Electron microprobe analyses are presented of chevkinite-group minerals from Canada, USA, Guatemala, Norway, Scotland, Italy and India. The host rocks are metacarbonates, alkaline and subalkaline granitoids, quartz-bearing pegmatites, carbonatite and an inferred K-rich tuff. The analyses extend slightly the range of compositions in the chevkinite group, e.g. the most MgO-rich phases yet recorded, and we report two further examples where La is the dominant cation in the A site. Patchily- zoned crystals from Virginia and Guatemala contain both perrierite and chevkinite compositions. The new and published analyses are used to review compositional variation in minerals of the perrierite subgroup, which can form in a wide range of host rock compositions and over a substantial pressure- temperature range. The dominant substitutions in the various cation sites and a generalized substitution scheme are described.

Zircon- and reidite-type ZrSiO4 produced by shock recovery experiments at different pressures have been studied using infrared (IR) and Raman spectroscopy. The v3 vibration of the SiO4 group in shocked natural zircon shows a spectral change similar to that seen in radiation-damaged zircon: a decrease in frequency and increase in linewidth. The observation could imply a possible similar defective crystal structure between the damaged and shocked zircon. The shock-pressure-induced structural phase transition from zircon (I41/amd) to reidite (I41/a) is proven by the occurrence of additional IR and Raman bands. Although the SiO4 groups in both zircon- and reidite-ZrSiO4 are isolated, the more condensed scheelite gives rise to Si–O stretching bands at lower frequencies, suggesting a weakening of the bond strength. Low-temperature IR data of the reidite-type ZrSiO4 show an insignificant effect of cooling on the phonon modes, suggesting that the structural response of reidite to cooling-induced compression is weak and its thermal expansion is very small.

The effects of ion irradiation in the ABO4-type compounds were compared by performing experiments on four materials that include the most common crystal structures (monazite vs. zircon) and chemical compositions (phosphates vs. silicates) for these phases. Pure synthetic single crystals of ZrSiO4, monoclinic ThSiO4, LaPO4 and ScPO4 were irradiated using 800 keV Kr+ ions. Radiation damage accumulation was monitored as a function of temperature in situ in a transmission electron microscope. The activation energies for recrystallization during irradiation were calculated to be 3.1–3.3 eV for the orthosilicates but only 1.0–1.5 eV for the isostructural orthophosphates. For the ion-beam-irradiated samples, the critical temperature, above which the recrystallization processes are faster than damage accumulation and amorphization cannot be induced, is >700°C for ZrSiO4 but it is only 35°C for LaPO4. At temperatures above 600°C, zircon decomposed during irradiation into its component oxides (i.e. crystalline ZrO2 plus amorphous SiO2). The data are evaluated with respect to the proposed use of the orthophosphates and orthosilicates as host materials for the stabilization and disposal of high-level nuclear waste. The results show that zircon with 10 wt.% Pu would have to be maintained at temperatures in excess of 300°C in order to prevent it from becoming completely amorphous. In contrast, a similar analysis for the orthophosphates implies that monazite-based waste forms would not become amorphous or undergo a phase decomposition.

The chemical and structural alteration of metamict zircon crystals from a 619 ±17 (2σ) Ma old, posttectonic granite in the southern part of the Eastern Desert, Egypt was studied. The crystals show simple oscillatory growth zones with metamictization–induced fractures, which provided pathways for fluid infiltration. Electron and ion microprobe analyses reveal that metamict, i.e. U and Th–rich, areas are heavily enriched in Ca, Al, Fe, Mn, LREE, and a water species, and have lost Zr and Si as well as radiogenic Pb. These chemical changes are the result of an intensive reaction with a low–temperature (120—200°C) aqueous solution. The chemical reactions probably occurred within the amorphous regions of the metamict network. During the zircon–fluid interactions the metamict structure was partially recovered, as demonstrated by micro-Raman and -infrared measurements. A threshold degree of metamictization, as defined empirically by an α–decay dose, Dc, was necessary for zircons to undergo hydrothermal alteration. It is proposed that Dc marks the first percolation point, where the amorphous domains start to form percolating clusters in the metamict network and where bulk chemical diffusion is believed to increase dramatically. The time of the hydrothermal alteration is determined by a lower intercept age of a U-Pb SHRIMP discordia of 17.9 (2σ) Ma, which is in good agreement with an apatite fission track age of 22.2 (2σ) Ma. The hydrothermal alteration event occurred contemporaneously with the main rifting phase of the Red Sea and widespread low- temperature mineralizations along the Red Sea coast.