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Mineral like oxides have amorphous stracture.Example alumunium oxide.
An allophane is an amorphous hydrous aluminium silicate clay mineral.
By definition, a mineral must have a definite crystalline structure. Calcite has this. Glass is amorphous, meaning it does not have such a structure.
Amorphous means not having a distinctive shape or formless. It is no particular mineral or element there for can have no particular melting point.
Amber is not a mineral, because it has an organic origin and amorphous structure (no orderly internal arrangement of atoms).