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Its crystals may be independent (evolved granites, pegmatites, nepheline
syenites) or be inclusions in other minerals (biotite, amphibole, cordierite).
In this case, pleochroic halos due to radiation from radioactive trace ele-
ments in zircon develop in the hosts: the dark halo in biotite and amphibole,
yellow in cordierite.
Pupin and Turco (1972) have shown some correlation between the form
of single crystals and type of magmatism. These results are far, however, of
having an absolute and systematic value.
Zircon is one of the heavy minerals easily reworked - sometimes con-
centrated - in the sediments. It is therefore commonly found in metamor-
phic rocks as well as ortho- as para-origin.
Zircons commonly contain 1 wt% of hafnium, and traces of heavy Rare
Earths, yttrium, uranium and thorium. Zircon is commonly used in geo-
chronology due to the presence of these radioactive elements.
Generally a single crystal of zircon records a complex history with
several episodes of growth (formation of zoned euhedral crystals) and ero-
sion (eroded grain boundaries intersecting the previous zonation). Zircon
growth occurs either when the zircon is incorporated into a newly formed
igneous rock, or during metamorphism. The core of the crystals is not re-
equilibrated. During the alteration of these rocks, the zircon crystals may be
eroded, re-sedimented and subsequently included in a new cycle (inherited
zircons): zircons are forever !
6
5
2
1
1
3
4
5
6
Figure 3.38 Image by catodoluminescence of a zircon from the orthogneiss of Canigou
(Pyrénées orientales, France; after Cocherie, 2005). The size of this crystal
is of the order of 250 microns. Zone 1 is the oldest part of the zircon. Then
comes a zone of growth (2). This first zircon is eroded, then comes a second
zone of growth (3). Then we notices the stages of erosion followed by growth
(4) and (5) and an area of subsequent marginal overgrowth (6).
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