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Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: HelloScience on November 07, 2020, 07:49:22 PM

Title: Need Help With A Little Geochemistry Problem
Post by: HelloScience on November 07, 2020, 07:49:22 PM

Hello, I need to see what others think about this situation.

I have a material said to be basalt. I have been given major element data in ppm and would l like to use the percentages of corresponding oxides moving forward. Here is the deal:

Al2O3 is 16.31% in the ball park. Given Al at 43,152.08 ppm.
Fe2O3 is 10.15% in the ball park.  Given Fe at 41,625.02 ppm.
SiO2 is 1.41%. Given Si at 4,498.08 ppm.

[(Element ppm)(Oxide mass/Element mass)/(10,000)] = % Oxide

Is this correct? Any ideas what this material might be?

Thank you,
Andrew H.

Title: Re: Need Help With A Little Geochemistry Problem
Post by: AWK on November 08, 2020, 01:51:11 AM

If you look at Wikipedia, you can read that the SiO₂ content in basalt is in the range of 45-52%. The Si content of SiO₂ is about 47% (for the estimate, I assume a value of 50% with an error of less than 10%). This means that you should expect a Si content in the basalt of ~250,000 ppm. Well, under no circumstances can the name basalt be used for your sample.

As for your strange calculations - with the Si content of around 4,500 ppm ("Given Si at 4,498.08 ppm.") and Si to O₂ mass ratio of 28:32 (which is slightly less than 50% for silicon content) the percentage of SiO₂ in your sample will be slightly less than 0.9%. Your quoted percentages of Al₂O₃ and Fe₂O₃ in the basalt are plausible but by no means correspond to the Al and Fe content in ppm in your sample.

Title: Re: Need Help With A Little Geochemistry Problem
Post by: HelloScience on November 08, 2020, 02:00:59 AM

Thanks, I see the error in my calculations now.