Darwin-L Message Log 1:188 (September 1993)

Academic Discussion on the History and Theory of the Historical Sciences

This is one message from the Archives of Darwin-L (1993–1997), a professional discussion group on the history and theory of the historical sciences.

Note: Additional publications on evolution and the historical sciences by the Darwin-L list owner are available on SSRN.

<1:188>From mhallbey@magellan.geo.usherb.ca  Tue Sep 21 18:20:30 1993

Date: Tue, 21 Sep 1993 19:20:07 -0400
From: mhallbey@magellan.geo.usherb.ca (Mryka Hall-Beyer)
To: darwin-l@ukanaix.cc.ukans.edu
Subject: RE: Classification in mineralogy

	Interesting reply about minerals. I think we are defining "observable"
in slightly different manners. I quite agree that a phenomenon such as colour
is not appropriate in a classification scheme. Noting the colour or the range
of possible colours or the most common colour of a mineral is useful in a rough
identification key, but that is as far as it goes. I guess the fact that I work
in remote sensing biases me into thinking of things as "observable" that are
not accessible to our senses. The main "observable" bases underlying the
standard mineralogical classification scheme are weight percents of the 14
or so most common elements, Si:O ratios, and crystallographic structure. The
structure may be observable to the extent to which it influences crystal
shape, but of course most minerals one sees do not have well-developed crystal
faces. That's what makes mineral-hounding fun, trying to find those few that
_do_ have such structure. The structure usually has to be observed using
x-ray crystallography to be really sure.
	I find your ideas about persistence and process interesting. I had not
thought of these concepts in relation to classification. Yet in some ways it
is the process of mineralogical change that underlies metamorphic petrology.
The question about chemical equilibria on various scales is crucial to using
chemical mineralogy to interpret paleopressures and temperatures. The presence
or absence of certain mineral species, or the state of transition between them,
is how we define metamorphic isograds. My master's work looked at partitioning
of Mg and Fe between coexisting minerals (on the mm scale) as a geobarometer.

	I'm not sure where this fits into your ideas - it just occurred to me
as I read them and I thought I'd toss it out.  Thanks for the discussion.
Lurking around a different but kindred field gets the brain cranking!

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