Stability of beryllium carbonate
Why and how is this stability maintained?


Beryllium carbonate decomposes into Beryllium oxide and Carbon dioxide.
$$\ce{BeCO3 &lt;=&gt; BeO + CO2}$$

According to Le Chatelier's principle one simply has to store it under a $\ce{CO2}$ atmosphere to push the equilibrium to the left.
$$\ce{BeCO3 &lt;&lt;=&gt; BeO + CO2}~\text{in excess}$$


  Why this compound decomposes when kept in atmosphere of oxygen?


When there is no $\ce{CO2}$ atmosphere the equilibrium is pushed to the right, which would be the "normal" case under standard conditions.
$$\ce{BeCO3 &lt;=&gt;&gt; BeO + CO2}$$