Sorry for the wrong citation in previous message:


The crystallographic relation between rutile needles and garnet host is given in the following paragraph in Hwang et al. (2007, JMG, 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."


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Dr. Tzen-Fu Yui
Institute of Earth Sciences
Academia Sinica
P.O. Box 1-55, Nankang,
Taipei-, Taiwan

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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