Dear Horst and all other working on epidote minerals,
unfortunately, until now there is no official nomenclature on epidote
minerals. There is an IMA working group on the nomenclature of
epidote minerals that will hopefully present their results at the
beginning of 2004 and there will be a RiMG volume on epidote minerals
appearing next spring/summer that will also adress the problem of
nomenclature.
Even the term pistacite is not used consistently. Some use it for
Ca2Fe3+Al2(Si2O7)(SiO4)OOH other for the hypothetical Al-free
endmember Ca2Fe3+3(Si2O7)(SiO4)OOH. As long as there is no official
nomenclature I would suggest to use the term pistacite only for the
hypothetical Al-free endmember, the term epidote only for the whole
mineral group, and the term clinozoisite for the whole monoclinic
Al-Fe3+ solid solution. Because the iron content in clinozoisite
normally does not exceed one Fe p.f.u. you can express the
composition of clinozoisite as mol% Al2Fe (=Fe/(Al+Fe-2)*100) or as
mol fraction Al2Fe (=Fe/(Al+Fe-2) or if you define pistacite as
described above as mol% or mol fraction of pistacite.
The distinction between clinozoisite and epidote based on their
optical character is somewhat arbitrary, because there are no
structural changes accompanying the change in optical character. It
is just a consequence of the different dependence of nx and ny on
increasing iron content. So therefore this distinction should be
avoided.
Cheers, Axel
P.S.: Horst, anything new from Heidelberg? Best greatings to all of you.
--
Dr. Axel Liebscher
GeoForschungsZentrum Potsdam
Sektion 4.1 "Experimentelle Geochemie und Mineralphysik"
Telegrafenberg
D-14473 Potsdam
Tel./Fax: +49 (0) 331 288-1848/1402
e-mail: [log in to unmask]
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