Dear Julie, Eric and Peter,

the problem certainly lies with the mass absorption coefficients. Some
twenty-five years ago, we used here a Cambridge instrument with a 70°
take-off angle, thereby minimising absorption effects, and using the set of
pure oxides that Julie uses was really fine, even for Fe-Ca-rich garnet. As
we moved to a Cameca instrument with 40° take-off angle (not to speak of
one with 18°), the results with this set of standards were depressing, for
instance with andraditic or calderitic garnets.
Best wishes to all.
Christian

At 07:22 10/01/03 -0500, you wrote:
>Julie,
>      In our experience, oxide standards are poor choices for light
>elements in unknowns with relatively high atomic numbers.  We would
>use an almandine standard for Fe, Al, Si; a rhodonite for Mn,
>synthetic geikielite for Mg, synthetic pyrophanite for Mn, the
>wollastonite may be OK if it is coarse enough.  I think the problem
>lies with some or all of the light elements, Mg, Al, Si, where the Z
>and A corrections will be very large with these standards.  Penn St.
>Univ. does have a good garnet standard (Ingamells almandine).  I
>think that using FeSiO3 would also be good for Si, but you still need
>a high Z standard for Al and Mg.  By the way, SiO2 is easily beam
>damaged if you use a point beam with currents greater than 10 nA.
>eric
>
>
>
>>Hi Eric
>>
>>We are heavily dependent on pure oxide standards for most of our work:
>>I've been using:
>>
>>SiO2 (synthetic)
>>Al2O3 (synthetic)
>>Fe2O3 (synthetic
>>MnO (synthetic)
>>MgO (synthetic)
>>Wollastonite (synthetic)
>>
>>Operating conditions:
>>I normally run the probe with the accelerating voltage at 15 kV.
>>I've tried juggling the sample current, everywhere from about 12 na
>>to 6 na.  That hasn't seemed to solve anything.
>>
>>If by getting in a couple of different garnet standards I could get
>>around the analytical problems I am encountering, I would be really
>>pleased if you could/would offer some suggestions...
>>
>>Thanks, Julie Vry
>>
>>
>>
>>P.S. I don't have any garnet standards in the minerals bar, just:
>>olivine (San Carlos, USNM 111312/444; Springwater meteorite, USNM 2566);
>>augite (Kakanui, New Zealand)
>>pyroxene (P.S.U. Px-1, Goldich et al. Can J. Earth Sci 4, 747, 1967)
>>Diopside (P.S.U. 63-1827)
>>Amphibole, Engels
>>Hornblende (P.S.U. 4-190)
>>Tremolite
>>Grunerite
>>Biotite (P.S.U. 5-110)
>>Biotite (U.M. R-2208)
>>Albite, Amelia Ab-1
>>Orthoclase Or-1
>>Ilmenite, Ilmen Mtns (USNM 96189)
>>Gahnite (Brazil) (USNM 145883)
>>Obsidian, comenditic, KN18, Naivasha, Kenya
>>Basaltic glass, VG-A99, Makaopuhi, Hawaii (USNM 113498/1)
>>Rhyolitic glass, VG-568, Yellowstone Nat. Park (USNM 72854)
>>
>>>Julie,
>>>     What standards are you using and what operating conditions when
>>>you get 103 wt%?
>>>eric
>>>
>>>>I encounter persistent difficulties in analysing the Fe-Ca-Mn garnets
>>>>(about 1/3 of each component) that are common on the NZ Southern Alps
>>>>(usually I get about 103% totals).  I suspect that the problem is in
>>>>the matrix correction factors, but that is not easy for me to sort
>>>>out.
>>>>
>>>>I wonder if using a garnet standard of similar composition might help...
>>>>(I don't have such a standard)
>>>>
>>>>Any suggestions as to how I might deal with the problem would be
>>>>appreciated.
>>>>The probe (a 20+ year-old JEOL JXA-733 SuperProbe, using Moran
>>>>Scientific software) works wonderfully except for this.
>>>>
>>>>Julie Vry
>>>>--
>>>>Dr. Julie Vry
>>>>Senior Lecturer - Geology (metamorphic petrology)
>>>>School of Earth Sciences
>>>>Victoria University
>>>>PO Box 600
>>>>Wellington, New Zealand
>>>>
>>>>
>>>>
>>>>phone, office:  64 4 463 6432 (please do NOT leave messages here)
>>>>please leave messages at: 025 644 5598
>>>>fax  64 4 463 5186
>>>>email:  [log in to unmask]
>>
>>--
>>Dr. Julie Vry
>>Senior Lecturer - Geology (metamorphic petrology)
>>School of Earth Sciences
>>Victoria University
>>PO Box 600
>>Wellington, New Zealand
>>
>>
>>
>>phone, office:  64 4 463 6432 (please do NOT leave messages here)
>>please leave messages at: 025 644 5598
>>fax  64 4 463 5186
>>email:  [log in to unmask]
>
>
=====================================================
Christian Chopin               [log in to unmask]
Laboratoire de Geologie        fax: +33 1 44 32 20 00
Ecole normale superieure
24 rue Lhomond
75005 Paris, France
http://www.geologie.ens.fr/~wwwminer/mineralogie.htm
        European Journal of Mineralogy website:
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=====================================================