Worked example of a calculation of a mineral formula
from the chemical composition.
A chemical formula tells the RATIO of the elements in a compound
From a chemical analysis we learn the WEIGHT PERCENT (wt %) of each element in the mineral. This tells the proportion, by weight, of each element.
NOTE - the total must always be 100% if the analysis is correct.
From the WEIGHT PERCENT we can calculate the relative number of atoms of each substance. Since each kind of atom has its own unique weight we calculate the relative proportions of each atom by dividing weight percent by atomic weight.
Example (for the mineral olivine):
|
Element |
weight % |
weight per atom |
weight %/atomic weight = atom proportions |
|
Mg |
34.56 |
24.32 |
1.421 |
|
Si |
19.96 |
28.09 |
0.710 |
|
O |
45.47 |
16.00 |
2.842 |
The next step is based on the recognition that there are no fractional atoms. The calculated atom proportions give us the RATIO of the different kinds of atoms to each other. The final step is to recalculate this RATIO so that all the atom proportions are small whole numbers.
To do this: divide each of the atomic proportion values by the smallest value (.710) to give the whole number ratio which is:
|
Element |
Atomic proportions |
Atomic ratio |
|
Mg |
1.421 |
2 |
|
Si |
0.710 |
1 |
|
O |
2.842 |
4 |
It is standard practice to write mineral formulas with elements on the left side of the periodic table (those that tend to lose one or two electrons and become positive ions) first, then elements in the right third (those that lose or gain 3 or 4 electrons) and finally, on the right of the formula, those that tend to gain 1 or 2 electrons and thus become negatively charged.
Following this guideline, and based on the calculation above the formula of olivine is: Mg2SiO4
Try this yourself
|
Element |
weight % |
weight per atom |
weight %/atomic weight = atomic proportions |
Atomic ratio |
|
Ca |
40.00 |
40.08 |
|
|
|
C |
11.98 |
12.00 |
|
|
|
O |
48.02 |
16.00 |
|