Hi all,
I'm trying to get rid of some bad points in some EMP data I collected on amphiboles a while ago. I figure there's a cut-off on what one might consider a "1st glance bad point" with regards to total wt. % but I don't know what people generally use. Any suggestions? I also figure I could eliminate any points outside of 2sigma, but I don't know if I should calculate the St. Dev before or after eliminating "first-glance" bad totals. Any suggestions would be greatly appreciated.
Thanks,
Nik Deems
Hi Nik,
Are you asking what is too low an acceptable total for an amphibole because it is a hydrous phase and one will usually get less than 100% summation because we don't analyze for hydrogen (water and hydroxl)?
Yes! That's exactly it.
I am not the right person to respond to this but in broad terms amphiboles have ~1.5 weight percent tied-up in hydroxyl, hence at 98.5% you may have a good analysis (and your cation values on the basis of 24 oxygens look reasonable as well). Others will hopefully give you more detailed guidance regarding limits.
Hi Nik,
Usually with hydrous phases, you need to recalculate the expected H2O content based on mineral formula (i.e., assuming a fixed atom per formula unit of OH-group). Such a mineral formula recalculation can be also helpful to check the site occupancy and possibly identify "bad" analysis (i.e. how much cation should be is site A, B, T... and compare with the ideal case). I'm sure you can find on the web or ask a colleague. I also have a website you can calculate mineral formula, and there is a specific entry for amphibole (http://cub.geoloweb.ch/index.php?page=mineral_formula&reset=true (http://cub.geoloweb.ch/index.php?page=mineral_formula&reset=true)). You can also output your results in Probe for EPMA as atomic proportion and recalculate these values based on a fixed amount of cation (e.g. 15 cations for amphibole, with 22 oxygens and 2 OH group), but I don't think there is an option there to recalculate the H2O wt-%... I also attach a spreadsheet for amphibole mineral formula calculation. You should also check the literature for amphibole classification (e.g., papers from Hawthorne 2007 [Reviews in Mineralogy and Geochemistry 67, pp. 55-88] or Leake 1997 [American Mineralogist 86, pp. 1019-1037]), I think these papers contain useful info on the variation of amphibole composition, which could be of some help for your issue, along with some help about mineral formula calculation / classification of the amphibole.
Once you have your mineral formula recalculated and you have an estimate of H2O content, you can then check for outliers based on the classical assumption that a "good" analysis should be between 98 and 102%. If data are below 98%, something is wrong. However, the mineral formula recalculation can be tricky for these nasty "garbage" amphibole for numerous reasons. Here are some...
a) F and Cl can substitute for OH (did you measure them?). The amount of F and Cl will have a direct influence on how much H2O could be present. There can also be some O2- present instead of OH-...
b) Fe is both Fe2+ and Fe3+ in amphibole, and you cannot reliably measure the Fe2+/3+ proportion by EMP. The proportion of Fe2+ and Fe3+ will influence slightly your total as there is more oxygen associated to Fe3+...
c) vacancies exist in amphibole. The presence of vacancy is problematic essentially when checking the site occupancy.
d) Are you sure you have analyzed every single element in these amphiboles? Any missing element will induce a slightly lower total, and might screw up your mineral formula recalculation. However, if you did analyze all elements you can see on the EDS, then your total (with H2O recalculated) should be close to 98-100%. For minor / trace element you could do a long WDS scan. However, in most cases, the simple analysis of Si, Al, Fe, Ti, Mn, Mg, Ca, Na, K, F, Cl and possibly Cr, too, should be enough...
e) Totals might be too low due to, for instance, a bad standardization of some elements or the alteration of the amphibole (e.g. chloritization => more OH-group than normally expected...). A way to know if the problem could be sample or instrument / calibration related is to analyze a secondary standard. Any chance you have a Kakanui hornblende or another hornblende standard (reference material) of known composition? If so, try to analyze such a standard, and compare your analysis with the certified values...
