\begin{table}%t3
\par
\caption{$d$-spaces of strongest diffraction peaks for crystalline cristobalite,
          quartz and Mg-silicates, different entries for orthorhombic enstatite correspond
          to variations in the size of the unit cell due to multiply-linked chain 
          structures.\label{tab3}}         % title of Table
      % is used to refer this table in the text
%\centering
                          % used for centering table
\begin{tabular}{r c  c}        % centered columns (4 columns)
\hline\hline                 % inserts double horizontal lines
Mineral & $d$ (\AA) &  $hkl$-index  \\    % table heading 
\hline
   Cristobalite & 4.0474 & 101 \\
   Quartz  & 3.3436  & 101     \\
\hline
   Forsterite & 2.4557  & 112 \\ 
\hline
   Monoclinic  &  & \\ 
   Enstatite   &  2.9808  & 22-1  \\
\hline
   Orthorhombic & & \\ 
     Enstatite 1  &  2.4338 & 112  \\
     Enstatite 2  & 3.1486  & 221 \\
     Enstatite 3&  3.0745  & 121 \\
\hline                                   %inserts single line
\end{tabular}
\end{table}