Vibrational Spectroscopic Characterization of the Phosphate Mineral Anapaite Ca 2 Fe 2+ (PO 4 ) 2 · 4(H 2 O)

ISSN： 0038-7010

Source： Spectroscopy Letters, Vol.46, Iss.6, 2013-08, pp. : 441-446

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Abstract

We have characterized anapaite Ca₂Fe²⁺(PO₄)₂ · 4(H₂O), a rare Ca and Fe phosphate, using a combination of electron microscopy and vibrational spectroscopy. The mineral occurs in soils and lacustrine sediments and is usually related to the diagenetic process in phosphorous-rich sediments. The phosphate anion is characterized by its Raman spectrum with an intense sharp band at 943 cm⁻¹, attributed to the ν₁ symmetric stretching mode. Three bands at 992, 1039, and 1071 cm⁻¹ are attributed to ν₃ antisymmetric stretching modes. The infrared spectrum of anapaite shows complexity with a series of overlapping bands. Water in the structure of anapaite is observed by OH stretching vibrations at 2777, 3022, and 3176 cm⁻¹ (Raman) and 2744, 3014, and 3096 cm⁻¹ (infrared). The position of these bands provides evidence for the strong hydrogen bonding of water in the anapaite structure and contributes to the stability of the mineral.