Good luck!
Julien
Julien's reply covers the most ground. Here is my two cents.
In practice, I would keep all analyses with total >96% for amphiboles when O is calculated, then check formula for each point base on O22(OH,Cl,F)2. Amphiboles contain various OH, up to 2.5 wt%? - check "Rock-forming Minerals" for info. Some also contain Li, which is not detectable by epma.
Amphiboles may show elemental zonation within single crystals. Do not eliminate points outside of 2sigma. Those points may be real. Do high-contrast BSE imaging using high current to check zonation.
Have fun!
Chi Ma
In case anyone is interested in amphibole calculations there is an option for site occupancy in amphiboles and biotites in PFE in the Calculation Options dialog as seen here:
(https://smf.probesoftware.com/oldpics/i41.tinypic.com/2guh01h.jpg)
These calculations are from Jay Ague and George Brimhall when Jay was a grad student at UC Berkeley. When this output option is run on say Kakanui Hornblende, one gets the following output:
St 468 Set 2 Hornblende (Kakanui) USNM 143965, Results in Oxide Weight Percents
ELEM: Na2O SiO2 K2O Al2O3 MgO FeO CaO S Cl TiO2 P2O5 F O H2O MnO SUM
141 2.530 40.074 2.069 14.921 12.741 10.924 10.250 .010 -.014 4.796 .069 .005 .328 .938 .090 99.732
142 2.647 40.163 2.049 14.768 12.885 10.856 10.207 .006 -.035 4.712 -.063 .012 .328 .938 .090 99.564
143 2.576 40.171 2.080 14.916 12.717 10.844 10.327 .021 .017 4.763 .012 .029 .328 .938 .090 99.830
144 2.642 40.068 2.111 14.872 12.903 10.823 10.211 -.018 -.026 4.681 -.061 -.013 .328 .938 .090 99.550
145 2.558 40.279 2.004 14.977 12.863 10.983 10.222 .043 -.010 4.640 -.076 -.012 .328 .938 .090 99.827
AVER: 2.590 40.151 2.063 14.891 12.822 10.886 10.243 .012 -.014 4.719 -.024 .004 .328 .938 .090 99.700
SDEV: .052 .086 .040 .078 .086 .066 .050 .022 .020 .063 .062 .017 .000 .000 .000 .137
SERR: .023 .039 .018 .035 .039 .030 .022 .010 .009 .028 .028 .008 .000 .000 .000
%RSD: 2.01 .21 1.93 .52 .67 .61 .48 178.26 -144.96 1.33 -263.26 414.50 .00 .00 .00
PUBL: 2.600 40.372 2.050 14.900 12.800 10.920 10.300 .000 .000 4.721 .000 .000 .328 .938 .090 100.020
%VAR: -.38 -.55 .61 -.07 .17 -.31 -.55 .00 .00 -.04 .00 .00 .00 .00 .00
DIFF: -.010 -.221 .012 -.010 .021 -.034 -.056 .000 .000 -.002 .000 .000 .000 .000 .000
STDS: 336 162 374 160 162 162 162 730 285 22 285 284 0 0 0
Amphibole Formula Calculations (from Jay Ague AMPHI.F code)...
