Hi all,
we studied a similar occurrence in:
Cesare B., Gomez-Pugnaire M.T., Sanchez-Navas A. & Grobety B. (2002)
Andalusite - sillimanite replacement (Mazarrón - SE Spain):
microstructural and TEM study.American Mineralogist, 87. 433-444.
Another partial pseudomorph of Sil on And, with the involvement of
staurolite inclusions, is in:
Cesare B. (1994) Hercynite as the product of staurolite decomposition in
the contact aureole of Vedrette di Ries, eastern Alps, Italy.
Contributions to Mineralogy and Petrology, 116, 239-246
For more on epitaxy among Al2SiO5 polymorphs see also:
Cesare B. and Grobety B. (1995) Epitaxial replacement of kyanite by
staurolite: a TEM study of the microstructures. American Mineralogist.,
95, 78-86
Regards,
Bernardo
--
Bernardo Cesare
Dipartimento di Geoscienze
Via Giotto, 1, I-35137 PADOVA ITALY
Tel: ++39-049-8272019 Fax: ++39-049-8272010
email: [log in to unmask]
web site: http://www.dmp.unipd.it/bernardo/bernardo.html
> Hi,
>
>
>
> Following on Bob Tracy’s comment, I attach two images of sillimanite after
> andalusite from the LPHT metamorphic rocks of the Peninsular Ranges
> batholith, Baja California, Mexico. As a side note, Ron Vernon was
> actually
> in the field with me the year (1997 or 1998?) that I collected this
> sample.
> We could actually see the large chiastolitic grain shapes in the samples,
> so
> assumed that it was andalusite. We also noted some blue coloration and I
> was
> excited about the possibility of finding kyanite. Turns out it is all
> sillimanite – barely a trace of remnant andalusite in the samples
> collected.
>
>
>
> The EBSD data show that sillimanite with three different c-axis
> orientations
> replaced the chiastolite grains. Note that the sillimanite crosses grew
> between the chiastolitic sector zone boundaries. Note also that the
> sillimanite replacing the main body of the andalusite has recrystallized.
>
>
>
> I would be interested to know if anyone has seen this type of
> microstructure
> before. We are considering possibly working this up for a short
> publication.
>
>
>
> Cheers - Scott
>
>
>
>
>
>
>
> *************************************************************************
> Scott E. Johnson
> Department of Earth Sciences
> 5790 Bryand Global Sciences Center
> University of Maine
> Orono, ME 04469-5790
> USA
> email: [log in to unmask]
> phone: (207) 581-2142
> Fax: (207) 581 2202
> <blocked::http://www.geology.um.maine.edu/user/scott_johnson/HM.html>
> http://www.geology.um.maine.edu/user/scott_johnson/HM.html
> <blocked::http://www.geology.um.maine.edu/geodynamics/>
> http://www.geology.um.maine.edu/geodynamics/
> *************************************************************************
>
>
>
> From: Metamorphic Studies Group [mailto:[log in to unmask]]
> On
> Behalf Of Robert Tracy
> Sent: Sunday, July 26, 2009 7:29 AM
> To: [log in to unmask]
> Subject: Re: oriented sillimanite after ?andalusite
>
>
>
> Hanan Kisch's comments are very interesting. When I first saw the
> photomicrographs, I immediately thought of sillimanite replacements of
> andalusite (sometimes quite large original andalusite crystals up to cm in
> scale) that occurs in rocks of southern New Hampshire and central
> Massachusetts USA. In many cases the replacement (a pseudomorphing of a
> kind) is complete, but the single large original chiastolite grain shapes
> (commonly even with carbonaceous crosses) are perfectly preserved by finer
> grained aggregates of sillimanites in multiple orientations, but all
> apparently sharing a common c-axis orientation. In thin sections cut
> across
> the foliation and lineation of the rock, the classic diamond shapes of the
> individual sillimanite grains can be seen in the replacements. John
> Rosenfeld wrote an admirable description of this phenomenon in a paper in
> American Journal of Science in 1969 (the Al-silicate issue), and several
> papers or field guides by Peter Robinson and his students in the late
> 1970s
> and early 1980s also describe these composite replacement textures.
