SEM, EDX and vibrational spectroscopy of the phosphate mineral vauxite from Llallagua, Bolívia

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Highlights

  • We have studied the mineral vauxite.

  • The chemical formula was determined.

  • Raman and infrared bands are assigned to phosphate stretching and bending modes.

  • Vibrational spectroscopy enables the assessment of the molecular structure of vauxite to be undertaken.

Abstract

We have undertaken a vibrational spectroscopic study of vauxite from Llallagua, Bolívia. This source is important source for rare and unusual secondary phosphate minerals and is the type locality for a number of rare phosphates such as vauxite, sigloite, metavauxite and for jeanbandyite. The chemical formula was determined as (Fe0.98Mn0.01)∑0.99(Al2.00)(PO4)∑2.03(OH)1.98·5.95(H2O).

The Raman spectrum is dominated by intense Raman bands at 978, 1000, 1009, 1027 cm−1 assigned to the PO43− and HPO42− stretching modes. Low intensity Raman bands are found at 1046, 1059, 1070, 1105, 1122, 1134 and 1150 cm−1 and are assigned to the PO43− ν3 antisymmetric stretching vibrations. Raman bands of at 498, 502, 517, 523 and 535 cm−1 are assigned to the ν4 PO43− bending modes while the Raman bands at 418, 451, 461 and 470 cm−1 are due to the ν2 PO43− bending modes. The Raman spectral profile of vauxite in the hydroxyl stretching region is broad with component bands resolved at 2918, 3103, 3328, 3402, 3555 and 3648 cm−1. Vibrational spectroscopy enables the assessment of the molecular structure of vauxite to be undertaken.

Section snippets

1. Introduction

The vauxite mineral is a rare hydrothermal phosphate and it‘s general chemical formula is given as FeAl2(PO4)2(OH)2·6H2O. This supergene phophate was first discribed by Gordon [1]. After this, he also discrebed in details about 70 minerals in 1944, in Llallagua mine. The study of mineralogy and petrology in this area was recently published [2]. Vauxite crystalizes with triclinic symmetry and unit cell parameters a = 9.13 Å, b = 11.59 Å, c = 6.14 Å, α = 98.3°, β = 92.0°, γ = 108.4°, V = 608 Å3, Z = 2 and space

2.1. Samples description and preparation

The vauxite sample studied in this work was collected from the Siglo XX mine (also named Siglo Veinte, Catavi or Llallagua), a tin deposit located in the Andes Mountain, Bustillo Province, northern of Potosí department, Bolivia. In the middle of the 20th century Siglo XX was the most productive tin mine in the world. The mine is also an important source for rare and unusual secondary phosphate minerals and is the type locality for a number of rare phosphates such as paravauxite, sigloite,

3.1. Chemical characterization

The SEM image of selected crystals of vauxite is reported in Fig. 1.

The quantitative chemical data were recalculated considering 28.54% of H2O in the structure, as expected for the vauxite end member. The H2O was distributed as OH and H2O according to the crystal structure. The Fe was considered as Fe2+. The chemical analysis is shown in Table 1. The chemical formula was calculated on the basis of 16 O atoms and can be expressed as: (Fe0.98Mn0.01)∑0.99(Al2.00)(PO4)∑2.03(OH)1.98·5.95(H2O).

4.1. Background

Interestingly Farmer in his book on the infrared spectra of minerals divided the vibrational spectra of phosphates according to the presence, or absence of water and hydroxyl units in the minerals [6]. In aqueous systems, Raman spectra of phosphate oxyanions show a symmetric stretching mode (ν1) at 938 cm−1, the antisymmetric stretching mode (ν3) at 1017 cm−1, the symmetric bending mode (ν2) at 420 cm−1 and the ν4 mode at 567 cm−1 [7], [8], [9], [10], [11]. Farmer reported the infrared spectra of

5. Vibrational spectroscopy

The Raman spectrum of vauxite over the 4000–100 cm−1 spectral range is displayed in Fig. 2a. This spectrum shows the position and relative intensity of the Raman bands. There are large parts of the spectrum where no or little intensity is observed. Thus, the spectrum is divided into subsections depending upon the type of vibration being studied. The infrared spectrum of vauxite over the 4000–600 cm−1 spectral range is displayed in Fig. 2b. As for the Raman spectrum, there are large parts of the

6. Conclusions

As vauxite is isostructural with laueite Mn2+Fe3+2(PO4)2(OH)2·8H2O [12], it could be indirectly concluded that the structure of vauxite is based on an infinite chain of vertex-linked oxygen octahedra, with Al occupying the octahedral centers, the chains oriented parallel to the c-axis. Chains are in turn connected to others by PO4 tetrahedra which also bridge through isolated octahedra (with Fe2+ as centers). Chemically, vauxite is closely related to its basic namesake paravauxite [13], [14],

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 instrumentation.

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