Dear All,                                       6 March '99
        I received about twenty responses to my original e-mail, many of
which asked me to post my results. Hence, I am now doing so. About half of
the responses provided specific observations or comments, either
petrographic or theoretical. A few provided references to papers which
specifically discuss some textural effects that can be attributed to the
presence of the graphite. I have included such responses below, but edited
them to include only the directly relevant portions.
        In my own case some petrographic features I've observed in
graphite-rich rocks are as follows.
         I've noted some things like clearly reduced grain size of the
matrix phases (mainly the layer silicates); better development of euhedral
porpyroblasts; better preservation of andalusite with regard to
pseudomorphing associated with polymetamorphism, hour glass Gr
concentrations in some sectors of St, but pushing it aside along other
sectors (the 010, I think?) so that a mound of graphite accumulates along
the external face bounding that sector;
        Obviously my observations have clear similarities with some of
those provided below by several repondents.
        In aggregate, the responses suggest to me that there may be some
systematic patterns as to abundant graphite effecting the textural aspects
of metapelites, and that there may also be theoretical reasons to expect
this to be the case. Moreover, the responses also suggest to me that
workers should take care in making their petrogenetic interpretations not
to merge uncritically observations made in graphite-rich rock with
observations made on graphite-poor/absent rocks.
        MANY thanks to all who responded to my original e-mail.
                                Cheers,  Chuck

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Date: Mon, 1 Mar 1999 15:44:37 -0800 (PST)
From: Bernard Evans <[log in to unmask]>
To: "Charles V. Guidotti" <[log in to unmask]>
Subject: Re: Effects of graphite on metapelite textures

Charlie,
    It has long been known that the presence of graphite tends to result
in smaller grain sizes. The only reference I could find quickly was Rast
on p.84 of "Controls of Metamorphism" eds. Pitcher and Flynn, Geol Journal
Special Issue No. 1, 1965. He implies that finely disseminated graphite is
like fine mica in creating high surface tension on mica-quartz interfaces.
    All the best,  Bernard
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Date: Tue, 2 Mar 1999 13:52:10 +1300 (NZDT)
X-Sender: [log in to unmask]
To: [log in to unmask] (Charles V. Guidotti)
From: Julie Vry <[log in to unmask]>
Subject: Re: Effects of graphite on metapelite textures

Dear Dr. Guidotti:

I'd also be really interested in knowing what you find out in response to
your question...

The texture that I've seen a lot of is a (commonly) star-shaped pattern of
inclusions (much like a chiastolite cross) developed in garnets from
graphitic layers. The garnets in less graphitic or graphite-free layers may
show evidence of having overgrown an earlier foliation (inclusion trails,
etc.), while garnets in the more graphitic layers of the same thin-section
may just show the star-shape pattern of inclusions. This can make it tricky
to tie the deformation history to specific episodes of garnet growth...
The samples are from the Alpine Schist, near the Alpine Fault in New
Zealand, so deformation is an important part of the story.

Any ideas you've got on these textures, or references that turn up in
response to your query, would be interesting to me as well.

Thanks!
Julie Vry
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Date: Tue, 2 Mar 1999 12:31:30 +1000
To: [log in to unmask] (Charles V. Guidotti)
From: Nick Oliver <[log in to unmask]>
Subject: Re: Effects of graphite on metapelite textures

Hi Chuck,
        the place I've most seen this is in your neck of the woods, in the
Bronson Hill Anticlinorium! Garnets there show apparently embayed textures,
elongate shapes like cereal biscuits (we call them weetbix here) with feet
coming off them, sometimes like a caterpillar (not the inclusions!). I
never followed this up in the literature. I mainly assumed it was a
combination of inhibiting grain growth by pinning the garnet grain
boundaries with the graphite impurities (much on this in the quartz
deformation literature - quartz grain boundary migration impeded by small
micas - and any metallurgy book - different iron phases with
insoluble/refractory impurities), and kinetic effects related to the lack
of carbon in the crystal structure of most silicates, i.e. these are not
inclusions that can be removed by slow grain growth but must be a special
category of "immobile" inclusions that chemically impede garnet growth -
even ilmenite, titanite etc (most other common inclusions in garnet) all
have components that can be "shared" with garnet so perhaps this is why
they don't drastically affect the external morphology.

