Why do people assume 28 oxygens in chlorite mineral formula?

I have been trying to calculate chlorite mineral formulas from microprobe data. However, all of the mineral formula spreadsheets on the internet, including those from the Open University assume 28 oxygens (<http://www.open.ac.uk/earth-research/tindle/AGT/AGT_Home_2010/Microprobe-2.html>)


However, all the formula I have ever seen for chlorite only have 18 oxygens? (see DHZ or <http://www.mindat.org/min-1016.html>)


By then calculating my formula assuming 28 oxygens, it means I have easily more cations than the mineral structure would have sites for?


Many thanks for any help




It doesn't mean that there are 28 oxygen atoms. It means a charge equivalent of 28 oxide anions, considering that each oxide anion has a -2 charge and each hydroxide anion has a -1 charge. 


This is based on the chlorite having an formula including $\ce{O20(OH)16}$


For example see $\ce{Mg10Al2(Si6Al2)O20(OH)16}$ in [The effect of Cr on chlorite stability and the appearance of a new Al-rich silicate](http://adsabs.harvard.edu/abs/2010EGUGA..12.7294F)


In otherwords, as explained in [OCTAHEDRAL OCCUPANCY AND THE CHEMICAL
COMPOSITION OF DIAGENETIC (LOW-TEMPERATURE) CHLORITES](http://www.minersoc.org/pages/Archive-CM/Volume_26/26-2-149.pdf)



> 
> All chlorite analyses were re-calculated as chlorite structural formulae on the basis of 28 oxygen equivalents (56 negative charges) i.e. assuming an ideal anion framework of $\ce{O20(OH)16}$
> 
> 
> 


 3