Final answer:
The distinction between a mineral and a naturally formed glass such as obsidian lies in the crystal structure; minerals have an orderly crystalline structure, while glasses like obsidian lack such a structure due to rapid cooling that prevents crystal formation.
Step-by-step explanation:
We distinguish between a mineral and a naturally formed glass like obsidian primarily because glass does not have a fixed crystal structure. Unlike minerals, glasses like obsidian do not have an orderly atomic arrangement that manifests as a crystalline structure. Instead, obsidian is formed by the rapid cooling of lava, which prevents the formation of crystals, leading to its characteristic glassy texture.
A mineral is defined as a naturally occurring, inorganic solid with a definite chemical composition and a characteristic crystalline structure. This strict definition excludes any synthetic materials and requires a mineral to be formed by geologic processes in a predictable structure. Compared to glasses, which can also be synthetically made, minerals have a fixed and orderly crystal lattice.
In contrast, obsidian, as an example of volcanic glass, is formed when high-viscosity lava cools so rapidly that atoms do not have time to arrange themselves into a crystal lattice, resulting in the glassy, non-crystalline amorphous solid we recognize.