The structure of mimetite, arsenian pyromorphite and hedyphane – A near-infrared spectroscopic study

Electronic and vibrational spectra of mimetite, arsenian pyromorphite and hedyphane minerals have been analysed and the spectra related to the mimetite and arsenian pyromorphite and hedyphane mineral structure. The chief spectral feature in the electronic spectra at ∼10 000 cm−1 (1.00 μm) with variable band position and intensity results from the ferrous ion. The splitting of Fe(II) band is large in mimetites with a separation of 1415 cm−1. An additional band shown by arsenian pyromorphite at 10 735 cm−1 (0.93 μm) is assigned to Cu(II) dd-transition. The substitution of Fe(II) causes a blue shift for Cu(II) band in mimetites and the intensity of this band is enhanced at ∼11 140 cm−1 (0.90 μm). The change in colour from brown to orange-yellow relates to the amount of Cu and/or Fe impurities in the mimetite minerals.The presence of OH-stretching overtones at 7000 cm−1 (1.43 μm) is the result of OH substitution for Cl. Phosphate ion bands are produced in arsenian pyromorphite, hedyphane and mimetite-3. Arsenian pyromorphite spectrum is a mixed profile dominated by PO stretching vibrations at 1100–900 cm−1. The spectrum of hedyphane is significant with arsenate bands at 900–700 cm−1 and weak bands appear on either side of this due to the phosphate ion. The absence of phosphate ion group of bands and the presence of (AsO4)3− bands near 800 cm−1 in selected mimetites is evidence for isomorphic substitution of (AsO4)3− by (PO4)3−.

A comparison of Raman spectra of pyromorphites with published infrared and Raman data of similar minerals has been made. Crystals of the pyromorphite- mimetite series are chemically zoned, resulting in changes in the crystal structure with the isomorphic replacement of phosphate by arsenate with the consequential change in crystal structure from monoclinic to hexagonal. Such dimorphic relationships are exemplified by the shift in the Raman spectral band positions of phosphate with increasing arsenate substitution.Figure optionsDownload as PowerPoint slide