alpha Quartz
alpha Cristobalite
Tridymite
Figure 1. Polymorphs of crystalline silica [5].
TABLE 1. PHYSICAL PROPERTIES OF QUARTZ, CRISTOBALITE AND TRIDYMITE.
| Silica Polymorph | Crystal Structure | Index of Refraction | Specific Gravity | Other Properties |
| Quartz | hexagonal | 1.544 | 2.645 - 2.660 | Mol.Wt. 60.08 m.p. °C 1610 |
| Cristobalite | cubic or tetragonal | 1.487 | 2.32 | Mol.Wt. 60.08 m.p. °C 1723 |
| Tridymite | rhombohedral | 1.469 | 2.26 | Mol.Wt. 60.08 m.p. °C 1703 |
In nature, quartz is the crystalline form most commonly encountered and is so abundant that the term quartz is often used in place of the general term crystalline silica [6, 7]. Quartz is abundant in most rocks and soils and is also present in sand, mortar, concrete, fluxes, abrasives, construction aggregate, porcelain, paints and brick [8]. In addition, quartz-containing dust may be generated in any process which involves movement of earth (e.g., mining, farming, construction), disturbance of silica-containing products such as masonry and concrete, or use of sand and other silica containing products (e.g., foundry processes). Consequently, workers are potentially exposed to quartz dust in many occupations and industries.
Cristobalite and tridymite can be found in volcanic rocks and soils. These polymorphs can also be produced in some industrial operations. For example, cristobalite transformations occur in foundry processes, calcining of diatomaceous earth, brick and ceramics manufacturing, and silicon carbide production [9-11]. Burning of agricultural waste or products such as rice hulls may also cause amorphous silica to become cristobalite [8,12].
The other crystalline polymorphs (i.e., keatite, coesite, stishovite, and moganite) are very rarely or never observed in nature and are formed only under very high pressures [2]. Since these polymorphs are generally formed in small quantities, they are therefore of limited industrial hygiene interest.
