Please give the chemical formula for:

sillimanite




For silicates, be sure to put the tetrahedral part of the formula in parentheses,
to emphasize the silicate type. Use 6 oxygens for single chain silicate formulas.
My apologies to non-Smith readers who use other formula conventions. A complete list
of formulas, with the required ones in bold can downloaded as a pdf file with this link.

Si:O = 1:4 for island silicates
Si:O = 1:3 for single chain silicates
Si:O = 1:3 for ring silicates
Si:O = 4:11 for double chain silicates
Si:O = 2:5 for sheet silicates
Si:O = 1:2 for framework silicates