# Practical Aspects of Mineral Thermobarometry

### Formula recalculation, including ferric iron estimation

#### Cations per formula unit

Although most microprobe software will calculate atomic proportions for you, it's quite likely that you'll want to recalculate analyses yourself.

The procedure for simple recalculation of a mineral analysis into
cations per formula unit is given in Deer, Howie and Zussman
(Appendix 1). The calculation is easy to set up in spreadsheet form.
Have a go! When you've got it working, compare it with my own
version, which is on the tools page as
`formula.xls`.

For recalculating many analyses at one time, for generating database files which can be imported into spreadsheet programs, and for producing neat lists for including in reports, the old DOS program MINCALC may still have its uses.

##### Reference

W.A. Deer, R.A. Howie, J. Zussman (1992). An introduction to the rock-forming minerals, 2nd ed.. Longman Scientific & Technical, Harlow.

#### Ferric iron recalculation

Although the microprobe cannot distinguish between atoms of different valence states, it is possible in many cases to calculate the proportion of ferric iron required to satisfy charge balance.

If all the following can be reasonably assumed to be true:

- Fe is the only element present with variable valency
- Oxygen is the only anion
- The cation sites are full (no vacancies)

then F, the number of Fe^{3+} ions per X oxygens in the
mineral formula, is given by

**F = 2X.(1 - T/S)**

where T is the ideal number of cations per formula unit, and S is
the cation total obtained when all iron is assumed to be
Fe^{2+}.

##### Reference

Droop G.T.R. (1987) A general equation for estimating
Fe^{3+} concentrations in ferromagnesian silicates and oxides
from microprobe analyses, using stoichiometric criteria. Min. Mag.
51, 431-435.

This page last modified 12 October 2004