### 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