Refractive index of barium sulfate
The index of refraction of $\ce{BaSO4}$ at 589 nm can be found in "Index of Refraction of Inorganic Crystals", in CRC Handbook of Chemistry and Physics, 90th Edition (CD-ROM Version 2010), David R. Lide, ed., CRC Press/Taylor and Francis, Boca Raton, FL.

Separate values are given for the indexes of the orthorhombic crystals:

$\begin{align}
n_x &amp;= 1.6362\\
n_y &amp;= 1.6374\\  
n_z &amp;= 1.6480
\end{align}$