Oxide to Element Conversion

How to convert oxide to element concentrations?

Why do we want to convert oxide to element concentrations (or vice versa)?

Element concentrations, particularly those of the major elements (Na, Mg, Al, Si, P, K, Ca, Ti, Mn, and Fe), are commonly reported in the oxide form (Na₂O, MgO, Al₂O₃, etc.). Sometimes even trace elements are reported as oxides, e.g., Cr₂O₃, V₂O₅, NiO, ZrO₂, etc. (see Table 2). Therefore, you may need an oxide to element conversion or vice versa.

The oxidation stage of the element determines the form of the oxide, such as that of iron, i.e., Fe^III₂O₃ or Fe^IIO. This has an implication of the environment the mineral development or deposition. For instance, Fe-minerals may oxidise in certain environments leading to Fe^III, while under anoxic or even euxinic conditions Fe^II might prevail. Thus, you may need to correct the elemental data se for the dominating oxidation states of certain elements.

Particularly trace elements can have several oxidation states and associated oxide forms. Commonly, trace elements are reported in the element form from chemical analyses, but sometimes this is not the case. It is recommended to convert these oxides back to elements.

Another reason for conversion to element formats can be mineral modelling (although I would recommend using mole for such).

How to convert oxide to element concentrations or vice versa?

The conversion from oxide to element or element to oxide is straight forward. All we need are the atomic weights of the involved element and oxygen, which can be easily been taken from any good Periodic Table of Elements (e.g., IUPAC.org).

As an example, let’s convert SiO₂ to Si:

1. Firstly, we need the molecular weight of SiO₂. This is calculated from the atomic weight of silicon (Si; 28.0855) and oxygen (O, 15.9994). Thus, the molecular weight of SiO₂ is 28.0855 (Si) plus 2×15.9994 (O₂ = 2xO) equals 60.0843 (28.0855 + 2×15.9994 = 60.0843). [The unit of the atomic weight is g/mol.]
2. Then, we divide the concentration of SiO₂ (either in % or ppm) by its molecular weight, i.e. 60.0855. Now we have the SiO₂ as mole.
3. The next step is to multiply SiO₂ mole with the atomic weight of Si (remember?) 28.0855. Now we have Si in % or ppm depending what you used at the beginning.

element = oxide/molecular weight x atomic weight

Similar to convert from element to oxide, e.g., Si to SiO₂ (to stay with our example):

1. Divide Si concentration (either % or ppm) by its atomic weight of 28.0855.
2. Then multiply with the molecular weight of SiO₂, 60.0855.

oxide = element/atomic weight x molecular weight

Oxide to element conversion factors

As you can see, we are just using constants, i.e., atomic weights. Thus we can calculate conversion factors for each element (or get them from the internet, but then possibly we won’t know if the latest or correct atomic weights have been used).

Calculating conversion factors follows the same method as above, but without using the actual element/oxide concentration.

So, here we go …

Tables 1 and 2 below provide conversion factor from oxides to elements (and vice versa).

Table 1: Conversion factors for most common oxides/elements.

Table 2: Conversion factors for oxides/elements in alphabetic order.