In the USA they use these sources to fluoridate their water. Heavy metals like lead at 100th part. No description of the 0.5p of water-insoluble matter. 

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We are a leading  Sodium Silicofluoride  supplier in China Now we’d like to offer you Sodium Silicofluoride   as follows

Specifications

1.Cas no.: 16893-85-9
2.Sodium Silicofluoride : 99% min

Items	Standard Index
Sodium Silicofluoride	99.00% MIN
Loss of weight on drying at 105	0.30% MAX
Free acid(as HCl)	0.10% MAX
Chlorides(as Cl-)	0.10% MAX
Sulphate(SO42-)	0.25% MAX
Iron(Fe)	0.02%MAX
Water-insoluble matter	0.5% MAX
Heavy metal(as Pb)	0.01%MAX