Re: Geology Question (KRS related)

On Sat, 04 Feb 2006 22:12:22 +0100, Hayabusa <peregrine@xxxxxxxxxxx>
wrote:

On Sat, 04 Feb 2006 21:14:17 +1300, Eric Stevens
<eric.stevens@xxxxxxxxx> wrote:


Biotite is not biotite no matter what. See:

http://groups.google.com/group/sci.archaeology/msg/cd7be37e5eca5544?dmode=source&hl=en

That's my quote. - Look there is a difference between igneous and
metamorphic biotite.

Presumably neither the biotite contained in greywacke or the the
biotite contained in slate would be igneous?


For that to decide I would need to see the TS. But as a rule of thumb,
I'd say if it is coarse (1-3 mm-size) it is igneous, if it is far
smaller it is metamorphic. (I am talking about single biotite grains
that form one sedimentary grain each. There might be bio in the rock
fragments too, and they should give the origin away.) But you can
easily tell them from one another in a TS because the igneous bio
starts to turn into chlorite by new metamorphic conditions very soon
whereas the metamorphic-derived biotite is quite happy.


My comment was mased only on the fact that the temperatures you cited
earlier for the metamorphosing of the greywacke were way below what I
would expect with an igneous rock. I seem to be missing something
(like a geology text book :-). Can you explain the apparent
contradiction?



Eric Stevens

.

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