Relationships between textural and photoluminescence spectral features of carbonado (natural polycrystalline diamond) and implications for its origin

Hidemi Ishibashi, Hiroyuki Kagi, Shoko Odake, Hiroyuki Ohfuji, Hiroshi Kitawaki

Research output: Contribution to journalArticlepeer-review

1 Citation (Scopus)

Abstract

Field Emission SEM (FESEM) textural observations, crystal size distribution (CSD) analyses, UV-excited luminescence imaging, and photoluminescence (PL) microspectroscopy excited by 488 nm laser were conducted on two texturally contrasting samples of carbonado, a kind of natural polycrystalline diamond from the Central African Republic (CAR). The investigated carbonado samples A and B show extremely different textures: sample A is made up of faceted crystals accompanied by abundant, small rectangular pores, whereas sample B has a granular texture with coarser crystals and scarce, large pores. Diamond crystals smaller than 2–3 µm are enriched in sample A but depleted in sample B. These textural features indicate that sample B diamonds were annealed under thermodynamically stable P–T conditions. The pore characteristics indicate that fluid permeability was higher for sample A than sample B. Photoluminescence (PL) spectra indicate that samples A and B correspond to Group A and B carbonados in the classification of Kagi et al. (1994), respectively, so that sample A reveals emissions from the H3 center without any N–V0 derived emission at 575 nm, whereas sample B shows emissions from the 3H center and the N–V0 defect. In addition, UV-excited luminescence images and photoluminescence spectra for sample B indicate that the rims of diamond crystals within several microns of a pore show luminescence features similar to those of Group AB carbonados (Kagi et al., 1994), indicating that this Group AB material was formed from Group B by irradiation from pore-filling, radioactive-element-bearing materials at a low temperature. The extent of the low-temperature irradiation is considered to depend on fluid permeability, and the Group A material was strongly irradiated due to its permeable texture whereas the Group B material was not significantly irradiated due to its less permeable granular texture. These results indicate that Group B carbonados have retained their original PL spectral features produced under high pressures and temperatures at mantle depths.

Original languageEnglish
Pages (from-to)882-889
Number of pages8
JournalGeochemistry International
Volume54
Issue number10
DOIs
Publication statusPublished - 2016 Oct 1
Externally publishedYes

Keywords

  • CSD
  • carbonado
  • diamond
  • photoluminescence
  • texture

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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