Re: Any crystallographers out there?

In article <qdb4s21hukecj7b3erdsblispsaen35uhr@xxxxxxx>,
Chris Hogg wrote:
See the following links for photos of the crossed
feldspar twins.
http://i5.tinypic.com/4g5f393.jpg
http://i7.tinypic.com/2n8tyiq.jpg
http://i7.tinypic.com/2qvxz14.jpg
http://i5.tinypic.com/2rrph1u.jpg

The pebbles have been wetted to enhance their colour
and the contrast between the matrix and the feldspar
phenocrysts. The coin is a UK 10 pence piece, ~25mm
(1 inch) in diameter.

IMHO, the 'crosses' are clearly feldspar phenocrysts.

Agree with your diagnosis.
I don't recognise the style of twinning though.
I believe they are an
exaggerated or extreme form of one of the common forms
of feldspar twinning, possibly Carlsbad as Will
suggests, but my understanding of the descriptions of
twinning in text-books etc. is not good enough to
be able to work out which one it is.

If my memories of twinning in feldspars is correct,
then the angle between twin a and a' axis (to choose an
axis at random) is constant and diagnostic of a particular
style of twinning. I certainly don't recognise it.
By coincidence I bumped into my former Ig.Pet. and
Min. professor at another professors retirement party last
year, and we had a good chin wag. I'm just wondering if he
could recognise the style, because he was always into the
derivation of granites.

Oh hang on - even better :
PhDs supervised (year of completion)

Aberdeen

[SNIP]
* 1990 JC Rushton (with NH Trewin) The diagenetic
formation of feldspars

So that's what JC at work did his PhD in. And since
I've got to go up to the office at some point this
afternoon ... printout time.
--
Aidan Karley, FGS,
Aberdeen, Scotland
A light wave is more like a crime wave than a water wave.

.

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