Volcanic glass, most commonly known as obsidian, possesses many physical properties associated with true minerals, such as hardness and a glassy luster. However, scientific classification is based on internal structure and chemical composition, which ultimately determines its status. The question of whether this naturally occurring material is a mineral hinges on a strict set of criteria established by mineralogists.
The Five Key Requirements for a Mineral
For any naturally occurring substance to be classified as a true mineral, it must satisfy five distinct and universally accepted requirements:
- It must be naturally occurring, meaning it is not manufactured or produced by human intervention.
- It must be inorganic, which excludes materials derived from living organisms.
- It must exist as a solid under normal geological conditions.
- It must possess a definite chemical composition, which can be expressed by a specific chemical formula.
- It must have an ordered internal atomic structure, also known as a crystal lattice, where atoms are arranged in a precise, three-dimensional, repeating pattern.
The Structure of Volcanic Glass and Why It Fails the Mineral Test
Volcanic glass forms when molten rock, or magma, is rapidly cooled or “quenched” at the Earth’s surface. This process often occurs during explosive eruptions or when lava flows quickly into water. The resulting rate of cooling is so fast that the atoms within the liquid material do not have sufficient time to organize themselves into a crystalline structure.
This lack of time for atomic arrangement results in a non-crystalline, or amorphous, internal structure, which is characteristic of all glasses. Obsidian, which is a silica-rich volcanic glass, fails to meet the fifth requirement for a mineral because its internal atomic arrangement is random and disordered.
Volcanic glass does satisfy most of the other criteria; it is a naturally occurring, inorganic solid. However, its chemical composition can also vary more widely than a true mineral, as it is essentially a supercooled liquid. The defining factor remains the failure to develop the ordered internal architecture of a crystal. This absence of a crystal lattice is also why volcanic glass fractures with a distinctive conchoidal pattern, producing smooth, curved surfaces rather than breaking along flat cleavage planes.
How Geologists Classify Volcanic Glass
Since volcanic glass is a naturally occurring, inorganic solid that lacks a crystalline structure, geologists classify it as a mineraloid. A mineraloid is defined as a mineral-like substance that fails to meet one or more of the strict criteria for a true mineral. The term specifically highlights its amorphous, or glassy, internal state.
Obsidian is the most common example of a volcanic glass and is sometimes called a natural glass to distinguish it from synthetic, man-made glass. Other varieties of volcanic glass include pumice, a highly vesicular (bubbly) form, and tachylite, which is a dark, basaltic glass with a lower silica content. All forms share the same fundamental characteristic of a non-crystalline atomic arrangement.
Volcanic glass is also broadly classified as an extrusive igneous rock, meaning it formed from magma that cooled above the surface. This glassy material is relatively unstable over geologic timescales, and most volcanic glass is geologically young. Over millions of years, the amorphous structure tends to spontaneously reorganize itself into fine-grained crystals through a process called devitrification. This process is why ancient volcanic glass, older than about 65 million years, is rarely found intact.