Microprobe analyses of 20 staurolites from many localities indicate that the range of composition of natural ferroan specimens is Fe_2.95-3.44 Al_17.07-17.60Mg_0.38-1.13Ti_0.09-0.14Zn_0-0.17Mn_0-0.09Cr_0-0.02V_≤.01.Co_≤.004 Si_7.64-7.91O₄₄ OH_~4. Our observations, together with chemical analyses and other experimental evidence from the literature, are sufficient to conclude that the staurolite unit cell contains ~4 H⁺ in agreement with the structural formula of Smith (1968): (7 octahedral sites, principally Al)_~18 (1 tetrahedral site, principally Fe)_≤₄ (1 tetrahedral site, principally Si)₈O₄₈H_≤₄ Assuming that Si and a small amount of Al fill the Si^IV site, we have used the method of principal component analysis to deduce that the primary substitution in the A1^VI sites is Al ⇄ (Fe+Mg+Ti) and that in the Fe^IV sites is Fe ⇄ (Zn+Al). Secondary substituents in the AI^IVsites are M and Zn, and it is likely that small amounts of and Ti are present in the Fe^IV site. The generally accepted view that Mg is the primary substituent for Fe in the Fe^IV site is not supported Lattice parameters determined by powder methods on 15 analyzed staurolites show the following ranges: a= 7.865 - 7.879Å; b = 16.600 - 16,633Å; c = 5.656 - 5.664Å; β = 90.0°. There are positive correlations of b and c with Fe content, while a is nearly independent of composition. These relationships are explained on the basis of the crystal structure and site occupancies. Natural specimens have uniformly shorter a cell edges {by ~0.015Å) than synthetic Fe-Mg staurolites. Improved curves showing the relationship of 2V and refractive indices to Fe-content have been established. / Master of Science