Why is glass not considered a mineral?

This is why glass is not truly a mineral. Glass does not have a crystal lattice structure. It is best described as an "amorphous solid" meaning that its atoms are rigidly fixed, but not in an orderly pattern (see the figure below to compare the structure of glass with a crystal lattice structure). read more

Glass is generally not a mineral because it is a manufactured product, not ‘natural’. However obsidian is composed of glass (along with entrained material). So in that occurrence the glass is considered to be a mineral. read more

Glass is not a mineral because it does not have a definite chemical composition, which means it does not always have the same chemical composition that can be expressed by a chemical formula. It also does not have a crystalline structure, so there is no specific order to the atoms. read more