Crystal Chemistry of Zircon and Monazite: Crystal Structure, Major and Trace Elements, and Radiation Damage
Creators: name: Zaman, Md. Mashrur
Publisher: name: Graduate Studies
Year: 2015
Subjects: subject: Geology
subject: FOS: Earth and related environmental sciences
subject: Mineralogy
subject: XRD
subject: Crystal

description: This study investigates the crystal structural variations and their relation to chemical composition and radiation effects of detrital zircon and monazite, and the elevated radionuclide concentrations in zircon and monazite-rich heavy mineral deposits in Cox’s Bazar, Bangladesh. Several experimental techniques such as electron-probe micro-analysis (EPMA), single-crystal X-ray diffraction (SCXRD), synchrotron high-resolution powder X-ray diffraction (HRPXRD), gamma-ray spectroscopy with hyper-purity germanium detector (GRS-HPGe), and gravity, magnetic, and electrical mineral separators have been used to carry out this research. In addition, several software programs including SHELX, WinGX, GSAS-EXGUI, Crystal Maker, and Gamma-W have also been applied for data processing and analysis. The unit-cell parameters for the eight zircon samples vary linearly with increasing unit-cell volume, V. The detrital zircon sample 7:PIF (Perry Island Formation) from the Canadian Arctic Islands has the lowest unit-cell parameters and bond distances, ideal stoichiometric composition, and is unaffected by α-radiation damage. Thus, sample 7:PIF is chemically and structurally pure zircon. Sample 8 from Jemaa, Nigeria shows the significant change throughout the synchrotron HRPXRD trace and reveals the largest structural parameters after the Rietveld refinement. Samples 1 to 7 show very good correlations between the V and Zr and Si apfu contents. They received α-radiation doses which are lower than ~3.5 × 1015 α-decay events/mg. Substitutions of other cations at the Zr and Si sites control the variations of structural parameters for samples 1 to 7. The sample 5 shows relatively long unit-cell parameters and bond distances because the Zr site accommodates other cations that have higher ionic radii. Geological age increases the radiation doses in zircon and it is also related to the V. The a and b unit-cell parameters for monazite samples 1, 2, 3, and 4 vary systematically with V, although each monazite sample contains several cations that occupy the Ce/Sm site in the monazite structure. However, the c unit-cell parameter shows limited variation. The increase or decrease of the average distances is dependent on the type of cations occupying the Ce site in the monazite structure but the average distances is independent showing a rigid body behavior obtained with SCXRD and EPMA. The HRPXRD data shows pegmatitic Ce-dominated monazite contains multiple phases and Sm-dominated monazite has a single phase. The multiple phases in sample 2a may not be crystallized at the same time because the average distances differ. Redistribution of Ce and P site cations with Y in sample 2a is also indicative late recrystallization of phases 2a and 2c. As the pegmatitic monazite sample 2a received a high amount of alpha-radiation doses, the phase changes occur for the effects of alpha-radiation. The activity concentrations for 238U, 235U, 232Th and 40K in bulk beach sand samples are found to be considerably high and positively correlated to the amount of heavy minerals present in the sands. In the separated mineral fractions, the highest activity concentration was found in the zircon followed by garnet, rutile, ilmenite and magnetite fractions. The determination of the radium activity, several radiation hazard indices, and absorbed and effective gamma doses allow to assess the related exposure of the coastal environment and the local population to elevated radioactivity. It becomes evident from the present study that if raw sands or mineral fractions mined in the study area are used for building purposes or industrial use, their activity concentrations have to be considered from a radio-ecological perspective and if mining and processing of the minerals is being considered, U and Th may become strategically significant by-products.