<p>If you are talking about Table 2 of Chapter 8 in <em>Principles of Igneous and Metamorphic Petrology</em> (Winter, 2013), p. 146 in the second edition, then you are misreading the column headers. 59.3 is in 'Wt. % Oxide', i.e., it is a mass fraction, while 21.2 is in 'Atomic %' (atomic percent, no mass involved). The way this is calculated is explained on the same page for Table 1, which has more columns showing the details of the calculation.</p>
<p>For what you want to do, going from wt% element to wt% oxide, all you have to do is multiply by a conversion factor, which is 2.1393 for silicon (the 0.4674 factor you mentioned is to go the other way around, from oxide to element). Be careful with iron, there are two different factors depending on the state of oxidation (<span class="math-container">$\ce{FeO}$</span> vs. <span class="math-container">$\ce{Fe2O3}$</span>), but I'm not sure how you can discriminate between the two with your elemental analysis...</p>

CC BY-SA 4.0
2023-02-07T09:51:44.427Z
Thank you for your reply, that is what I wanted to know
Ferreira
CC BY-SA 4.0
2023-02-06T16:58:13.033Z
2023-02-06T16:58:13.033Z
Jean-Marie Prival
<p>I never trust conversion factors. Deriving it on your own is super easy:</p>
<h3>Conversion from element to oxide</h3>
<p>Divide by the molar mass of the element, and multiply by the molar mass of the oxide. For example, molar mass of silicon is 28.085, and of the oxide is 28.085 + 2×15.999 = 60.08. Note that you need this to be on a single cation basis. For oxides like Na<sub>2</sub>O or Al<sub>2</sub>O<sub>3</sub>, you actually need the molar mass of NaO<sub>0.5</sub> and AlO<sub>1.5</sub>.</p>
<h3>Conversion from oxide to element</h3>
<p>Do the opposite. Divide by the molar mass of the oxide, and multiply by the molar mass of the element. For you example of 59.3% SiO<sub>2</sub>, the answer is 59.3/60.08×28.085=27.72%.</p>

CC BY-SA 4.0
2023-02-13T06:38:12.123Z
But then you have to trust molar masses. ;)
Jean-Marie Prival
CC BY-SA 4.0
2023-02-13T12:02:26.423Z
@Jean-MariePrival I trust well known molar masses better than I trust conversion factors published in some obscure place by a someone. Besides, knowing how to do it solves the problem of conversion factors not existing for some oxides, or polyvalent oxides (Mn, V, U) for which factors are for an unknown oxide, or not the oxide in question.
Gimelist
CC BY-SA 4.0
2023-02-13T02:30:46.067Z
2023-02-13T02:30:46.067Z
Gimelist
<p>So, I did some measurements in ICP-OES and the results were given to me in elemets. However, I need to convert the results to oxides. After some googling I found out that I need to use simple conversion factors that are based on the molecular weight and amount of cations in the molecules. The problem is, I found some data on a book (Winter's book on Petrology) and apparently he did calculations that I could not understand.
For example : He converted 59.3% SiO2 to 21% of Si.
If I use the conversion factor of 0.4674, I would get that 59.3% of SiO2 is equal to 27.71 of Si.</p>
<p>What am I missing here?</p>

<geology><geochemistry><biogeochemistry>
How do I convert an element measurement to oxides?
Ferreira
# How do I convert an element measurement to oxides?

(Asked by: username_0 on 2023-02-06)

So, I did some measurements in ICP-OES and the results were given to me in elemets. However, I need to convert the results to oxides. After some googling I found out that I need to use simple conversion factors that are based on the molecular weight and amount of cations in the molecules. The problem is, I found some data on a book (Winter's book on Petrology) and apparently he did calculations that I could not understand.
For example : He converted 59.3% SiO2 to 21% of Si.
If I use the conversion factor of 0.4674, I would get that 59.3% of SiO2 is equal to 27.71 of Si.

What am I missing here?

**Tags:** geology, geochemistry, biogeochemistry


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## 2 answers

### Answer by username_1 on 2023-02-06. Score: 4

If you are talking about Table 2 of Chapter 8 in *Principles of Igneous and Metamorphic Petrology* (Winter, 2013), p. 146 in the second edition, then you are misreading the column headers. 59.3 is in 'Wt. % Oxide', i.e., it is a mass fraction, while 21.2 is in 'Atomic %' (atomic percent, no mass involved). The way this is calculated is explained on the same page for Table 1, which has more columns showing the details of the calculation.

For what you want to do, going from wt% element to wt% oxide, all you have to do is multiply by a conversion factor, which is 2.1393 for silicon (the 0.4674 factor you mentioned is to go the other way around, from oxide to element). Be careful with iron, there are two different factors depending on the state of oxidation ($\ce{FeO}$ vs. $\ce{Fe2O3}$), but I'm not sure how you can discriminate between the two with your elemental analysis...

> **Comments:**
> **username_0**: Thank you for your reply, that is what I wanted to know  

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### Answer by username_2 on 2023-02-13. Score: 1

I never trust conversion factors. Deriving it on your own is super easy:

### Conversion from element to oxide

Divide by the molar mass of the element, and multiply by the molar mass of the oxide. For example, molar mass of silicon is 28.085, and of the oxide is 28.085 + 2×15.999 = 60.08. Note that you need this to be on a single cation basis. For oxides like Na2O or Al2O3, you actually need the molar mass of NaO0.5 and AlO1.5.

### Conversion from oxide to element

Do the opposite. Divide by the molar mass of the oxide, and multiply by the molar mass of the element. For you example of 59.3% SiO2, the answer is 59.3/60.08×28.085=27.72%.

> **Comments:**
> **username_1**: But then you have to trust molar masses. ;)  
> **username_2**: @username_1 I trust well known molar masses better than I trust conversion factors published in some obscure place by a someone. Besides, knowing how to do it solves the problem of conversion factors not existing for some oxides, or polyvalent oxides (Mn, V, U) for which factors are for an unknown oxide, or not the oxide in question.  

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