What is the weight per square foot for each tile type?

The weight of your mosaic depends on the material it's made from

Vitreous Glass – 1.35 lbs per sq/ft
Sintered Glass – 1.35 lbs per sq/ft
Glazed Porcelain – 2 lbs per sq/ft
Natural Stone – 5 lbs per sq/ft

This is for the tile only. You'll also want to allow for the weight of and surface treatment, tile adhesive and grout.