Use Oxygen From Halogens (F, Cl, Br and I) Correction
This option will allow the program to calculate the equivalent oxygen from the measured or specified halogen concentrations (F, Cl, Br and I) and subtract that amount from the amount of calculated stoichiometric oxygen.
During the calculation, since it requires two halogen atoms to replace one oxygen atom, one-half (by atom) of the halogens present are converted to equivalent oxygen and that amount is subtracted during the compositional iteration procedure. The adjustment is iterated along with the modified matrix correction factors, due to the change in overall composition. This option is applicable for ZAF/phi-rho(z), Bence-Albee and calibration curve matrix correction calculations. This option applies only to samples where oxygen is calculated by stoichiometry and contain acquired or specified F, Cl, Br or I.
If this option is NOT used for samples where oxygen is calculated by stoichiometry and halogens are present, the software will simply report the oxygen equivalent of the halogens without subtracting the calculated amount. In this case, the user may then manually subtract the oxygen equivalent from the stoichiometric calculated oxygen; however, since the matrix correction is not adjusted for the change in oxygen concentration, the calculation of the other elements (especially F, due to its large correction factor in the presence of oxygen) will be slightly in error.
Finally it should be noted that to be internally consistent in the matrix calculations, all oxide standard compositions used in halogen analyses should reflect the same adjustment for equivalent oxygen in the standard database as is used for the unknown (or standard) analysis in Probe for EPMA.
For example, the following standard composition is entered with the assumption that all cations have a full complement of stoichiometric oxygen (and no correction for F or Cl):
St 112 biotite #3
TakeOff = 40 KiloVolts = 15
Oxide and Elemental Composition
Average Total Oxygen: 40.474 Average Total Weight%: 101.488
Average Calculated Oxygen: 40.474 Average Atomic Number: 13.413
Average Excess Oxygen: .000 Average Atomic Weight: 21.358
Oxygen Equiv. from Halogen: 1.697
ELEM: SiO2 Al2O3 FeO MgO CaO Na2O K2O TiO2
XRAY: ka ka ka ka ka ka ka ka
OXWT: 38.622 10.721 18.131 14.011 .020 .690 9.210 2.290
ELWT: 18.053 5.674 14.093 8.449 .014 .512 7.646 1.373
KFAC: .1368 .0382 .1205 .0528 .0001 .0024 .0689 .0119
ZCOR: 1.3201 1.4841 1.1692 1.6008 1.0860 2.1594 1.1093 1.1506
ATWT: 13.527 4.426 5.311 7.316 .007 .469 4.115 .603
ELEM: MnO BaO Rb2O Cl F H2O O
XRAY: ka la la ka ka ka
OXWT: .950 .111 .030 .020 4.020 2.663 .000
ELWT: .736 .099 .027 .020 4.020 .298 40.474
KFAC: .0062 .0008 .0002 .0002 .0112 .0030 .1826
ZCOR: 1.1891 1.3161 1.4008 1.2064 3.6053 .0000 2.2167
ATWT: .282 .015 .007 .012 4.453 6.222 53.236
Note that the total for the above composition is actually greater than 100% due to the fact that in reality the fluorine and chlorine actually replace some of the cation oxygen in this mineral. Note also, the oxygen equivalent from all halogens (F, Cl, Br and I) is reported (in the header), but not subtracted from the stoichiometric oxygen (oxygen from cations).
Calculations using this standard composition and an adjustment for equivalent oxygen from halogens in the matrix correction procedure will be slightly in error due to the fact that the standard k-factor calculation will not reflect the proper reduction of stoichiometric oxygen due to the presence of halogens.
Since the default mode of this analysis option (unchecked) is to only display the equivalent oxygen and not actually utilize it in the matrix corrections, then results calculated using typical standard compositions will at least be internally consistent.
However, if it is desired to use this analysis option by reducing the calculated stoichiometric oxygen in the matrix correction then for internally consistent results, the user should make an adjustment (reduction) in the amount of stoichiometric oxygen in the standard composition. This is easily done by noting the actual amount of stoichiometric oxygen (adjusted) in the Standard Composition dialog (see menu Standard | Modify) and entering that value for the concentration of oxygen as seen here:
St 112 biotite #3
TakeOff = 40 KiloVolts = 15
Oxide and Elemental Composition
Average Total Oxygen: 38.777 Average Total Weight%: 99.791
Average Calculated Oxygen: 40.474 Average Atomic Number: 13.505
Average Excess Oxygen: -1.697 Average Atomic Weight: 21.480
Oxygen Equiv. from Halogen: 1.697
ELEM: SiO2 Al2O3 FeO MgO CaO Na2O K2O TiO2
XRAY: ka ka ka ka ka ka ka ka
OXWT: 38.622 10.721 18.131 14.011 .020 .690 9.210 2.290
ELWT: 18.053 5.674 14.093 8.449 .014 .512 7.646 1.373
KFAC: .1367 .0382 .1207 .0529 .0001 .0024 .0689 .0119
ZCOR: 1.3208 1.4841 1.1681 1.5982 1.0858 2.1546 1.1091 1.1499
ATWT: 13.836 4.527 5.432 7.483 .008 .479 4.209 .617
ELEM: MnO BaO Rb2O Cl F H2O O
XRAY: ka la la ka ka ka
OXWT: .950 .111 .030 .020 4.020 2.663 -1.697
ELWT: .736 .099 .027 .020 4.020 .298 38.777
KFAC: .0062 .0008 .0002 .0002 .0113 .0030 .1731
ZCOR: 1.1881 1.3153 1.4016 1.2073 3.5626 .0000 2.2398
ATWT: .288 .016 .007 .012 4.555 6.364 52.168
Note that if excess oxygen from Fe or another variable valence oxide is also present and reported, then that concentration needs to be added to the actual oxygen concentration (in Elements/Cations stoichiometry, e.g. Fe:O of 2:3 rather than 1:1) after subtraction of the oxygen equivalent from the halogens.