Mineral formula calculator: up and running

Adularia.app is a simple web-based application that calculates mineral formulas based on the results of chemical analysis. The application was made in collaboration with a group of my friends, Peter Frolov and Nikita Krupnik, in attempt to simplify and exemplify mineral formula calculations that are usually done in MS Excel. It should also help undergraduate students that are often tasked to perform the calculations on paper by hand. This application could be used to check the results of such calculations. 

It features two ways of calculating mineral formulas. One is anion-based method that uses prior knowledge of how many oxygens (and other negatively charge ions) are present in the formula. For example, in the formula of olivine, Mg₂[SiO₄], 4 oxygens. Imagine a grain of olivine was analyzed by electron microprobe yielding 42.06 % MgO, 18.75 % FeO and 39.19 % SiO₂. Knowing that the mineral has 4 units of oxygen, the rest of the formula will be calculated as Mg_1.6Fe_0.4[SiO₄]. See this implemented in the screenshot. 

This method also offers calculating mineral formulas for hydrous phases in which water is present as OH^-. We allow user to choose micas and amphiboles assuming they are fully hydrous or containing other anions like F^- and Cl^- in complementing amount. In case of amphiboles, the fully anhydrous option is available. In such amphiboles, the water-containing position is occupied by additional oxygens. Such example is kaersutite NaCa₂Mg₃AlTi[Al₂Si₆O₂₂]O₂. 

Here is an example of actinolite analysis borrowed from webmineral.com  (http://webmineral.com/data/Actinolite.shtml). The calculated formula is Na_0.22Ca_1.83Mg_3.41Fe²⁺_1.26Al_0.23Fe³⁺_0.05Mn²⁺_0.02Ti_0.02[Al_0.21Si_7.79]O₂₂(OH)₂. Note all aluminum is kept together since the calculator does not know how to distribute it. That is left for a human to figure out (subtract the amount of Si from 8, add complementary amount of Al, place of Al with the rest along with Fe, Mg, etc.)

The second method is for figuring the ratio of ferrous to ferric iron in a mineral formula, Fe²⁺ and Fe³⁺, respectively. The method uses prior knowledge of cations and anions per formula and maintains neutral balance at expense of valency of Fe. In other words, Fe²⁺/Fe³⁺ ratio is calculated to keep zero charge of the formula. The assumption is that iron is the only element with variable valency and that oxygen is the only anion. The algorithm is fully adopted from the paper by Droop (1987) published in Mineralogical Magazine. 

Working options are amphiboles (13), garnets (8) and pyroxene (4). The numbers in parenthesis indicate number of cations per formula used as a basis for calculations. For amphiboles that may contain water as OH^-, the method uses anhydrous basis, i.e. 23 oxygen per formula. Thus, the formula needs to be entered without analysis of water, which is the case for electron microprobe analysis, and all iron needs to be entered as FeO. If iron is reported as Fe₂O₃, the number needs to be converted to the proportional amount of FeO by multiplying the weight percents by 0.899. This particular calculator uses 13 cation basis for amphiboles, which is best suitable for calcic amhiboles, tremolite-actinolite series for example.