cheers, Nick Oliver
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X-Sender: [log in to unmask]
Date: Tue, 2 Mar 1999 14:56:36 +1000
To: [log in to unmask] (Charles V. Guidotti)
From: Tim Bell <[log in to unmask]>
Subject: Re: Effects of graphite on metapelite textures

Chuck
Hi! We talk about it in Bell & Brothers 1985 JMG and in Rubenach & Bell
1988 JMG. The interesting thing is that the strain energy is commonly
preserved in cren hinges with graphite and if the graphite burns off, it
gets removed and the micas recrystallize as two or three grains instead of
bent ones.
Cheers
Tim
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From: [log in to unmask]
To: [log in to unmask]
Subject: Graphitic pelite textures
Date: Tue, 2 Mar 1999 09:33:55 -0000
MIME-Version: 1.0

Dear Chuck,

No doubt you have already been inundated with identical messages...

I worked in interlayered semi-pelites and graphitic pelites in southern
Spain, collecting numerous samples from garnet grade to granulite.  The
dichotomy in textures is very marked; graphite appears to act as a strong
control on crystal (particularly porphyroblast) growth.  Miyashiro (1994:
Metamorphic Petrology book) discusses graphite at length, but not with
respect to textures.  However, there is a paper by Kevin Burton (1986:
Mineralogical Magazine vol.50, 611-620), which explains the oriented
intergrowth of tubular quartz rods in garnet by reduced SiO2 solubility in a
C-O-H fluid.

I haven't tested this theory, but the field evidence seems to support it.
Similar quartz tubules occur in andalusite (chiastolite), garnet and
staurolite from graphitic schists, with characteristic graphite 'drifts' on
crystal faces in many cases.  These textures were absent from graphite-poor
schists, where irregular poikiloblasts generally formed instead of
idioblastic crystals.

Although the slow growth suggested by these textures implies an inhibitor of
some sort, it should be noted that graphitic layers were also
correspondingly more aluminous than graphite-poor layers, and there may be
other factors involved.

Hope this is some help - I'd be grateful if you could post any other
references you receive, for interest.
Cheers,

Tom
Dr T.W.Argles, NERC Research Fellow
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X-Sender: [log in to unmask]
Date: Tue, 02 Mar 1999 11:51:09 +0100
To: [log in to unmask] (Charles V. Guidotti)
From: [log in to unmask]
Subject: Re: Effects of graphite on metapelite textures

dear chuck, perhaps this is obvious to you and everyone else,
so i won't share this pearl of wisdom with the whole list, but
in materials science they frequently observe an effect known
as "grain boundary pinning" in which the prescence of a secondary
phase inhibits coarsening of the primary phase. i know that there
is at least one paper on this effect with a geological material
(calcite) in contributions ca '90, one of the authors being dave
olgaard. cheers, jamie.
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Date: Tue, 02 Mar 1999 11:59:52 +0000
From: Dave Waters <[log in to unmask]>
Reply-To: [log in to unmask]
Organization: Earth Sciences, University of Oxford
To: "Charles V. Guidotti" <[log in to unmask]>
CC: [log in to unmask]
Subject: Re: Effects of graphite on metapelite textures

Yes, this seems to be a general feature.

Three papers which discuss the effect are (grabbed out of Georef)

Garnet-quartz intergrowths in graphitic pelites; the role of the fluid
phase.
Burton-K-W
Mineralogical Magazine. 50, Pages 611-620. 1986.

Porphyroblast textural sector zoning and matrix displacement.
Rice-A-H-N; Mitchell-J-I
Mineralogical Magazine. 55; 380, Pages 379-396. 1991.

