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Hematite colour revisited: Particle size and electronic transitions
Isabela F S Dos Santos1, Howell G M Edwards2, Dalva L A de Faria3
1Paulista Museum, University of São Paulo, 04263-000 São Paulo - SP, Brazil; Institute of Chemistry, University of São Paulo, 05508-000 São Paulo - SP, Brazil.
Spectrochimica Acta. Part A, Molecular and Biomolecular Spectroscopy
|January 17, 2024
Summary
The purple color of hematite, a pigment, originates from its crystallinity, not particle size. This crystallinity alters light absorption, particularly below 500 nm, creating the distinct deep hue.
Area of Science:
- Materials Science
- Solid-State Chemistry
- Spectroscopy
Background:
- Hematite (α-Fe2O3) is a widely used natural pigment known for its color range.
- The purple variety, caput mortuum, has historically been attributed to particle size.
- Understanding the origin of its color is crucial for pigment applications.
Purpose of the Study:
- To investigate the origin of the purple color in hematite.
- To differentiate between particle size and crystallinity as the cause of the purple hue.
- To elucidate the optical mechanisms responsible for the color.
Main Methods:
- Diffuse Reflectance Spectroscopy (DRS) for optical properties.
- Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS) for morphology and composition.
- Raman Microscopy for structural and vibrational analysis.
Main Results:
- Purple hematite's color is linked to crystallinity, not particle size.
- Crystallinity causes decreased absorption below 500 nm and enhanced d-d transitions around 880 nm.
- Distorted FeO6 octahedra likely influence oxygen-to-iron charge transfer transitions.
Conclusions:
- The purple color of hematite is primarily due to its crystalline structure.
- Structural distortions in FeO6 octahedra play a significant role in the observed optical properties.
- Further experiments are needed to confirm the exact role of charge transfer transitions.
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