\begin{table}%t1
%\centering
\par
\caption{Chemical surface reactions $r$ assumed to form one new unit
   of the solid material s in the grain mantel. The efficiency of the
   reaction is limited by the key species, which has the lowest
   abundance among the reactants. The notation $\half$ in the
   r.h.s.~column means that only every second collision (and sticking)
   event initiates one reaction of kind $r$ to form the dust
   species $s$ (see $\nu^{\rm key}_r$ in Eqs.~(\ref{eq:chinet}),
   (\ref{eq:verbrauch})).}\label{tab:chemreak}
%\centering
\par
\begin{tabular}{ccll}\hline \hline 
{Index $r$} & {Solid s} & {Surface reaction} & {Key species (growth)} \\
\hline 
1 & TiO$_2$[s]         & TiO$_2$ 
       $\longrightarrow$ TiO$_2$[s]                  & TiO$_2$ \\ 
2 & rutile             & TiO + H$_2$O  
       $\longrightarrow$ TiO$_2$[s] + H$_2$          & TiO     \\ 
3 &                    & TiS + 2 H$_2$O 
       $\longrightarrow$ TiO$_2$[s] + H$_2$S + H$_2$ & TiS     \\
\hline 
4 & SiO$_2$[s]         & SiO$_2$ 
       $\longrightarrow$ SiO$_2$[s]                  & SiO$_2$ \\ 
5 & silica             & SiO + H$_2$O 
       $\longrightarrow$ SiO$_2$[s] + H$_2$          & SiO     \\ 
6 &                    & SiS + 2 H$_2$O 
       $\longrightarrow$ SiO$_2$[s] + H$_2$S + H$_2$ & SiS     \\
\hline 
7 & Fe[s]              & Fe 
       $\longrightarrow$ Fe[s]                       & Fe      \\ 
8 & solid iron         & FeO + H$_2$ 
       $\longrightarrow$ Fe[s] + H$_2$O              & FeO     \\
%9 &                    & FeS + H$_2$ 
%       $\longrightarrow$ Fe[s] + H$_2$S              & FeS     \\ 
9&                    & Fe(OH)$_2$ + H$_2$ 
       $\longrightarrow$ Fe[s] + 2 H$_2$O            & Fe(OH)$_2$ \\ 
\hline 
10& Mg$_2$SiO$_4$[s]       & 2 Mg + SiO + 3 H$_2$O
       $\longrightarrow$ Mg$_2$SiO$_4$[s] + 3 H$_2$      & $\min\{\half$Mg, SiO$\}$\\
11& forsterite          & 2 Mg(OH)$_2$ + SiO 
       $\longrightarrow$ Mg$_2$SiO$_4$[s] + H$_2$O + H$_2$        & $\min\{\half$Mg(OH)$_2$, SiO$\}$ \\ 
12&                    & 2 MgOH + SiO + H$_2$O
       $\longrightarrow$ Mg$_2$SiO$_4$[s] + 2 H$_2$        & $\min\{\half$MgOH, SiO$\}$\\ 
\hline 
13& Al$_2$O$_3$[s]       & 2 AlOH + H$_2$O 
       $\longrightarrow$ Al$_2$O$_3$[s] + 2 H$_2$   & $\half$AlOH \\ 
14& corundum             &  2 AlH + 3 H$_2$O 
       $\longrightarrow$ Al$_2$O$_3$[s] + 4 H$_2$   & $\half$AlH\\
15&                      & Al$_2$O + 2 H$_2$O
       $\longrightarrow$ Al$_2$O$_3$[s] + 2 H$_2$   & Al$_2$O\\ 
16&                      & 2 AlS + 3 H$_2$O
       $\longrightarrow$ Al$_2$O$_3$[s] + 2 H$_2$S + H$_2$
                                                    & $\half$AlS\\
17&                      & 2 AlO$_2$H 
       $\longrightarrow$ Al$_2$O$_3$[s] + H$_2$O    & $\half$AlO$_2$H\\ \hline 
\end{tabular}
\end{table}