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Frost, Ray, Xi, Yunfei, Scholz, Ricardo, Lopez, Andres, Belotti, Fernanda, & Chaves, Mario (2013) Raman and infrared spectroscopic characterization of the phosphate min- eral lithiophilite - LiMnPO 4. Phosphorus, Sulfur and Silicon and the Related Elements, 188(11), pp. 1526-1534.
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lithiophilite – LiMnPO4
Ray L. Frost a, Yunfei Xia, Ricardo Scholz b, Andrés Lópeza, Fernanda M. Belotti c,
Mário L. S. C. Chaves d
a School of Chemistry, Physics and Mechanical Engineering, Science and Engineering
Faculty, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001,
Australia.
b Geology Department, School of Mines, Federal University of Ouro Preto, Campus Morro
do Cruzeiro, Ouro Preto, MG, 35,400-00, Brazil
c Federal University of Itajubá, Campus Itabira, Itabira, MG, 35,903-087, Brazil
d Geology Department, Federal University of Minas Gerais, Belo Horizonte, MG, 31,270-
901, Brazil
Author to whom correspondence should be addressed ([email protected]) P +61 7 3138 2407 F: +61 7 3138 1804 1
Abstract:
The metal lithium is very important in industry, including lithium batteries. An important
source of lithium besides continental brines is granitic pegmatites as in Australia.
Lithiophilite is a lithium and manganese phosphate with chemical formula LiMnPO4 and forms a solid solution with triphylite, its Fe analog and belongs to the triphylite group that includes karenwebberite, natrophilite and sicklerite.
The mineral lithiophilite was characterized by chemical analysis and spectroscopic techniques. The chemical is: Li1.01(Mn0.60, Fe0.41, Mg0.01, Ca0.01)(PO4)0.99 and corresponds to an intermediate member of the triphylite-lithiophilite series, with predominance of the lithiophilite member. The mineral lithiophilite is readily characterized by Raman and infrared spectroscopy.
Keywords: lithium batteries, Raman spectroscopy, lithiophilite, triphylite, phosphate, lithium
2
Introduction
Triphylite-lithiophilite, amblygonite-montebrasite, spodumene and polylithionite-trilithionite
(lepidolite) are the dominant lithium minerals in granitic pegmatites. Lithium is an important
and strategic mineral resource [1]. The mineral has application in aluminum-alloys for the
aerospace equipments, in ceramics and lubricants. In the pharmaceutical industry it is an important compound in psycho-therapeutics medications and, recently, with the environmental tendency to decrease the world petroleum demands, lithiophilite has become
as an essential issue in an increasing battery market [2-5]. Triphylite has special interest in
view of its application as a storage cathode for rechargeable lithium batteries [6, 7].
The most important lithium world deposits are related to continental brines, located in Bolivia
(Salar de Uyuni), Chile (Salar de Atacama) and Argentina (Atacama Desert) and pegmatites
(Australia). In Australia, the most important deposits are related to granitic pegmatites of
New South Wales, associated to the phosphate amblygonite [8]. In Brazil, lithium minerals
occur in pegmatites of the Borborema Pegmatite Province (BPP) and in the Eastern Brazilian
Pegmatite Province (EBP). However the only Li production is restricted to spodumene ore
from the Cachoeira pegmatite in the EBP, North of Minas Gerais. With increased lithium
consumption, other minerals in addition to spodumene will be required for the lithium
industry.
Lithiophilite is a lithium and manganese phosphate with chemical formula LiMnPO4 and
forms a solid solution with triphylite, its Fe analog [9] and belongs to the triphylite group that
includes karenwebberite, natrophilite and sicklerite. Minerals of this series occur in granitic
pegmatites that are enriched in both Li and P [10]. The mineral was first described by Brush
3
and Dana in 1878 [11] from Fairfield County, Connecticut, USA. Members of thiphylite
group are isostructural with olivine [12] and a solid solution was established [13]. The crystal
structure of synthetic LiMnPO4 was refined by Geller and Durand [14] and of triphylite was
refined by Finger and Rapp [15]. Lithiophilite crystallizes in the orthorhombic crystal system,
space group Pbnm and unit cell parameter a = 4.7383Å, b = 10.429Å and c = 6.0923Å [10].
Mössbauer spectroscopic study of triphylite was carried out by Fehr et al. [7] and Van
Alboom et al. [16]. Magnetic properties of the lithiophilite-triphylite mineral series was
determined [17]. In recent studies, Libowitzky et al. [12] have described a hydrous
component in triphylite-lithiophilite gemstone with support of infrared and Raman
spectroscopy. Minerals of the lithiophilite-triphylite series are the main primary Li phosphate
in granitic pegmatites [18] and are often altered to a wide variety of secondary hydrous
phosphate minerals such as phosphosiderite, hureaulite, strengite, variscite and eosphorite
[19-22].
