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Dear Frank:
        I completely agree with your opinion on "pseudosection", which I heard in a
seminar by a European petrologist. I thought it is used as did A.B.
Thompson, but then could not follow the lecture.


-----Original Message-----
From:   Frank Spear [SMTP:[log in to unmask]]
Sent:   Thursday, March 01, 2001 4:59 AM
To:     [log in to unmask]
Subject:        "Pseudosections"

Hey all you etymology freaks...can anyone enlighten me as to the
origin of the term "pseudosection", or as to its proper usage.

The term is currently in common usage to describe P-T or T-X
assemblage stability diagrams drawn for specific bulk compositions
(e.g. Powell, et al., 1998, JMG, 557-588).  The context here is a
diagram that outlines the fields in which a particular mineral
assemblage is stable, and lines on the diagram are defined where the
mode of one phase is zero.

Many authors refer to Hensen (1971; CMP, 33, 191-214) as the
originator of these types of diagrams, and he does, indeed, seem to
have published the first such diagram (his Fig. 8), but he doesn't
(that I can find) ever use the word "pseudosection".

Alan Thompson (1976; AJS, 276 401-424) uses the term "pseudo-binary
diagram" to describe T-X or P-X sections drawn at a constant Al
content (reference the AFM diagram). As I understand it, he used the
term "pseudo" because the diagram is not a rigorous projection, such
that crossing tie lines do not necessarily reflect reaction
relationships. This usage of "pseudo" seems fine.

In my dictionary, the adjective "pseudo" means "an imitation intended
to deceive or mislead" (i.e. fake, counterfeit, spurious, bogus,
phoney, false). Is this the connotation we wish for these types of
diagrams? I guess I would prefer a term such as "mineral or
assemblage stability diagram".

Cheers to all the pedants out there.


Frank
----------------------------------------------
Frank Spear
Professor and Chair
Department of Earth and Environmental Sciences, JRSC 1C25
Rensselaer Polytechnic Institute
Troy, New York 12180
Phone (office): 518-276-6103
           (lab)     : 518-276-4899
Fax:                  518-276-6680
http://www.rpi.edu/dept/geo/spear/spear.html