For a mineral . `liAl (SiO_3)_2`, what is the charge on `SiO_3` unit ? What is the arrangement of oxygen atoms around the silicon atom ?

Explanation

Since lithium has one unit `+ve` charge and aluminium has three units +ve charge, therefore, total +ve charge on cations is 4. Since the mineral `LiAl(SiO_(3))_(2)` is electricity neutral, therefore, total -ve charge on two `SiO_(3)` units is `-4` or each `SiO_(3)` unit must carry two units -ve charge and must be represented as `SiO_(3)^(2-)`.
We know that all silicates consist of `SiO_(4)^(4-)` tetrahedra.
But from the formula of the given silicate, it appears that there are only 3 oxygen per silicon atom. This can be explained if two of the oxygen atoms of each `SiO_(4)^(4-)` unit share with each other giving a simple chain structure for the silicate, `LiAl(SiO_(3))_(2)` as shown below: