Hydrated Amorphous Silica is used in toothpaste for as an abrasive agent and rheology modifier. Addition of Silica changes the viscosity of the toothpaste.
In toothpowder it is used to also act as an anticaking agent.
Choice of properties of Silica and dosing depends upon the choice of RDA (Relative Dentin Abrasivity) level and viscosity.
It is practically inert and compatible with active ingredients including Floride making it a first choice for the purpose.