In this work, spectroscopic investigation of a pure, monomineral lithiophilite sample from the
Cigana pegmatite, Conselheiro Pena, Minas Gerais, Brazil has been carried out. The analysis
includes spectroscopic characterization of the structure with infrared and Raman
spectroscopy. Chemical analysis was applied to support the mineral characterization.
Results and discussion
Chemical characterization
The quantitative chemical analysis of one lithiophilite mineral (sample SAA-086) is presented in Table 1. Composition is the result of measurements by EMP in the WDS mode.
Li2O content was calculated by stoichiometry and the chemical formula was calculated on the
4
basis of 4 oxygen atoms on the structure. The chemical composition indicates a partial
substitution of Mn by Fe2+ cations. Minor amounts of Ca and Mg also were found. The
chemical formula can be expressed as: Li1.01(Mn0.60, Fe0.41, Mg0.01, Ca0.01)(PO4)0.99 and the
mineral correspond to a intermediate member of the triphylite-lithiophilite series, with
predominance of the lithiophilite member. The mineral can be written as Tr0.40-Li0.60.
Table 1. Chemical composition of lithiophilite from Cigana pegmatite. Li2O calculated by stoichiometry.
Number of atoms Probe Standard/ Constituent wt.% Crystal
FeO 18.48 0.41 Magnetite/TAP
MnO 26,67 0.60 Rodhonite/LIF
P2O5 44.24 0.99 Ca2P2O7/PETJ
MgO 0.27 0.01 MgO/TAP
CaO 0.51 0.01 Ca2P2O7/PETJ
1.01 Calculated by Li2O 9.50 stoichiometry
Total 99.66 3.03
Spectroscopy
The Raman spectrum of lithiophilite in the 100 to 4000 cm-1 spectral range is displayed in
Figure 1a. This figure clearly shows that the position of the peaks and the relative intensities
of the Raman bands. The spectrum in the 2600 to 3800 cm-1 displays no intensity when
compared with the phosphate spectral region around 1000 cm-1. Clearly there are parts of the
spectrum where no intensity is observed. Thus, the Raman spectrum is divided into
5
subsections depending upon the type of vibrations being observed. In comparison, the
infrared spectrum as shown in Figure 1b shows no intensity in the 2500 to 3600 cm-1 spectral
range. Again, there are parts of the spectrum where no intensity is found, and as a consequence the spectrum is divided into sections for further analysis. Consequently, the
spectra both Raman and infrared are subdivided into sections for more detailed examination.
The 800 to 1400 cm-1 Raman spectral range
The Raman spectrum of lithiophilite in the 800 to 1400 cm-1 region is displayed in Figure 2a.
-1 3- The Raman spectrum shows a very sharp band at 950 cm assigned to the phosphate PO4 symmetric stretching mode. A very low intensity shoulder is observed at 944 cm-1. The two
Raman bands at 1000 and 1068 cm-1 with additional bands on the higher wavenumber side at
-1 3- 1018 and 1081 cm are assigned to the PO4 antisymmetric stretching modes. The authors
consider that these small low intensity peaks may be due to the presence of triphylite.
Triphylite is the other end member of the lithiophilite-triphylite solid solution series. S. D.
Ross in Farmer’s treatise on the infrared spectrum of minerals [23] reported the infrared spectrum of both lithiophilite and triphylite. Galliski et al. [24] reported the relationship between these two minerals and qingheiite and beusite. The symmetric stretching mode was reported as 990 cm-1 in Farmer; however this vale differs from that reported in this work.
There is a spectrum of lithiophilite published on the RRUFF data base
(http://rruff.info/lithiophilite/R070303). This RRUFF Raman spectrum seems to show a very
sharp and intense peak at ~950 cm-1; although it is difficult to define the exact position of the
band from the shown spectrum. The position of this band in this data base supports the
position of the band in this work.
