For 82 staurolite analyses taken from the literature, a strong correlation (r = −0.96) exists between Fe + Mg + Zn and Al′, where Al′ is the aluminum remaining after any Si deficiency below 8.0 atoms is made up by Al. The equation of the best fit line [Al′ = 20.5 – 0.80(Fe+Mg+Zn)] and charge balance requirements result in the generalized chemical formula
(Fe,Mg,Zn)25.61.25xAl1.5x8.2Si16.20.5xO48H4

where 16.58 ≤ x ≤ 18.61 for the 82 staurolites used. The average chemical formula for those specimens is (Fe,Mg,Zn)4.0Al17.3(Si7.6Al0.4)O48H∽4. Although it has not been possible to place rigid limits on the composition of natural staurolite, the availability of only 8 sites suitable for Si leads to a lower limit of 16.4 for x and the formula (Fe,Mg,Zn)∽5.1Al∽16.4Si8.0O48H∽4, which represents an upper limit for Si and (Fe+Mg+Zn) and a lower limit for Al.

Some staurolites of unusual compositions do not fit the curves of lattice parameters vs. unit cell Fe content proposed by Griffen and Ribbe (1973). By recasting the curves in terms of 〈r〉 (= mean radius of the cations in the Fe2Al0.7O2(OH)2 monolayer) instead of Fe content, the crystal chemical rationale (Griffen and Ribbe, 1973) for lattice parameter variations in staurolite is found to be satisfactory.

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