11th International GeoRaman Conference (2014)
5069.pdf
REFERENCE RAMAN SPECTRA OF CaCl2.nH2O solids (n= 0, 2, 4, 6). L.Martinez-Uriarte1, J. Dubessy1, I.
Bihannic2, P. Boulet3, P. Robert1
1
Universite de Lorraine, CNRS, Georessources Laboratory, BP 70239, F-54506, F-Vandoeuvre-lès-Nancy,
([email protected]), 2Université de Lorraine, CNRS, LIEC, F-54500, F-Vandoeuvre-lès-Nancy,
([email protected]), 3Université de Lorraine, CNRS, IJL, Parc de Saurupt, F-54011, Nancy
X-ray diffraction measurements were carried out in
transmission mode using a Bruker D8 Advance diffractometer. The diffractometer is equipped with the monochromated Mo Kα1 wavelength (λ = 0.7093 Å), a
capillary sample stage and a PSD (positive sensitive
detector).
X-Ray diffractograms: The experimental X-Ray
diffractograms were compared with the database PDF2
(2010) For CaCl2.nH2O solids n = 0,2, 4 and 6, there is
no doubt of the only presence of one phase even if
there is a slight offset of the peak positions between
the experimental and the reference data. Experimental
data also confirms the only presence of the for
CaCl2.4H2O hydrate polymorph.
Raman spectra of bending and stretching modes
water (figure 1)
T=21C
T=-172C
1664
3431
3385
Intensity (AU)
1660
1665
1643
1550
1600
3411
3406
3399
3383
3245
1626
1500
3242
1650
1700
1750
1800
3100
3200
3300
Raman Shift (cm-1)
3400
3500
3600
3700
Raman Shift (cm-1)
Antarticite CaCl2·6H2O
T=21C
T=-172C
1657
1645 1662
3411
3406
33843399
1625
3400 3435
3460
3431
3452
T=21C
T=-172
Intensity (AU)
Intensity (AU)
3364
1643
1627
3350
3400
3511
3450
Raman Shift (cm-1)
3242
1500
1550
1600
1650
1700
1750
1800
3100
3200
3498
3350
3242
3300
Raman Shift (cm-1)
3400
3500
3600
3700
Raman Shift (cm-1)
CaCl2·4H2O
T=21C
T=-172C
1638
T=21C
T=-172C
3452
3437
3211
3215
1635
Intensity (AU)
3486
Intensity (AU)
Experimental methods: The spectroscopic cells
used for the synthesis of the salt hydrates are hematochrit capillaries (1 mm I.D.; 1.6 mm O.D.; ®Hirschmann Laborgerate). CaCl2 solution at 6.98 mol.kg-1
H2O concentration was prepared. Capillaries, previously sealed at one end, were loaded in a two-step procedure using a microsyringe. First, a measured volume of
solution is loaded and then heated up to 140°C for
water evaporation and formation of CaCl2 (non hydrated solid, controlled by Raman spectroscopy). Then, a
second solution loading is carried out with an amount
such that the bulk composition (CaCl 2 salt + second
loaded solution) corresponds exactly to the composition of a salt hydrate (CaCl2.nH2O, n = 1,2, 4 or 6).
Raman spectra were acquired at 21°C and -172°C
using a CAP500 heating-freezing stage (®Linkam)
with modified silver lids) coupled with the microRraman spectrometer (Labram HR, 1800 grooves/mm;
liquid N2 cooled CCD; exciting radiation: 0=514.532
nm, Wl0= 60 mW onto the sample provided by an Ar+
laser ® Spectraphysics).
T=21C
T=-172
3430
1647
Intensity (AU)
Introduction: The H2O-NaCl-CaCl2 system is
one of the major reference chemical system of geological fluids. Its topology and the expected solid phases
(salt hydrates and non-hydrated end-member salts)
coexisting with aqueous solutions and vapor as a function of temperature are well known [1], [2]. The phase
diagram is used for the interpretation of dissolution
temperature to estimate the composition of fluid inclusions. Inspite of frequent metastability [3], the determination of CaCl2.nH2O solids nucleating at different
temperature is critical, especially for fluids with high
Ca/Na ratios. Micro-Raman spectroscopy carried out in
fluid inclusions at a given temperature is a powerful
tool for the identification of CaCl2-bearing phases
[3,4]. Using synthetic fluid inclusions, the Raman
spectra of CaCl2.4H2O and CaCl2.2H2O in the OHstretching region at liquid N2 temperature [3] exhibit a
large number of bands, which did not seem to agree
with numbering of bands. In addition, the bands appeared over an intense background. Thus, it seems
necessary to study in details the Raman spectra of all
the CaCl2.nH2O solids to establish reference spectra for
fluid inclusionists in the 100-3800 cm-1 spectral range.
To be sure of the assignment of these spectra to a well
identified solid, X-ray diffraction was also carried out.