>
> The replacement occurs at sillimanite + K-feldspar grade and commonly with
> migmatization, probably at T of ca. 750C at roughly 0.6 GPa. Our
> interpretation was of an early low-P contact metamorphic phase related to
> magmatism in a Taconic back-arc setting, succeeded by an increase in P as
> well as T in a more regional event , although still dominated by magmatic
> heating (so-called "anti-clockwise P-T trajectory" similar to that
> described
> in the New England terrane of Australia by Ron Vernon and Bill Collins).
> My
> impression is that whereas Hanan's case from Norway represents two
> distinct
> events separated in time, the New England USA occurrence is more likely a
> continuum that starts in the ca, 440-445 Ma time frame with contact
> metamorphism and the regional phase is likely at 430-435. Looking back
> again
> at his message, however, there is a striking similarity of timing and
> overall character (not uncommon for Caledonian phenomena in Norway,
> northern
> UK and in the northern Appalachians).
>
> Bob Tracy
>
> Hanan Kisch wrote:
>
> Dear Dr Homam,
>
> A belated comment.
>
> Coarse sillimanite porphyroblasts (Kisch 1962, Plate VII, fig. 2 and Plate
> VII, fig. 1) along the western contact of the Fongen-Hyllingen gabbro
> complex in the Tydal area, central Caledonides of the eastern Trondheim
> region, contain sillimanite in two orientations (ibid., Plate VII, fig.
> 2).
> This silimanite was probably formed as replacement of andalusite, which
> occurs further south in the contact aureole of the southern termination of
> the gabbro (Vogt 1941; Birkeland & Nilsen 1972). The gabbro was dated as
> late Silurian; together with its contact aureole, it was overprinted by
> late-Caledonian staurolite-kyanite grade regional metamorphism.
>
> References :
> Kisch, H.J. (1962) Petrographical and geological investigations in the
> southwestern Tydal region, Sör-Tröndelag, Norway. Acad. Proefschr., Univ.
> of Amsterdam, 136 pp.
> Vogt, Th. (1941). Geological notes on the Dictyonema locality and the
> upper
> Gauldal district in the Trondheim area. Norsk Geol. Tidsskr. 20, 171-192.
> Birkeland, T. & Nilsen, O. (1972) Contact metamorphism associated with
> gabbros in the Trondheim region. Norges Geol. Unders. 273, 13-22.
>
> I hope that this is some help.
>
> Regards,
>
> Hanan Kisch
>
> Dear All
>
> As Freshte (Ranjbar) supervisor I would like to express my honest
> acknowledgement for your valuable comments regarding andalusite query.
> What
> we have in region seems to be a very complex metamorphic case but most
> probably a contact metamorphism followed by a regional deformation.
> Andaluasit as medium to large fully oriented grain, garnet, fibrolite and
> cordierite (?) can be seen in thin sections. No staurolite has been found.
> Using different direction sectioning from the sample helped us to find
> what
> was the mineral. As a matter of fact it was noting but an oriented growth
> of
> sillimanite in andalusite, very similar with that reported by Prof. Vernon
> (1987) from Juan Tabo area, USA.
>
> Best Regards
> Dr. S. M. Homam
>
>
>
> _____
>
>
>
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>
>
> Prof. Hanan J. Kisch
> Department of Geological and Environmental Sciences
> Ben-Gurion University of the Negev
> P.O.B. 653, Beer-Sheva 84105, Israel
> Tel.: +972 8 646 1290
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> E-mail: [log in to unmask]
>
>
>
>
>
> --
>
>
>
> Dr. Robert J. Tracy
>
> Department of Geosciences
>
> 5064 Derring Hall
>
> Virginia Tech
>
> Blacksburg VA 24061-0420
>
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>
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