Here's another anecdote. The first time I analyzed hogbomite on the probe the chemical analysis was completely mystifying as to what the phase was. Didn't make any sense at all. So I looked around the thin section for a suitably oriented grain to get a good interference figure (not easy, with the deep red-brown color of the mineral and the small grain size). But with persistence I got a uniaxial negative nearly centered OA figure and looked in the trusty old Larsen-Berman tables under uniaxial negative, with a relatively high average refractive index (ca. 1.8). Out popped hogbomite as the obvious answer. So optical mineralogy and petrography combined, as Pavel points out, can be very useful if one has a suitable printed or on-line database to look through.
Bob
Dr. Robert Tracy
Professor of Geosciences
Associate Department Head
Director, Museum of Geosciences
Virginia Tech
Blacksburg VA 24061-0420
540-231-5980
540-231-3386 (F)
On Jan 15, 2016, at 10:55 AM, Pavel Pitra <[log in to unmask]> wrote:
> Dear Peter and all,
>
> I obviously totally agree with you concerning the
> absolute necessity of good petrographic
> observation.
> It can happen, though, that even with good
> petrograpic observation you sometimes don't know
> what you are dealing with (in particular since
> convergent light microscopy is mostly not taught
> anymore or at least most people have difficulties
> using it in a routine way).
> The first time I found large crystals of
> wagnerite (Mg,Fe)2(PO4)F in one of my rocks, I
> was not able to identify it optically - and then
> webmineral.com was useful for interpreting the
> EPMA data. Incidentally, this experience, in
> particular analysing the F (vs. OH) in wagnerite
> made me realise the problems with standards for
> fluorine and the dependence of the analysis on
> the crystal orientation that you mention...
>
> All the best,
>
> Pavel
>
> At 14:31 +0000 15/01/16, Treloar, Peter J wrote:
>> Dear All
>>
>> I read Pavel's comments with interest and felt
>> that I needed to add something here. There might
>> be a bit of a rant here - but please read on and
>> enjoy it.
>>
>> I have just taken early retirement after a life
>> time working as a mineralogist and petrologist.
>> When I did my first degree in Bristol we were
>> taught how to use the optical microscope as the
>> primary means of identifying minerals. I first
>> used the EMPA as a PhD student and then spent
>> four years as a post-doc in Jim Long's lab in
>> Cambridge running the then state-of-the-art WDS
>> and EDS microprobes. I have followed with real
>> fascination how these various have evolved and
>> have been involved in buying three SEM based
>> EPMA systems that deliver both WDS and EDS data.
>> We have widely published precise, accurate data
>> from these instruments and I will argue to the
>> end that EDS is unfairly criticised by those who
>> do not understand the counting statistics.
>>
>> WDS is no different now. Count rates are still
>> constrained by flow counters but it is now all
>> computer driven but you still need to know what
>> you are doing. Peak searches, P/B ratios etc,
>> proper standards.
>>
>> EDS is more interesting. Peter Statham (who cut
>> his teeth in Cambridge in developing the first
>> EDS system with Jim Long and is no at Oxford
>> Instruments) said to me a few years ago that "we
>> now finally understand the physics of the EDS
>> spectrum and how to resolve the overlaps".
>> People are still "sniffy" about EDS data - but
>> it is great. With the new Pentier cooled EDS
>> detectors (which do not need liquid Nitrogen)
>> count rates are faster, P/B rates are better,
>> precision is better and mapping is faster. The
>> growth of non quantitative EDS mapping of
>> mineral assemblages reflects this. For those who
>> wish to ask -about Pentier cooled detectors I
>> did test this when we bought our latest system
>> and they do work. Peak shift and peak broadening
>> is no longer an issue with these systems.
>>
>> EPMA today begs new questions. How best can we
>> (if we can at all) generate accurate (precise is
>> potentially not possible) analytical data using
>> a variable vacuum SEM with no carbon coating of
>> the target? Our data look good - if not
>> encouraging.
>>
>> What effect does the sample orientation have on
>> analytical totals? Rephrase that into a
>> discussion of electron channeling and then think
>> of an orthorhombic or tetragonal mineral.
>>
>> BUT and this is a BIG BUT: I know that there are
>> minerals that you can not easily identify using
>> optical techniques. But the bottom lone before
>> any scientist puts a sample into an EPMA they
>> need to know what they are looking at. This is
>> basic science. Why spend grant money identifying
>> quartz or calcite (or even differentiating
>> between pyroxene and amphibole) when you should
>> be able to do it on the optical microscope.
>> There is a need to do optical microscopy not
>> just to identify mineral assemblages, but do the
>> whole paragenesis thing.
>>
>> As Pavel says - MINDAT is the best of the
>> various chemical packages that you can use. BUT
>> - please do the optical mineralogy.
>>
>> After all:
>>
>> You do need to use chemical stoichiometry to
>> differentiate between so why not start with the
>> optics.
>>
>> My final message is to anybody who thinks that
>> they can cut corners: it is not just us old
>> guys, but the best petrologists working today do
>> optics first, second and third and then, and
>> only then, EPMA.
>>
>> Regards
>>
>> Peter Treloar
>
> --
> Pavel PITRA
> Géosciences Rennes email: [log in to unmask]
> Université Rennes 1 tel: (++33) 2.23.23.65.06
> Campus de Beaulieu - Bat. 15 fax: (++33) 2.23.23.60.77
> F - 35 042 RENNES CEDEX
> FRANCE http://www.geosciences.univ-rennes1.fr/
> http://www.geosciences.univ-rennes1.fr/spip.php?article67
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