The unit cell of any substance will contain one or and integral multiple of chemical formula units. Mineral formulas are based on the relationship to unit cell volume and the positions of atoms within the unit cell.
The calculation of the chemical composition is straight-forward. The molecular weight from the empirical formula divided by the sums of the atomic weights of each element to yield the percentage of each element. This is done for each element in the empirical formula and tabulated as the chemical composition. Please note the sort order is based on the electro-negativity of the element.
Additional information provided is the chemical composition expressed as the % oxide of the element. These data are provided to help in the comparison of mineral analyses which use oxide percent. The convention of reporting the oxide percent dates back to when minerals and rocks were analyzed using wet-chemical methods to precipitate an element. The precipitate was oxidized under high temperature and the results were weighed as the oxide.
Example Subject Searches
Example: Use potassium sodium calcium for minerals that contain those elements without regard to order of occurrence.
Example: To find minerals containing Li, Ba, without Ca, try +lithium +barium -calcium. (note: the "+" sign is optional, the "-" sign is not)
Example: Use copper-26 -"Dana Class-3" or -"Dana Class-2" to find all non-sulfide minerals containing 26.00% to 26.99% copper.
Example: Use "Silver" "Gold-2*" to find all silver minerals containing 20.0% to 29.99% gold.
The safest way to search for elements is to spell out the element name in full since every page has a chemical composition. Be sure to use sulfur and not sulfate since sulfate is not reported or analyzed. Use chlorine and not chloride, carbon and not carbonate, etc.
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