2
14 . 0 2 5
18.239
22.728
27.410
7 . 10 1
9.660
12 . 4 0 7
15 . 2 6 9
18.192
3.081
4. 365
s.833
7. 458
9.218
1 10 . 0 0
12 0 . 0 0
13 0 . 0 0
14 0 . 0 0
15 0 . 0 0
53.494
s9.121
64.lr58
69.s13
7q.302
32.225
37.123
42.067
47.031
5 1. 9 9 2
2t.135
24.058
2 5 . 9 71
2 9. 8 3 1
32.638
11 . 0 9 0
13.055
15 . 0 9 6
17.200
19 . 3 5 4
16 0 . 0 0
17 0 . 0 0
18 0 . 0 0
19 0 . 0 0
200.00
78.850
8 3 .18 0
87.317
91.282
9 5 . 0 91
56.933
61.845
6 6. 7 1 7
71.545
76.325
35.385
38.0?0
4 0. 6 9 2
4 3 . 2 51
45.748
21.548
23.775
26.02s
2 8 . 2 94
30.576
210.00
220.OO
230.00
240.00
250.00
98.756
102.282
I 05.575
108.940
112.085
81.053
85.729
90.3s1
94.918
99.430
4 8 . 18 6
5 0 .5 6 5
5 2 .8 8 8
5 5 . 15 6
57.370
32.868
3 5 . 16 4
3 7 .q 6 3
39.762
42.059
260.O0
270.00
280.00
290.00
300.00
115.128
118.084
12 0 . 93 9
123.657
126.396
103.885
10 8 . 2 8 6
1 12 . 6 3 2
116.92t|
121.161
59.534
6 1. 6 q 8
53.?15
65.735
67.711
44.352
46.638
48.918
51.189
s3.451
2 73 . 1 5
2 98 . 1 5
118.996
12 5 . 8 5 7
1.37
109.661
120.381
r.37
62.304
6 7 .3 4 8
r.20
47.357
53.033
r. 16
HEAT
Table 6.
CAPACITIES:
HYDROMAGNESITE
1779
T h e r m o d y n a m ipcr o p e r t i e s o f n e s q u e h o n i t e .M q C 0 ^ . 3 H ^ 0
-Jl
TEMPERATURE
r
NESSUEHONITE,
HEAT
CAPACITY
c!
KELVINS
ENTROPY
rsi - sfil
ENTHALPY
FUNCTTON
( H ;- H ; ) / r
GIBBS ENERGY
FUNCTION
-(c; - H;)/r
CALORIES,/MOL.KELVTN
5.00
10 . 0 0
15.00
20.00
25.00
0.016
0.139
0.422
0 . 9 18
1. 6 0 1
0.006
0.045
0 . 14 8
0. 330
0.606
0.005
0.034
0.112
0.244
0.447
0.001
0.011
0. 036
0.086
0.159
30.00
3 5 .0 0
{0.00
45.00
50.00
2.46',1
3.463
4.575
5. 735
6. 9 1 2
0.973
1.427
1.960
2.566
3.231
0.708
1.031
1. 4 0 1
1.819
2.269
0.265
0.396
0.559
0.748
o.962
60.00
70.00
80.00
90.00
10 0 . 0 0
9.297
r 1. 6 5 0
1 3. 9 2 4
16.105
18.189
u.702
6.313
8.018
9. ?85
11 . 5 9 0
3.242
q.275
5.340
6.416
7.q90
1.461
2.O37
2.678
3.369
4.100
110 . 0 0
12 0 . 0 0
130.00
14 0 . 0 0
15 0 . 0 0
2 0 . 1 71
2 2. 0 4 6
23.827
25.537
27.201
1 3 . r r1 8
15 . 2 5 4
17 . 0 9 0
18.918
20.737
8.554
9. 601
10.627
11 . 6 3 1
12.614
4.864
5.654
6. 463
7.287
I .123
160.00
170.00
18 0 . 0 0
19 0 . 0 0
2 0 0 .0 0
28.828
3 0 . 4 11
31.9s0
33.462
34.982
22.54s
24.340
26.122
27.890
29.645
13.577
14 . 5 2 1
I 5. 445
16.35s
17.248
8.968
9.820
10.676
11.535
12 . 3 9 7
210.00
220.OO
230.00
240.00
250.00
36.524
38.073
3 9 . 6 04
41.143
42.729
31.389
3 3 . 12 4
34.850
36.568
38.280
1 8. 1 2 9
19.001
19 . 8 6 3
2 0. 7 1 8
2 1. s 6 6
13 . 2 6 0
14 . 1 2 4
1 4 .9 8 7
15 . 8 5 1
16.714
260.00
2 70 . O 0
280.00
290.00
3 0 0 .0 0
4 4 . 5 17
46.305
49.217
52.662
57.976
39.989
41.598
43.432
45.216
47.107
2 2 . 4 13
2 3. 4 7 I
2 4 .3 s 0
2 5- 2 7 2
26.297
1 7. 5 76
18 . 2 2 0
19 . 0 8 2
19 . 9 4 4
2 0 .8 1 0
2 7 3 . 15
298.15
47.11e
56.808
r.17
42.241
46.756
t.1 4
23.749
2 6. 1 0 6
r.08
18.492
20.650
r.06
1780
R. A. ROBIE AND B. S. HEMINGWAY
have reported valuesfor the entropy changeSornr.ru- Soorather than
an absolutevalue for the entropy of hydromagnesiteand nesquehdrrite.
Until the crystal structure of hydromagnesiteis determined,its absolute
entropy will remain in doubt. We shall,however,tentatively assumethat
hydromagnesiteis an ordered compound and thus Srnr.,uis 120.38 +
0.37 cal mol-' K-'. We have also measured(Robie and Hemingway,
unpublished data) the enthalpy of formation of MgCOa'3H,O by
solution calorimetry. The agreementbetweenthe Gibbs free energy of
formation obtained from the measuredenthalpy and entropy data and
that obtained from the solubility studies of Kline (1929) and P. B.
Hostetler (written communication,April 1970) indicated that Soofor
MgCOr'SHrO is zero. Accordingly the correct value for the entropy'
,So2ss.ru,
foruseinthermodynamic
+ 0.14calmol-'K-'.
calculationsis46.T6
AcKNoWLEDGMENTS
We wish to thank Dr. S. A. Bilgrami, Pakistan Chrome Mines Ltd., Hindubagh, West Pakistan, for his kindness in providing the crystals of hydromagnesite used in our measurements, and for the description of their occurrence,
and Professor Donald L. Graf of the University of Illinois for providing a
sample of hydromagnesite from Coalinga, California, which we used in some
preliminary studies. We also are much indebted to M. E. Mrose, I[. T. Evans,
Jr., and Daniel Appleman of the U.S, Geological Survey, for allowing us to
use their unpublished data for the unit cell dimensions of hydromagnesite. Bruce
S. Hemingway wishes to thank the National Science Foundation for support
from N.S.F. grants GA-1651 and GA-453. This research was also supported
in part by the Advanced Research Projects Agency (A.R.P.A. order 1813).
Rnrnnorcns
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HEAT
CAPACITIES:
NESQUEHONITE,
HYDROMAGNESITE
t781.
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