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Re: Coprolites (or "Why X-ray paleo-poop?")



<<<Absolutely. Powder X-ray diffraction analysis always has worked for
me! (provided no replacement has occured, there should be nice sharp
peaks for fossil hyroxy-fluor-apatite). Take your sample to the nearest
university geology department, and have them "zap it" for you.>>>

> Is there such a thing a diagenetic hydroxyflourapatite?

Yes indeedy.

> Could a
> phosphate-rich dropping precipitate apatite inside itself?

Yes.

> I don't know for
> sure but I did think that most coprolites were preserved by phosphatic

> mineralisation of the groundmass. If so wouldn't a thin section
(looking for
> fragments with micro features such as lacunae, haversian canals, etc.)
be
> more diagnostic?

If the bones in the cop's are from small birds, small lizards, etc,
there may be no diagnostic biogenic features observable in a
thin-section, but I agree that a thin-section would be the first thing
to try if the suspected bone-material in the cop' is macroscopic.

Many moons ago, (15+ years ago) I did an independant study on how (or
if) both hydroxyfluorapatite and carbonate fluorapatite in fossil bone
differed in their X-ray diffraction pattern from the abiotically-formed
apatites of similar compositions.  I found consistent and striking
differences in the relative peak intensities, peak widths, peak skewness
and peak kurtosis between the biotic and abiotic apatite groups.

Fossil biogenic apatites that have not been subsequently altered
(replaced or recrystallized) always had a uniform crystallite diameter
and have a preferred crystallite orientation.  When the specimen is
ground-up and placed in a powder mount, these characteristics are
maintained (the xls self-align slightly due to the handling process).
The pattern was skewed slightly and sometimes had a moderate degree of
peak kurtosis.  In contrast, mechanically-powdered
non-biologically-formed apatites show the typical random cleavage planes
and shapes of the igneous (and diagenetic) mineral apatites.  These
feature showed up on the X-ray diffraction pattern in the form of much
more symmetrical diffraction peaks.

I also discovered an interesting feature when I did X-ray diffraction
comparisons between fossil bone and recent bone.  Fossil bone always had
much sharper and better-defined peaks than did recent bone.  At first I
attributed this phenomenon to a possible re-crystallization in the
fossil bone, but later I concluded that the bone collagen that is
ubiquitously disseminated between the crystallites in recent bone was
hampering me from grinding the bone fine enough for the X-ray
proceedure. (The powders have to be ~30-40 microns or less to yield nice
sharp diffraction patterns).

Sorry for being slightly off-topic, but perhaps this little bit of
esoterica may be useful to someone.

                           <pb>