The 500 to 1300 cm-1 Infrared spectral range
6
The infrared spectrum of lithiophilite in the 500 to 1300 cm-1 spectral range is reported in
Figure 2b. The infrared spectral profile in this region is complex with a series of overlapping
bands. Infrared bands may be resolved at 972 (with a broad shoulder at 879 cm-1) and an
intense band at 1046 cm-1. Other quite sharp infrared bands are observed at 1077, 1095,
-1 3- 1107, 1137 and 1145 cm . These latter bands are attributed to the PO4 antisymmetric
2h stretching modes. The minerals lithiophilite and triphylite have the space group D16 . The spectra recorded in this work are consistent with this space group. Ross in Farmer [23] reported the antisymmetric stretching modes at 1052, 1094 and 1138 cm-1. These values
agree in reasonable agreement with the bands reported in this work. It is noted that very little
has been published on the Raman or infrared spectroscopy of lithiophilite and triphylite, apart
from the work of Ross in Farmer’s work [23]. The Raman spectrum of lithiophilite in the
published on the RRUFF data base (http://rruff.info/lithiophilite/R070303) appears to show two sharp bands at around 1025 and 1170 cm-1. The observation of these two bands helps to confirm that a second phase may be present in the Raman spectrum of lithiophilite.
The 300 to 800 cm-1 and 100 to 300 cm-1 Raman spectral range
The Raman spectrum of lithiophilite in the 300 to 800 cm-1 and in the 100 to 300 cm-1 spectral range are displayed in Figures 3a and 3b. Three sharp bands are observed at 575,
-1 3- 591 and 627 cm and are attributed to the ν4 out of plane bending modes of the PO4 units.
Ross in Farmer [23] reported these bending modes at 551, 578 and 638 cm-1. These bands
were extracted from the infrared spectra [23]. The position of these infrared bands is in good
agreement with the position of the Raman bands, tabulated in this work. It is interesting to
compare the position of these vibrational modes for triphylite. Ross reported these infrared
7
bands at 500, 550, 578 and 640 cm-1. The intensity of these bands in the Raman spectrum of
lithiophilite published on the RRUFF data base (http://rruff.info/lithiophilite/R070303) shows
to low intensity to be certain of the position of these bands.
-1 3- The three bands at 403, 424 and 443 cm are assigned to the ν2 PO4 bending modes. Ross
in Farmer [23] reported these bending modes at 463 and 490 cm-1. These bands were
extracted from the infrared spectra [23]. The position of these infrared bands is in reasonable
agreement with the position of the Raman bands, tabulated in this work. The Raman band at
317 cm-1 is considered to be due to the MnO stretching vibration. A series of bands are found
in the far low wavenumber region with intense Raman bands found at 146, 235 and 247 cm-1.
These bands are simply described as external vibrations or vibrations of the lattice.
The spectra recorded in this work do not show the presence of any OH or water stretching vibrations. No intensity exists even after scale expansion in the Y direction in either the infrared or Raman spectrum. However the work of Libowitsky et al. [12] seems to indicate that a hydrous component could be present in gem quality specimens of lithiophilite. These authors found polarised infrared spectra of oriented triphylite sections show a weak, pleochroic band in the O-H stretching region that is centred at ~3444 cm-1. Attempts have
been made to determine the Fe content in lithiophilite using infrared spectroscopy [25]. A
direct linear correlation was found between the chemical composition and the bands in the
455 to 475 cm-1 region. Frost and Erickson [26] published the near-infrared spectroscopy of
phosphates and hydroxylated phosphates and near-infrared spectroscopy is probably more
suitable for distinguishing between the pegmatitic phosphates especially of the lithiophilite-
triphylite series.
8
Experimental
Samples description, occurrence and preparation
Brown greenish to gray lithiophilite crystals from the Cigana mine, a granitic pegmatite from
Minas Gerais were obtained from the collection of the Geology Department of the Federal
University of Ouro Preto, Minas Gerais, Brazil, with sample code SAA-086. The lithiophilite
crystals occur in association with hureaulite, reddingite, triphylite and vivianite. The Cigana
mine, located in the municipality of Conselheiro Pena, is a well-known pegmatite in Brazil,
being an important source of rare phosphates, industrial feldspar and with minor importance
gemstones and samples for the collectors market Chaves et al. [27].
The sample was gently crushed and the associated minerals were removed under a
stereomicroscope Leica MZ4. The lithiophilite crystals were phase analyzed by X-ray
diffraction. Scanning electron microscopy (SEM) was applied to support the chemical
characterization and indicate the elements to be analysed by EMP.
Electron micro probe analysis (EMP)
EMP was performed in a lithiophilite single crystal. The chemical analysis was done in a Jeol
JXA8900R with four WDS spectrometers at the Physics Department of the Federal
University of Minas Gerais, Belo Horizonte. For each selected element the following
standards were applied: Fe (Magnetite/TAP), Mn (Rodhonite/LIF), Ca and P (Ca2P2O7/PETJ) and Mg (MgO/TAP). Samples of lithiophilite embedded in an epoxy resin were coated with a thin layer of evaporated carbon. The EMP analysis was performed at 15 kV of accelerating voltage and beam current of 10 nA.
