Pyrosilicate

A pyrosilicate is a type of chemical compound; either an ionic compound that contains the pyrosilicate anion Si
₂O⁶⁻
₇, or an organic compound with the hexavalent ≡O
₃Si-O-SiO
₃≡ group. The anion is also called disilicate or diorthosilicate.

Ionic pyrosilicates can be considered salts of the unstable pyrosilicic acid, H
₆Si
₂O
₇. Unlike the acid, the salts can be stable. Indeed, pyrosilicates occur widely in nature as a class of silicate minerals, specifically the sorosilicates - though some sorosilicate minerals, such as gehlenite, replace one of the silicon atoms with tetracoordinated aluminium or boron, giving the isostructural anions

Some notable synthetic pyrosilicates include

• sodium pyrosilicate Na
  ₆Si
  ₂O
  ₇, a possible component of water glass.
• sodium iron(II) pyrosilicate Na
  ₂Fe
  ₂Si
  ₂O
  ₇, a potential cathode material for batteries.
• sodium manganese(II) pyrosilicate Na
  ₂Mn
  ₂Si
  ₂O
  ₇, another potential cathode material.

The pyrosilicate anion can be described as two SiO
₄ tetrahedra that share a vertex (an oxygen atom). The vertices that are not shared carry a negative charge each.

The structure of solid sodium pyrosilicate was described by Volker Kahlenberg and others in 2010.

Yuri Smolin and Yuri Shepelev determined in 1970 the crystal structures of pyrosilicates of rare earth elements with generic formula Ln
₂Si
₂O
₇, where "Ln" stands for either one of lanthanum, cerium, neodymium, samarium, europium, gadolinium, dysprosium, holmium, yttrium, erbium, thulium, or ytterbium. They were found to belong to four distinct crystallographic classes, determined by the size of the cation. Other researchers also studied yttrium pyrosilicate Y
₂Si
₂O
₇., gadolinium pyrosilicate Gd
₂Si
₂O
₇., and scandium pyrosilicate Sc
₂Si
₂O
₇.

Rare earth pyrosilicates Ln
₂Si
₂O
₇ can be obtained by fusing the corresponding oxide Ln
₂O
₃ with silica in 1:2 molar ratio, Single crystals can be grown by the Verneuil process or the Czochralski method.

Industrial pyrosilicate can be produced by the depolymerisation of metasilicate by alkali, which releases water on breaking theSi−O−Si bond. This proceeds according to the idealised equation

2 SiO2−3 + 2 OH⁻ → Si₂O6−7 + H₂O

Additional alkali will degrade the bonds further, instead yielding orthosilicate.