Textural and twin sector-zoning and displacement of graphite in
chiastolite and pyralspite and granite garnets in the Variscides of
South-west England.
Rice-A-H-N
Proceedings of the Ussher Society. 8, Part 2; Pages 124-131. 1993.

Dave Waters
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Date:          Tue, 2 Mar 1999 15:24:38 -0600 (CST)
To:            [log in to unmask]
From:          John Valley <[log in to unmask]>
Subject:       Effects of graphite on metapelite textures

Hi Chuck

I can't put my hand on the reference, but I have read that small amounts of
graphite "poisons the grain boundaries". I think this means that the large
number of small refractory crystals inhibits grain growth by other phases.
Does this accord with your observations. Dunn and Valley 1992 show that in
marbles graphite often coarsens at ca. 500C and then the marble can coarsen
as well.

I have been discussing the amounts of graphite necessary to make a rock
gray lately with the deep crust electrical conductivity folks. Yardley and
Valley 1997 argue that graphite is not commonly present at 1 wt %. other
than in metamorphosed black shales. I have measured the carbon content (for
isotope analysis ) of dark gray rocks with finely dispersed graphite and
found as little as 0.1 wt % is enough to markedly darken the color.
Graphite is opaque down to thicknesses of 100nm as evidenced from the thin
coats we put on thin sections for the probe.

cheers,   John
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From: Peter Thompson <[log in to unmask]>
To: "[log in to unmask]" <[log in to unmask]>
Subject: Graphite-rich textures
Date: Wed, 3 Mar 1999 08:46:53 -0600
MIME-Version: 1.0

Dear Chuck,
I certainly have noticed textural differences in graphite-rich pelites, most
recently in the interlayered graphitic and silver-green phyllites/fine
schists of the "Foot Brook slice" in the northern Green Mountains (similar
to Ottauquechee and Stowe Formations).  Overall the graphitic rocks tend to
be finer grained, with more pervasive foliation, despite apparently similar
tectonic histories.  They are also more susceptible to development of later
spaced cleavage.  I don't know if others have written about this in the
literature.  My own rather naive interpretation is that glide within the
graphite crystal lattice, and between graphite grains, results in a
different strain response than in micaceous and chlorite-rich rocks under
the same stress field.  I would hazard the observation that chlorite-rich
phyllites, such as phyllonites derived from sheared greenstones, also show
this more pronounced development of closely-spaced foliation than the more
micaceous phyllites.

Not sure if this jives with what you're talking about.  Perhaps we can
discuss this more at GSA?  Peter.
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From: "Sally Goodman" <[log in to unmask]>
Organization: McGill University - EPS
To: [log in to unmask]
Date: Wed, 3 Mar 1999 10:41:50 EST5EDT
Subject: Re: graphite in metapelites

Chuck -

The main effect I've observed in graphitic pelites is that the
graphite appears to inhibit recrystallisation, so that graphitic
pelites tend to preserve more primitive textures/structures than
their graphite-free counterparts. Is this the effect you have in
mind?

I've always assumed that the chemical inertness and general
immobility of carbon allowed it to pin grain boundaries by slowing
diffusion - though I don't know that I've ever read any reference to
this...

Sally Goodman
McGill University, Montreal
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Date: Wed, 03 Mar 1999 12:01:17 -0700 (MST)
From: Peter Buseck <[log in to unmask]>
Subject: Re: Effects of graphite on metapelite textures; 3/3/99
X-Sender: [log in to unmask]
To: [log in to unmask] (Charles V. Guidotti)
Cc: [log in to unmask] (Bertrand)

Hi Chuck
We have found, and are working on, unusual textures/growth features in
phyllosilicates that grew in the presence of graphite. It is interesting
that you are also finding unusual features. I am now curious to see whether
this a widespread phenomenon.
Regards,  Peter
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C.V. GUIDOTTI                                Univ. Phone (207) 581 2153
UNIVERSITY OF MAINE                          Univ. Fax (207) 581 2202
DEPARTMENT OF GEOLOGICAL SCIENCES            Home Phone (207) 947 1388
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