Oxide - Carbonate - Element Conversion Table

Updated 13 November 2021

The following table is used when weight based analytical results need to be converted. For example 56.0% CaO might be obtained during a microprobe calcite analysis. The sample is likely to be a carbonate. To convert the oxide to carbonate look at the table below. Find the conversion for CaO and CaCO₃ and use the multiplying factor beside CaCO₃. The result is 56.0 x 1.7848 = 99.94% CaCO₃.

A factor is the sum of the atomic weights of the elements in the required formula divided by the sum of the atomic weights in the original formula.

For example the factor to convert Fe₂O₃ to 2FeO is 2 x (55.85 + 16)/(55.85 x 2 + 16 x 3) = 0.8998 while the factor to convert 2FeO to Fe₂O₃ is 1/0.8998 = 1.1113.

An example was recently sent to me which was correct, but there was some uncertainty. An electron microprobe result was Na₂O = 0.035%. What would the result be for Na in ppm?

• Na₂O = 0.035 x 1000000 / 100 ppm = 350 ppm.
• There are two atoms of sodium for each atom of oxygen.
• The atomic weight of sodium is 22.99 and oxygen is 16.
• Na = 350 x (22.99 x 2) / (22.99 x 2 +16) = 259.6 ppm
• O = (350 x 16) / (22.99 x 2 + 16) = 90.35 ppm
• So Na₂O = 259.6 + 90.35 = 350 ppm.
• This checks the calculation above is good.

Using the table factors:

• Na₂O = 350 ppm.
• Na = 350 x 0.7419 = 259.7 ppm.
• O = 350 x (1 - 0.7419) = 90.34 ppm

It would probably help to round the numbers to be comparable to the precision of the original data. A check calculation is always useful.