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Subject:

Re: wollastonite in volcanic ash

From:

Robert Tracy <[log in to unmask]>

Reply-To:

No title defined <[log in to unmask]>

Date:

Mon, 19 Feb 2001 10:50:48 -0500

Content-Type:

text/plain

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Parts/Attachments

text/plain (74 lines)

Eric's response over the weekend prompts me to mention that we have
found similar material, in this case products from combustion inside
country-rock spoil piles at an abandoned coal mine near here (SW
Virginia).  The pile apparently contained quite a bit of coal,
spontaneously ignited and smoldered for a long time and probably
reached >1000C temperatures, sufficiently high to promote substantial
low-P melting.  It's superficially similar to the Mottled Zone rocks
from the Negev that Eric mentioned in his message and the
coal-combustion products Mike Cosca studied some years ago.  I guess
it's a philosphical issue as to whether you call this occurrence
"geological" or not - the ignition was apparently natural, as was the
formation of igneous and metamorphic minerals, but the starting
material was obviously there through human intervention.  The
resemblance to Obenholzner's xenoliths in El Popo ash is striking.

The "rock" occurs as a fine-grained, highly vesiculated and slag-like
matrix cementing fragments of various size of fissile limestone and
carbonaceous shale.  These aggregate masses occur in huge chunks up
to several meters in size.  In thin section, the vesicular material
is texturally much like a basalt, with laths of plagioclase in a
heterogenous fine-grained matrix that includes Fe-Ti oxides and
pyroxene-like minerals (based on very cursory EDS examination).  The
baked limestone and shale fragments have a wide range of high-T
minerals (including wollastonite, a ferrobustamite-like phase, calcic
pyroxene, spinel and other as-yet unidentified minerals) that we have
not studied in any detail, as well as apparent amazing short-range
fO2 gradients.  We hope to study our samples in much greater detail
in the next year or so.  Sadly, the US Bureau of Mines came in last
year and "remediated" the property and the spoil piles now look like
a golf course, so a good "hard-rock" field trip locality in the midst
of a sea of soft rocks has gone away.

Bob T.

>Dear colleagues,
>I am writing a manuscript on new textural, mineralogical and chemical results
>on volcanic ash and aerosols: SEM and FE SEM examinations. All data are from
>the recent activity of Popocatepetl volcano, Mexico. One part is dedicated to
>the occurrence of Fe-rich wollastonite, hercynite and buchites
>(glasses with an
>origin of contact metamorphism [CM]) as a population of fragments of the
>volcanic ash. The wollastonite is not homogeneous, it shows Fe-rich
>and Fe-poor
>parts which are intergrown. Some grains show  vesiculation-like textures.
>Does anybody know SEM studies of similar wollastonites etc.? Also the buchites
>are highly heterogeneous.
>The main question is: Are these fragments products of ongoing CM underneath
>Popocatepetl, or are they fragments of an old aureol. Popocatepetl is situated
>on a ca. 3 km thick sequence of lime/dolostones/evaporites etc. of Cretaceous
>age.
>Buchites show wild compositions, they are rich in S, Cl and P. Anhydrite and
>inheritated zircons are enbedded in this glassy/devitrified matrix.
>Ideas, references and coauthorship for essential contributions are welcome.
>Thanks and best wishes
>Johannes H. Obenholzner, PhD
>MNH-Mineralogy- Vienna/Austria
>email: [log in to unmask]

--
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Dr. Robert J. Tracy
Professor of Geological Sciences
Virginia Tech
Blacksburg VA 24061-0420

540-231-5980
[log in to unmask]
(FAX: 540-231-3386)

"We can't solve problems by using
the same kind of thinking we used
when we created them."
-- Albert Einstein

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