ALL FE2 NORM 1 NORM 2 NORM 3 NORM 4 NORM 5
------ ------ ------ ------ ------ ------
Si 5.8886 5.9916 5.8292 5.7065 5.5664 5.8646
Ti .5205 .5296 .5152 .5044 .4920 .5183
Al 2.5744 2.6194 2.5484 2.4948 2.4336 2.5639
Fe3+ .0000 -.8040 .4644 1.4234 2.5172 .1883
Fe2+ 1.3352 -1.0000 .8573 -1.0000 -1.0000 1.1415
Mg 2.8025 2.8515 2.7742 2.7158 2.6492 2.7911
Mn .0112 .0114 .0111 .0109 .0106 .0112
Ca 1.6097 1.6379 1.5935 1.5599 1.5217 1.6032
Na .7366 .7495 .7292 .7139 .6963 .7336
K .3859 .3927 .3821 .3740 .3648 .3844
F .0019 .0020 .0019 .0019 .0018 .0019
Cl -.0034 -.0034 -.0033 -.0033 -.0032 -.0034
NORM 1: TOTAL-(NA+K)=15 NORM 2: TOTAL-(NA+CA+K)=13
NORM 3: TOTAL-K=15 NORM 4: SI+AL=8.0
NORM 5: TOTAL=15.8
-----------------------------------------------------------
STRUCTURAL FORMULA:
SI 5.8589
ALIV 2.1411 ALVI .4203
TI .5178
FE3+ .2322
FE2+ 1.0963
MG 2.7884
MN .0112
CA 1.6016
NA .3322 NA .4007
K .3840
F .0019
CL -.0034
OH 2.0015
-----------------------------------------------------------
MOLE FRACTIONS AND LOGARITHMS OF ATOMIC RATIOS:
X-FE2+= .157 X-MG= .398
X-FE3+= .033 X-ALVI= .060
X-MN= .0016 X-TI= .0740
X-CA= .229 X-NAM4= .0475
MG / (MG + FE2+) = .718
FE2+/(FE2+ + FE3+)= .825
MG/(MG+FE2+ + FE3+)= .677
X-OH= 1.001 X-F= .001 X-CL= -.002
LOG(X-MG/X-FE2+)= .405 LOG(X-F/X-CL)= .000 LOG(X-F/X-OH)= -3.023
LOG(X-MG/(X-FE2+ + FE3+))= .322 LOG(X-FE2+/X-FE3+)= .674
A-SITE= .785 TOTAL VI= 7.000
ALVI+2TI+A-SITE+FE3+= 2.473 ALVI+2TI+FE3+= 1.688
Schmidt (1992) Pressure (All FE2+): 9.03 KBar, (FE2+ -FE3+): 8.96 KBar
The output is also saved to a text file in your user data folder (containing your MDB probe database), called AMPHI.OUT in a "machine readable" format as seen here:
Sample "467"
41.6150 6.05719 5.93890 Si
1.39705 .152929 .149900 Ti
15.4665 2.65373 2.60190 2.06110 .540800 Al 9.42825 9.16756
.000000 .000000 .898900 Fe+3
11.4627 1.39535 .469200 Fe+2
14.2514 3.09144 3.03110 Mg
.149784 .018467 .018100 Mn
11.5703 1.80451 1.76930 Ca
1.90354 .537216 .526700 .122700 .404000 Na
.219696 .040797 .040000 K
-.00216 -.00099 -.00100 F
-.01291 -.00319 -.00310 2.00410 Cl
-.49136 .810242 .865954 .067029 .021414 .077257 .002586 .000000 2.18350 1.73950 2.60190 .444000 .342957 5.93890
.122700 .404000 7.00000 1.76930 0
Sample "468"
40.1509 5.88858 5.85890 Si
4.71855 .520451 .517800 Ti
14.8907 2.57440 2.56140 2.14110 .420300 Al 9.02922 8.96384
.000000 .000000 .232200 Fe+3
10.8859 1.33522 1.09630 Fe+2
12.8218 2.80249 2.78840 Mg
.090387 .011229 .011200 Mn
10.2433 1.60972 1.60160 Ca
2.59042 .736635 .732900 .332200 .400700 Na
2.06269 .385949 .384000 K
.004190 .001943 .001900 F
-.01359 -.00338 -.00340 2.00150 Cl
.000000 .405426 .717790 .156614 .073971 .060043 .001600 -3.0226 2.47280 1.68810 2.56140 .784700 .825216 5.85890
.332200 .400700 7.00000 1.60160 0
If anyone needs the source code for these calculations, please let me know.