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The crystallographic relation between rutile
needles and garnet host is given in the following paragraph in Hwang et al.
(2007, CMP, 25, 349-362):
"The pronounced needle-like morphology of
rutile also suggests that they could have grown along a specific lattice
direction of rutile, i.e. the long axis of rutile needle could be parallel to a
specific lattice direction of rutile. For this reason, seven needles from
samples 96Y5A/96Y5B were analysed in detail in order to define their growth
directions. Five needles are within ~7o off from the normal of {101}rt (cf. Fig.
2f for example), one needle is ~23o off from the normal of {101}rt and is nearly
parallel to the <101>rt, the other is ~61o off from the normal of {101}rt
and is nearly parallel to the normal of (210)rt. The preferred {101}-normal
growth explains the oblique extinction of the rutile needles in OM observations.
Note that the oblique extinction of oriented rutile inclusions in garnet was
previously taken as a probable indication of the presence of high-pressure
polymorphs of titania in other UHP rocks (Song et al., 2004). By plotting the
long axes of all needles along the [-11-1] direction of garnet, stereographic
projections of the rutile a- and c-axes show their random orientation relative
to the garnet host (Fig. 3). In other words, except for the gross alignment
between the <111>grt and {101}-normal of rutile, there are no specific
crystallographic orientation relationships between garnet and
rutile."
========================
Dr. Tzen-Fu Yui
Institute
of Earth Sciences
Academia Sinica
P.O. Box 1-55, Nankang,
Taipei-,
Taiwan
TEL: 886-2-27839910 ext 621
FAX: 886-2-27839871
e-mail:
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-----Original
Message-----
From: Metamorphic Studies Group [mailto:[log in to unmask]]
On Behalf Of Horst Marschall
Sent: Friday, December 09, 2011 11:38 PM
To:
[log in to unmask]
Subject: Re: inclusions in garnet
On 9
Dec 2011, at 10:21, Jim Eckert wrote:
> Actually, a seemingly
'characteristic' feature of these seems to be anomalous extinction...i.e., not
parallel to c.
I assume that the extinction is not parallel to
the elongation of the needles? That doesn't necessarily mean that it is not
parallel to the c axis of the rutile. It could mean that the exsolution of the
rutile is entirely controlled by the host garnet, and the habitus of the
lamellae doesn't express their crystallographic orientation. Any insight from
the TEM folks on this?
cheers,
Horst