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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 - 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 [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 ,

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, , strengite, variscite and

[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 4000525 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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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