1630
1555 O2
1620
3200
3454
3475
3220
Raman Shift (cm-1)
3388
1616
3216
1556 O2
1500
1550
3491
1600
1650
Raman Shift cm
1700
1750
1800
3100
3200
3300
-1
3400
3500
3600
Raman Shift (cm-1)
Sinjarite CaCl2·2H2O
Figure 1: Raman spectra of bending and stretching
modes water at 21°C (black) and – 172°C (red).
Abstract for 11th GeoRaman International Conference, June 15-19, 2014, St. Louis, Missourri, USA
3700
11th International GeoRaman Conference (2014)
Raman spectra: low frequency region for the
different CaCl2 Hydrates (Figure 2).
CaCl2·6H2O
CaCl2·4H2O
CaCl2·2H2O
CaCl2·0H2O
Intensity (AU)
Bending and stretching modes of water.
Two main effects should be considered for the interpretation of the vibrational modes of water in salt
hydrates: the magnitude and symmetry of the static
potential in the site occupied by water molecules and
intermolecular coupling between molecules [5]. Although isotopically dilution techniques is a powerful
tool for unambiguous band assignment, some preliminary interpretation of data are given.
The small range of the intramolecular bending
mode of water (2) among the different hydrates
(1616-1665 cm-1) first indicates small variations in the
environment of water molecules. The highest values of
2 are found for hydrates with higher water molecules
number (6, and 4) probably result from higher strength
hydrogen bonding between water molecules [4]. On
the other hand, the lowest value of 2 corresponds to
sijarite (n(H2O)= 2), which is indicative of metal(Ca)oxygen interaction [5,6].
Chloride is the second highest strength proton acceptor after F-. The site symmetry for a water molecule
in an hydrate is determined by the relative strength of
the two O-H bonds. If the O-H bonds have equal
strengths the site symmetry in the hydrate can be C2v
or C2, with possible intermolecular coupling reduced
separation of the bands: this could be the case for the 4
and 6 hydrates. For unequal intramolecular O-H bonds
the site symmetry must be a Cs or C1. However, the
wavenumber differences between higher and lower
wavenumbers, smaller than 130 cm-1, does not suggest
highly distorted water molecules. In addition, the presence of weak OH bands, which could be the antisymmetric mode, also supports a quasi symmetric force
field for water molecules [7].
The number of bending modes in a hydrate suggests the number of crystallographically distinct water
molecule in the unit cell but also may indicate intermolecular coupling. CaCl2·6H2O displays four bending
modes identified by curve fitting and five streching
bands suggesting C2v symmetry for the water molecules. CaCl2·4H2O show three bending modes and
eight stretching bands. This could indicate possible
overlapping of two bending modes within the band
with peak maximum at 1662 cm-1. CaCl2·2H2O displays three bending modes, two intense stretching
modes and three weak bands which could correspond
to antisymmetric stretching vibrations. For the three
hydrates the weak bands in the 3210-3240 cm-1 range
are assigned to the overtone of the bending mode [5,7].
5069.pdf
100
200
300
400
500
600
700
800
-1
Raman shift (cm )
Figure 2. Raman spectra from 100 to 900 cm-1 of the
different CaCl2·nH2O (n=0,2,4 and 6) at -172°C.
Ca-O bands are also expected in hydrate phases. In
Ca++-bearing aqueous solutions, such bands are weak
and around 280 cm-1 [9]. Following comparison with
aqueous solutions, restricted translation band sOH…Cl- occur at 190 cm-1 [9] and could account for
some bands in hydrates. By comparaison with libration
bands in barium chloride monohydrate [10], bands
around 400-500 cm-1 are considered as libration bands
of water [10]. Other studies on salt hydrate suggests
the same assignment for the bands up to 900 cm-1 [5].
For anhydrous CaCl2, Ag band at 209 cm-1, 2 B1g
bands at 113.5 and 250.5 cm-1, and a single B2g+B3g
band have been found [11].
References:
[1] Schiffries C.M. (1990) Geochim. et Cosmochim.
Acta, 54, 611-619. [2] Steele-MacInnis, M., Bodnar,
R.J., Naden, J. (2012). Geochim. et Cosmochim. Acta,
75, 21-40. [3] Baumgartner M.. and Bakker R. (2009)
Chemical Geology, 265, 335-344.[4] Dubessy J., et al.
(1982). Chemical Geology, 37, 137-150. [5] Lutz H.D.
(1988). Structure and Bondings, 69, 97-125. [6] Fifer
R.A. et al. (1971). J. Chem. Phys., 54, 5097-5102. [7]
Lutz H.D. and Christian H. (1982). J. Mol. Struct. 96,
61-72. [8] Schiffer J. and Hornig D.F. (1968). J. Chem.
Phys., 49, 4150-4160. [9] Rudolph W.W. and Irmer G.
(2013). Dalton Trans., 42, 3919-3935. [10] Lutz H.D.
and Christian H. (1983) J. Mol. Struct., 101, 199-212.
[11] Unruh H.G. et al. (1992) Z. Phys.B-Condensed
Matter, 86, 133-138.
Abstract for 11th GeoRaman International Conference, June 15-19, 2014, St. Louis, Missourri, USA