\begin{table}%t1
\caption {Average elemental ratios for Product I, II and III silicates determined from EDX measurements (Thompson et~al. \cite{Thompson07}). \label{tab1} }         % title of Table
%\centering
                          % used for centering table
\begin{tabular}{l c c}        % centered columns (4 columns)
\hline\hline                 % inserts double horizontal lines
            &  Mg:Si & Si:O \\  
\hline
Product I   &  1.08:1 & 1:4.94 \\
\par
Product II  & 0.52:1 & 1:2.89 \\ 
\par
Product III & 0.40:1 & 1:2.82 \\ 
\hline                                 
\end{tabular}
\end{table}