Question

Amorphous form of silica is: (a) Cristobalite (b) Tridymite (c) Asbestos (d) Fumed silica

Short answer

The amorphous form of silica is (d) Fumed silica.

Step-by-step solution

Understand the Question

The question asks us to identify which option represents the amorphous form of silica. Knowing the forms of silica is essential as silica can exist in both crystalline and amorphous forms.

Analyze the Options

The options given are different forms of silica and related minerals: (a) Cristobalite, (b) Tridymite, (c) Asbestos, and (d) Fumed silica. We need to determine which of these is an amorphous form.

Recall Definitions

Cristobalite and Tridymite are considerations as they are known crystalline forms of silica. Asbestos is a group of fibrous minerals, not primarily associated with silica in this context. Fumed silica, however, is known to be an amorphous form created via flame pyrolysis of silicon tetrachloride or similar processes.

Conclusion

From the analysis, fumed silica is the option that represents the amorphous form of silica, as the others are either crystalline or unrelated to amorphous silica.

Key concepts

Cristobalite

Cristobalite is one of the well-known crystalline forms of silica. It is a mineral that is formed at high temperatures and can be found naturally in volcanic rocks. The structure of cristobalite is characterized by a specific arrangement of silicon and oxygen atoms in a lattice formation. This structure gives cristobalite its stability at high temperatures, making it an important component in certain industrial applications.
While cristobalite is crystalline, its high-temperature stability must not be confused with amorphous silica, which lacks the orderly atomic structure that defines crystalline forms. Cristobalite can be found in different variants, such as low and high cristobalite, each having unique properties based on temperature and pressure conditions.

Tridymite

Tridymite, similar to cristobalite, is another crystalline form of silica. It typically occurs in igneous rocks and is stable at temperatures higher than those at which quartz forms but lower than the temperatures required for cristobalite. Tridymite can be found in several symmetry variations, known as polymorphs, such as monoclinic, orthorhombic, and hexagonal.
The formation of tridymite is often associated with regions experiencing volcanic activity, and like cristobalite, its crystalline nature means that it does not possess the amorphous quality of fumed silica. These crystalline forms of silica like tridymite and cristobalite play distinct roles in geological and industrial contexts, distinct from amorphous silica materials.

Fumed Silica

Fumed silica, unlike cristobalite and tridymite, is an amorphous form of silica. This means that its silicon dioxide molecules do not have the regular repeating structure seen in its crystalline counterparts. Instead, fumed silica is created through the process of flame pyrolysis, where silicon tetrachloride reacts in a flame to produce tiny, non-crystalline spherical particles.
These particles, due to their amorphous nature, have unique properties like low bulk density and high surface area, making fumed silica an excellent thickening agent in a wide range of products, including paints, coatings, cosmetics, and adhesives. Fumed silica’s versatility and ability to provide thixotropic properties enhance its significance in various manufacturing processes.

Silica Forms

Silica is found in both crystalline and amorphous forms, each having distinct properties and applications in nature and technology. Crystalline forms such as quartz, cristobalite, and tridymite, are characterized by their well-ordered, repeating atomic structures. These are often found in rocks and are crucial in geological studies.
Amorphous forms, like fumed silica, do not exhibit a regular structure. This lack of order results in different physical properties and industrial uses. Amorphous silica is especially valued for its properties in applications that require non-crystalline materials, such as stabilizers and rheology modifiers.

• Crystalline Forms: Ordered, stable, found in rock formations.
• Amorphous Forms: Disordered, versatile, used in industry.

Understanding the differences between these forms helps in selecting the right type of silica for specific scientific or industrial needs.