Glass 101: Fused Silica vs. Quartz

Quartz, fused quartz, quartz glass, silica, fused silica… the list of terms used to describe various silica-based materials is long, confusing, and often misunderstood. In this article, we take a close look at the unique properties of quartz and fused silica (and a few related materials), and clear up the confusion surrounding these terms.

Quartz vs. Silica
The first important thing to know about quartz and fused silica is that they both primarily consist of the same ingredient: silica, also known as silicon dioxide. Silica has the chemical formula SiO2 and is the primary constituent of most types of glass. The main form in which silica is found in nature is the mineral quartz: a hard, transparent crystalline material that makes up an appreciable fraction of the Earth’s crust. While quartz primarily consists of silica, it also contains naturally occurring impurities in various proportions depending on its geological origin.

So, silica is a specific chemical compound, silicon dioxide, with the chemical formula SiO2. On the other hand, quartz is a naturally occurring crystalline mineral, which consists primarily of silica but contains some impurities.

Crystalline and Amorphous Solids
To fully understand the differences between different silica-based materials, we first need to review the fundamental differences between crystalline solids and amorphous solids.

The distinction comes down to how atoms are arranged inside the solids. In a crystalline solid, the constituent atoms are arranged in regular, repeating patterns known as crystal lattices. Quartz is an example of a crystalline silica-based material: silicon and oxygen atoms are arranged in a well-defined ordered structure.

However, in an amorphous solid, the atoms have no long-range order. The seemingly random arrangement of molecules in an amorphous solid resembles that of a liquid, except that they are fixed in place and don’t move around. Most materials that we think of as “glass” are amorphous solids: in fact, any material with an amorphous atomic structure can be described as “glassy”.

Whether atoms are arranged in an orderly manner or oriented randomly can profoundly influence a material’s characteristics. One of the most striking examples is the glass transition effect exhibited by amorphous solids. Outside the world of silica or other oxide-based materials, disordered “glassy” metals are often used for their unusual mechanical characteristics compared to conventional metals.1

Silica-based materials – like quartz – can be characterized both in terms of their chemical composition and whether they are crystalline or amorphous.

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