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Delafossite structure of heterogenite polytypes (HCoO₂) by Raman and infrared micro-spectroscopy
C Burlet1, H Goethals1, Y Vanbrabant1
1Royal Belgian Institute for Natural Sciences, Geological Survey of Belgium, Jenner Street, 13, BE-1000 Brussels, Belgium.
Heterogenite, a cobalt mineral, is better described as HCoO2 with a delafossite-type structure. New methods distinguish its 2H and 3R polytypes using Raman spectroscopy and electronic backscattered diffraction.
Area of Science:
- Mineralogy
- Crystallography
- Spectroscopy
Background:
- Heterogenite is traditionally identified as cobalt oxy-hydroxide (CoO(OH)).
- Existing literature lacks clarity on its precise crystallographic and vibrational characteristics.
- Natural mineral samples exhibit complex structural variations.
Purpose of the Study:
- To re-evaluate the chemical formulation and structural model of heterogenite.
- To investigate the vibrational properties of heterogenite using Raman and infrared spectroscopy.
- To differentiate and map natural polytypes of heterogenite for the first time.
Main Methods:
- Acquisition and analysis of Raman and infrared spectra from natural heterogenite samples.
- Utilizing electronic backscattered diffraction (EBSD) for crystallographic analysis and mapping.
- Comparing spectral data with EBSD mapping results.
Main Results:
- Raman and infrared spectra suggest a delafossite-type structure (ABO2) rather than CoO(OH).
- Absence of free OH- modes and presence of hydrogen bonding in spectra support the HCoO2 formulation.
- EBSD successfully distinguished and mapped the 2H and 3R heterogenite polytypes.
- A distinct Raman peak at 1220 cm(-1) was identified for the 2H polytype.
Conclusions:
- The chemical formula HCoO2 and delafossite-type structure accurately represent heterogenite's vibrational behavior.
- Raman spectroscopy, in conjunction with EBSD, provides an efficient method for distinguishing heterogenite polytypes.
- This study clarifies heterogenite's mineralogy and offers a new tool for its characterization.
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