\begin{table}%T3
%\centering
\par
\caption{Activation energies of crystallization
$E_{\rm a}/k$ determined for differently produced magnesium silicates.}
\label{anneal} 
\begin{tabular}{cccc}
\hline\hline Material & Material properties &  $E_{\rm a}/k$ (K)&Crystalline
phase
\\
 \hline
Mg$_2$SiO$_4$& Smoke, inhomogeneous   &  39~100& forsterite  \\
MgSiO$_3$ & Smoke, inhomogeneous   & 42~040& forsterite, tridymite
\\ MgSiO$_3$ &Glas, homogeneous, grain sizes $\le$1$\mu$m
&40~100--43~300\\ Mg$_{0.95}$Fe$_{0.05}$SiO$_3$ & glass,
homogeneous, grain sizes $\le$1$\mu$m& 38~525 & enstatite\\
% &  &  \\
Mg-silicate           & Sol-gel, homogeneous, Me--OH bondings&
26~600--31~700 & \\ MgSiO$_3$             & &                &
enstatite\\ Mg$_{1.5}$SiO$_{3.5}$ & &                & enstatite,
forsterite \\ Mg$_{1.8}$SiO$_{3.8}$ & & dependent on &
forsterite\\ Mg$_2$SiO$_4$ & & Me--OH content &forsterite\\
Mg$_{2.4}$SiO$_{4.4}$ & & & forsterite\\ Mg$_{2.6}$SiO$_{4.6}$ & &
& forsterite, MgO\\ \hline
\end{tabular}
\end{table}