How do you calculate the
weight % of elements from a mineral (molecular) formula?
To do so, you will first need to determine the molecular weight of the mineral.
For example: KAlSi_{3}O_{8}
1. Set up a table with 5 columns, placing the element name in the first column.
2. In column 2, enter the number of atoms of the element in the mineral formula.
3. In column 3, enter the atomic weight of each of the elements in the mineral (if you don't know these, consult a periodic table).
4. Multiply the value in column 2, by the atomic weight in column 3, and enter the results in column 4 (in the case of O, 8 x 15.999 = 127.992).
5. Sum the values in the column 4 (and enter the sum at the bottom; e.g. 278.330)
6. To calculate the weight % of the element in the mineral, divide the value for the contribution to the molecular weight (for example, O 127.992), divided by the sum of the contributions (278.330), and multiply by 100 (to arrive at 45.99 weight % for O)
Element 
Number of Atoms in Formula 
Atomic Weight 
Contribution to Molecular Weight 
Weight % Element in Mineral 
K 
1 
39.098 
39.098 
14.05 
Al 
1 
26.982 
26.982 
9.69 
Si 
3 
28.086 
84.258 
30.27 
O 
8 
15.999 
127.992 
45.99 



278.330 
