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: KAlSi₃O₈

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