Dear Howard,
We have a paper in press (Mineralogical Magazine) that examines microprobe analytical issues, including Na loss, for the 'severe cases' of zeolites. There might be something useful in there that is applicable to omphacite. But for any analytical method,
I would recommend trying to find closely matching reference minerals, in line with what Alex was saying about Na-silicate standards.
A pre-proof full-text of the above paper, Campbell et al (2016) "Determination of zeolite-group mineral compositions by EPMA", is available at
http://www.researchgate.net/profile/Linda_Campbell10/
I hope this helps a little.
Regards,
Linda Campbell.
Dr. Linda S. Campbell,
Academic Visitor,
Sch. Earth, Atmospheric and Environmental Sciences, The University of Manchester, U.K.
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Date: Mon, 25 Jan 2016 19:41:51 +0200
From: "Proyer, Alexander (Prof)" <[log in to unmask]>
Subject: Re: Na loss in pyroxene
Dear Howard,
as nobody has come up yet with a published systematic study of the matter, let me contribute some of my experience at this point.
Analysing feldspars and omphacites routinely for years on a SEM equipped with an EDX system at 15kV, 5nA beam current (EMPAs usually use higher beam current), we were of course concerned about potential Na loss
and made a series of tests on our albite and jadeite standards: between 5 and 10 measurements subsequently on the same spot on the mineral, to see if later analyses show a trend in the Na/Si ratio that might indicate Na-loss.
Measurement times were 20 sec on peak, 10 sec on each background.
The jadeit worked perfect under these conditions, and even the albite which was considered a sure candidate for Na-loss did not show significant changes (given the EDX precision) after 5 measurements.
The setup was not perfect, e.g. I don't think we had freshly polished surfaces of the standards, etc., but I stopped being worried about Na-loss at that point.
As mentioned by others, non-stoichiometry in high-temperture/high-pressure pyroxenes due to significant amounts of Ca-Escola endmember might be an issue.
I have summarized some of the references around this in a Contrib. Mineral. Petrol. paper in 2004 (Pitfalls in geothermobarometry of eclogites... - CMP 147:305-318) - that study was done, hoever, with a high precision microprobe at 15 kV, 20nA and used micro-Moessbauer
analysis to determine ferrous and ferric iron.
What never worked was trying to analyse Na in pyroxenes with an albite standard or Na in a feldspar with the jedeite standard.
Hope that adds something you did not know yet.
Alex
Alexander Proyer
Professor of Petrology
Department of Geology
University of Botswana
________________________________________
From: Metamorphic Studies Group [[log in to unmask]] On Behalf Of day [[log in to unmask]]
Sent: Sunday, January 24, 2016 5:07 AM
To: [log in to unmask]
Subject: [geo-metamorphism] Na loss in pyroxene
Colleagues
Does anyone know of accessible studies of sodium loss (or not) from omphacitic pyroxene during microprobe analysis?
Thanks
Howard
Howard W. Day, Professor Emeritus
The Dept. Formerly Known as Geology
University of California Davis
One Shields Ave.
Davis CA 95616
