OH incorporation in nearly pure MgAl₂O₄ natural and synthetic spinels

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• 作者：Davide Lenaz ; Henrik Skogby ; Fabrizio Nestola ; Francesco Princivalle
• 关键词：pollen ; Neogene ; palaeoclimate ; biostratigraphy ; lithostratigraphy ; fluvial deposits
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2008
• 出版时间：15 January 2008
• 年：2008
• 卷：72
• 期：2
• 页码：475-479
• 全文大小：225 K

文摘

Four nearly pure MgAl₂O₄ spinels, of both natural and synthetic occurrence, have been studied by means of X-ray single crystal diffraction and FTIR spectroscopy in order to detect their potential OH content. Absorption bands that can be assigned to OH incorporated in the spinel structure were only observed in spectra of a non-stoichiometric synthetic sample. The absorption intensity of two bands occurring at 3350 and 3548 cm⁻¹ indicate an OH content of 90 ppm H₂O. Based on correlations of OH vibrational frequencies and O–HO distances, the observed absorption bands correspond to O–HO distances of 2.77 and 2.99 Å, respectively, which is close to the values obtained by the structure refinements for ^VIO–O_unsh (2.825 Å) and ^IVO–O (3.001 Å). This indicates that one probable local position for hydrogen incorporation is the oxygens coordinating a vacant tetrahedral site. The present spectra demonstrate that the detection limit for OH in Fe-free spinels is in the range 10–20 ppm H₂O. However, at appreciable Fe²⁺ levels, the detection of OH bands becomes hampered due to overlap with strong absorption bands caused by electronic d–d transitions in Fe²⁺ in the tetrahedral position.