Analytical Suite Metadata

SIGÉOM lithogeochemical major oxides

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The SIGEOM database includes a very large number of lithogeochemical analyses.  These analyses are occasionally retrieved and re-published, possibly alongside re-analyses of the same samples, or data from other sources (GSC, universities, etc.).  Analytical techniques varied over the years.  Original publications should be consulted to determine analytical details.

Fe is expressed in different formats.  Many analytical techniques cannot distinguish between Fe!sup!2+!/sup! and Fe!sup!3+!/sup!.  In XRF analysis, Fe is conventionally reported as Fe!sub!2!/sub!O!sub!3!/sub! (or Fe2O3T, for total).  Fe!sup!2+!/sup! can be measured by titration, and may then be reported as FeO.  This then allows one to calculate a “true” value for Fe!sup!3+!/sup!, or Fe!sub!2!/sub!O!sub!3!/sub! (or Fe2O3V, for vrai (true in French)).  Sometimes, Fe is reported as FeOT rather than Fe2O3T.  Note that the different conventions for reporting Fe will have an effect on the total sum of the major oxides.  Theoretically, this sum should be slightly less than 100.  But reporting Fe entirely as Fe!sub!2!/sub!O!sub!3!/sub! rather than a mix of Fe!sub!2!/sub!O!sub!3!/sub! and FeO may lead to sums of greater than 100.

Fe calculations

Using atomic weights of Fe = 55.845(2) and O = 15.9994(3), we obtain molecular weights for Fe!sub!2!/sub!O!sub!3!/sub! = 159.688 and 2(FeO) = 143.689.  This gives a conversion factor of 159.688 / 143.689 = 1.1113.

We can then use the expressions:

• Fe2O3T – FeO * 1.1113 = Fe2O3V
• Fe2O3T = FeOT * 1.1113