Tetraboric acid
Tetraboric acid or pyroboric acid is a chemical compound with empirical formula . It is a  colourless water-soluble solid formed by the dehydration or polymerization of boric acid. Tetraboric acid is formally the parent acid of the tetraborate anion . Preparation
Tetraboric acid can be obtained by heating orthoboric acid  above about 170 °C:

 4  →  + 5

References 

Borates
Inorganic polymers