I have done lots of SEM work on Triassic tuffs from the Southern Alps
(Austria & Italy). The rhyolitic tuffs contain many zircons. Some of them
show spongy texture and various etching features. The backscattered image of
the SEM could also see  Zr- and Y-rich clouds around altered zircons. The
tuffs are altered by burial diagenesis und may be a postdepositional
hydrothermal event.

See also:
Obenholzner, J. H. & Heiken, G.: Textural studies of altered/metamorphosed
tuiffs and secondary REE,-, Zr- and Y-minerals. Annalen des
Naturhistorischen Museums in Wien, 100A, 13-38, Vienna 1999.

There is also an abstract at AGU Spring meeting 2000.

best wishes
Johannes H. Obenholzner
Postfach 417
A-1014 Vienna

Dr Sandeep Singh wrote:

> Dear All
> Happy New year.
> I am working on SHRIMPING (ion probe mass-spectrometry) the Himalayan
> zircon having metamorphic overgrowth. During this I have come across some
> of the zircons which have spongy texture. Can anybody give me some
> references on that which explains the phenomenon.
> Sandeep
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> Dr. Sandeep Singh
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