Raman microprobe spectroscopy
9
Crystals of lithiophilite were placed on a polished metal surface on the stage of an Olympus
BHSM microscope, which is equipped with 10x, 20x, and 50x objectives. The microscope is
part of a Renishaw 1000 Raman microscope system, which also includes a monochromator, a
filter system and a CCD detector (1024 pixels). The Raman spectra were excited by a
Spectra-Physics model 127 He-Ne laser producing highly polarised light at 633 nm and
collected at a nominal resolution of 2 cm-1 and a precision of ± 1 cm-1 in the range between
200 and 4000 cm-1. Repeated acquisitions on the crystals using the highest magnification
(50x) were accumulated to improve the signal to noise ratio of the spectra. Raman Spectra were calibrated using the 520.5 cm-1 line of a silicon wafer. The Raman spectrum of at least
10 crystals was collected to ensure the consistency of the spectra.
Infrared spectroscopy
Infrared spectra were obtained using a Nicolet Nexus 870 FTIR spectrometer with a smart
endurance single bounce diamond ATR cell. Spectra over the 4000525 cm-1 range were
obtained by the co-addition of 128 scans with a resolution of 4 cm-1 and a mirror velocity of
0.6329 cm/s. Spectra were co-added to improve the signal to noise ratio. The infrared spectra
are given in the supplementary information.
Spectral manipulation such as baseline correction/adjustment and smoothing were performed
using the Spectracalc software package GRAMS (Galactic Industries Corporation, NH,
USA). Band component analysis was undertaken using the Jandel ‘Peakfit’ software package
that enabled the type of fitting function to be selected and allows specific parameters to be
fixed or varied accordingly. Band fitting was done using a Lorentzian-Gaussian cross-product
function with the minimum number of component bands used for the fitting process. The
Gaussian-Lorentzian ratio was maintained at values greater than 0.7 and fitting was
10
undertaken until reproducible results were obtained with squared correlations of r2 greater than 0.995.
Conclusions
We have characterized the mineral lithiophilite using vibrational spectroscopic techniques.
EMP was applied to support the mineral characterization and to determine the position in the solid solution. The mineral corresponds to an intermediate member and can be written as
Tr0.40-Li0.60.
The mineral lithiophilite is readily characterized by Raman spectroscopy. The mineral is
-1 3- characterized by a very sharp band at 950 cm assigned to the phosphate PO4 symmetric
stretching mode. Two Raman bands at 1000 and 1068 cm-1 with additional bands on the
-1 3- higher wavenumber side at 1018 and 1081 cm are assigned to the PO4 antisymmetric stretching modes. The small low intensity peaks on the high wavenumber side are due to the
-1 presence of triphylite. Three sharp bands at 575, 591 and 627 cm are attributed to the ν4 out
3- -1 of plane bending modes of the PO4 units. Three bands at 403, 424 and 443 cm are
3- assigned to the ν2 PO4 bending modes. The spectra recorded in this work do not show the presence of any OH or water stretching vibrations.
Acknowledgements
The financial and infra-structure support of the Discipline of Nanotechnology and Molecular
Science, Science and Engineering Faculty of the Queensland University of Technology, is gratefully acknowledged. The Australian Research Council (ARC) is thanked for funding the
11 instrumentation. R. Scholz thanks to FAPEMIG – Fundação de Amparo à Pesquisa do estado de Minas Gerais, (grant No. CRA - APQ-03998-10).
12
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14
List of figures
Figure 1 (a) Raman spectrum of lithiophilite over the 100 to 4000 cm-1 spectral range (b)
Infrared spectrum of lithiophilite over the 500 to 4000 cm-1 spectral range
Figure 2 (a) Raman spectrum of lithiophilite over the 800 to 1400 cm-1 spectral range (b)
Infrared spectrum of lithiophilite over the 500 to 1300 cm-1 spectral range
Figure 3 (a) Raman spectrum of lithiophilite over the 300 to 800 cm-1 spectral range (b)
Raman spectrum of lithiophilite over the 100 to 300 cm-1 spectral range
15
Figure 1a Raman spectrum of Lithiophilite (upper spectrum) and Figure 1b infrared spectrum of Lithiophilite (lower spectrum)
16
Figure 2a Raman spectrum of Lithiophilite (upper spectrum) in the 800 to 1400 cm-1 spectral range and Figure 2b infrared spectrum of Lithiophilite (lower spectrum) in the 500 to 1300 cm-1 spectral range
17
Figure 3a Raman spectrum of Lithiophilite (upper spectrum) in the 300 to 800 cm-1 spectral range and Figure 3b Raman spectrum of Lithiophilite (lower spectrum) in the 100 to 300 cm-1 spectral range
18
19