Due to radioactive decays of U and Th families, in particular in the form of alpha radiation with energy [greater than or equal to] 4,6 MeV whose emission is accompanied by relevant recoil interactions, the structure of zircon can be heavily damaged over geological times, resulting in a partially aperiodic state, the so-called metamict
This is supported by the Raman spectrum of this zircon (Figure 30), which is characterised by very sharp and pronounced vibrational peaks, as would be expected in metamict
and translucent zircon only after heat treatment (Zhang et al., 2000; Nasdala et al., 2002; Wang et al., 2006; Krzemnicki, 2010) or in well-crystallized (non-metamict) and commonly transparent zircon.
Before isotopic analysis, in addition to microscopic observation, back-scattered electron (BSE) images and cathodoluminescence (CL) images were taken with an EPMA (electron probe micro-analyzer) to check for metamict
regions, inclusions, and compositional zoning patterns.
Some zircon grains of Mata Oscura Granite have metamict
Two collections, entitled The Metamict
State (1987) and Gaps and Verges (1990), were published by the University of Florida Press.
It is also found as partially metamict
crystals in cavities in the lower sill.
1988), which was based on a single analysis of highly fractured and metamict
grains, may be erroneously young due to significant Pb loss from these zircons.
Also, the diffusion rate may increase due to strong radiation damage in metamict
It is radioactive, and the XRD pattern has broad, low peaks, indicating that the structure is partly metamict
(broken down by internal radioactive bombardment).
One highly discordant fraction of partly metamict
zircon grains plots to the left of this discordant trend, likely due to ancient Pb-loss, the age of which cannot be estimated reasonably from the data at hand.
The crystals can show etch figures indicating that conditions changed near the end of the crystallization; these figures could be confused with signs of incipient metamictization, but the mineral is not metamict
. The crystals are invariably pleochroic, as are all crystals of the minerals of the epidote group.
In general, smaller grains were clear and free of inclusions, whereas larger grains tended to be slightly metamict
. Seven zircon fractions were analyzed, of which five consisted of one or two grains and the remaining two were multi-grain fractions.