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

In otherwords, as explained in OCTAHEDRAL OCCUPANCY AND THE CHEMICAL
COMPOSITION OF DIAGENETIC (LOW-TEMPERATURE) CHLORITES


  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 $\ce{O20(OH)16}$