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Infra-red microspectroscopy of diamond in relation to mantle processes

Cooper, Guy Ian; (1990) Infra-red microspectroscopy of diamond in relation to mantle processes. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

Infrared spectra of diamond reveal the presence of broad absorption bands in the one-phonon region (1332 cm-1 to 900cm-1) which vary substantially within and between specimens in both structure and magnitude. These bands are sometimes accompanied by peaks in the vicinity of 1368cm-1 and/or in the C-H region of the spectrum (3200 - 2800 cm-1). Previous research has shown that the 1-phonon bands are associated with the presence of nitrogen in diamond, largely in the form of defects whose structures are generally agreed to form an aggregation sequence from single nitrogen atoms ('N') present in synthetic diamonds to 2-nitrogen ('A') or 4-nitrogen ('B') aggregates, and some type of extended defects of variable size known as "platelets" associated with the 1368cm-1 absorption. The defect aggregation state can be altered in laboratory experiments only at high temperatures, because the rates are slow enough to result, in many diamonds, only in partial aggregation under Mantle conditions even in 3Gy, but the activation energy involved in the 'A'->'B' nitrogen aggregation process had not been determined, and it was not known whether a first or second-order process was involved. This thesis reports the results of a series of experiments carried out at temperatures between 2250°C and 2650°C under a stabilizing pressure of 100 kb in order to determine this activation energy. The results indicated a second order process with an activation energy of 6.99eV for the 'A' to 'B' aggregation process. It has been shown that replicate measurements of infrared spectra taken using an infra-red microscope lead to a substantial reduction in the errors involved in determining the degree of aggregation which takes place during the experiment The ratio of the 'B':'A' nitrogen absorption coefficients (.177) was in good agreement with values obtained from infrared measurements on large diamonds by other workers. The relationship between the platelet peak intensity and the extent of 'B'-nitrogen aggregation has been altered by the laboratory experiments, but no explanation for this effect has been found. The nature of the graphite formed from the diamonds has been investigated, and its preferred orientation has been determined in relation to the morphology of the specimens. The preferred orientation is not necessarily related to the faces present on the specimen, but always includes the expected parallelism of the hexagonal graphite axis and the trigonal diamond axes. It has been shown that the activation energy obtained can be used to calculate the storage temperatures for different diamond populations from their infrared spectra, for comparison with results obtained from inclusion geothermometry, and that better comparisons can be made from diamond plates polished to expose inclusions and permit infra-red spectra to be obtained nearby.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Infra-red microspectroscopy of diamond in relation to mantle processes
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest.
Keywords: Earth sciences; Activation energy
URI: https://discovery.ucl.ac.uk/id/eprint/10111738
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