The crystal chemistry of staurolite

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Virginia Polytechnic Institute and State University

Microprobe analyses of 20 staurolites from many localities indicate that the range of composition of natural ferroan specimens is Fe2.95-3.44 Al17.07-17.60Mg0.38-1.13Ti0.09-0.14Zn0-0.17Mn0-0.09Cr0-0.02V≤.01.Co≤.004 Si7.64-7.91O44 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)₈O₄₈H₄ Assuming that Si and a small amount of Al fill the SiIV site, we have used the method of principal component analysis to deduce that the primary substitution in the A1VI sites is Al ⇄ (Fe+Mg+Ti) and that in the FeIV sites is Fe ⇄ (Zn+Al). Secondary substituents in the AIIVsites are M and Zn, and it is likely that small amounts of and Ti are present in the FeIV site. The generally accepted view that Mg is the primary substituent for Fe in the FeIV 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.