Cristobalite the Occurrence and Effect

Cristobalite

The Occurrence and Effect of Cristobalite in the Manufacture of Purified Silica: Overview and Analysis

This paper examines the mechanisms and effects of cristobalite formation in the manufacturing process of silica products. The basic formation and structure of cristobalite, a polymorph form of quartz, is discussed, before an identification and evaluation of its presence in manufacturing processes is conducted. The two primary forms of cristobalite formations, alpha- and beta-cristobalite, are found to differ in their formation and effects on the manufacturing process due to temperature and structural differences.

Cristobalite is a polymorph form of quartz occurring at high temperatures and transitioning to a less regular form when crystals are allowed to form at lower temperatures. The generally microscopic crystal of cristobalite that form under certain conditions have long been recognized in geological literature, and occur naturally in certain areas and rock formations (Claeys 1998). These crystals also form in manufacturing processes, especially in attempts to manufacture purified silicon crystal and/or sheets for use in industrial and technological applications (Claeys 1998; Tamura et al. 1998). This paper examines current and relevant literature to determine the conditions that lead to the occurrence of cristobalite in the manufacture of purified silica, and discusses the effects of this occurrence and the potential remedies for cristobalite formation.

Composition and Formation

Like all forms of quartz, cristobalite has a chemical composition of SiO2 (Claeys 1998). The crystals that form the cristobalite structure, however, technically have a composition of SiO4, with each tetrahedron or microcrystal sharing oxygen atoms at a single point with adjacent tetrahedrons in the crystal structure, in a state of constant tumbling and reorganization (Claeys 1998; Wright & Leadbetter 1975). This can create certain irregularities and disordered patterns and twinnings within the crystals; cooling at lower temperatures forms an even more disordered form of cristobalite wherein the tetrahedrons tilt irregularly on their axes in relation to each other, retaining a basic structure that is still crystalline but one that is highly irregular (Tamura et al. 1998; Wright & Leadbetter 1975).

When pure fused silica is subjected to temperatures between 1470 degrees Celsius and 1723 degrees Celsius for a sufficient time, the silica will undergo complete devitrification into cristobalite (Tamura et al. 1998). Such total devitrification does not naturally occur, at least not in large and discovered quantities, but natural formation of cristobalite does occur at lower temperatures, given sufficient time and certain other conditions (Claeys 1998; Kalpokaite-Dickuviene et al. 2009; Tamura et al. 1998). Cristobalite formation is both less certain and less stable at lower temperatures than it is in extreme and prolonged conditions above 1470 degrees Celsius, but it naturally occurs more often in lower temperature circumstances, along with other polymorphs of quartz (Claeys 1998).

Occurrence in Manufacturing Settings

The occurrence of cristobalite crystals in the manufacturing process is generally more of a problem than a purposeful process; the formation of cristobalite is representative of the difficulties encountered in the manufacture of pure silica; the very devitrification that leads to cristobalite formation is by definition an impurity and imperfection in the formation of silica sheets, and cristobalite forms gross imperfections and disorders in silica crystals (Tamura et al. 1998; Kalpokaite-Dickuviene et al. 2009). Not only the emergence of cristobalite itself but also the very irregularities and disorders of its crystalline structure, especially when the cristobalite has formed at lower temperatures, creates specific notable irregularities and disturbances in the silica products being manufactured during the formation of the cristobalite (Kalpokaite-Dickuviene et al. 2009; Claeys 1998).

In a certain manufacturing process involving the spraying of plasma -- superheated molten rock -- the formation of cristobalite was observed to occur at temperatures of approximately one-thousand degrees Celsius, while the formation of the crystals that were the target of this manufacturing process occurred at nine-hundred degrees Celsius (Kalpokaite-Dickuviene et al. 2009). In the formation of silica glass, the two factors of nucleation rate and growth rate -- both of which can be controlled but not entirely eliminated in the manufacturing process -- determine the extent and timing of cristobalite formation (Tamura et al. 1998). The relative ease with which cristobalite forms is unfortunate given its relative lack of use.

As the emergence of cristobalite in the processes for manufacturing various silica products has become a bigger problem in the technology and industrial sectors, efforts to understand the actual structure of the crystal have been ongoing; prior to more recent technologies for examinations, success was found using X-ray technology to identify the structure of the crystalline shape that cristobalite forms under different circumstances, yielding dome of the first conclusions regarding the shape of the crystal that truly matched the inferences made from measured data (Wright & Leadbetter 2009). The actual structure of the cristobalite crystal is now fairly well understood, but understanding the mechanisms of its development and the circumstances under which this occurs -- as well as the methods that can possibly be put into place to prevent this occurrence -- require further research.

Process Mechanisms

Kalpokaite-Dickuviene et al. (2009) demonstrated one process by which cristobalite forms, and though the manufacturing process utilizing plasma spray technologies was rather sophisticated, the incidental formation of the cristobalite crystals during the cooling process was rather simplistic. The formation of the cristobalite in this instance occurred largely as it might in natural circumstances, at temperatures lower than those required for stable cristobalite and over a longer time process (Claeys 1998; Kalpokaite-Dickuviene et al. 2009).

This information is still somewhat useful in understanding the mechanisms and processes behind cristobalite formation, but others have been more useful.

One particular circumstance that has been observed to lead to increased cristobalite formation in a manufacturing process is the prolonged exposure of manufactured silica glass sheets to liquid silicon during the heating and formation process (Tamura et al. 1998). In a controlled experimental study, it was found that detrivification of the silica -- or the vitrification of the cristobalite, which is the same thing in this instance, just stated in reverse -- occurred more rapidly and more extensively on areas of a silica glass plate that had had liquid silicon applied than on areas without such an application (Tamura et al. 1998). As the creation of silica glass via certain methods necessitates the exposure of the glass to silicon liquid during the manufacturing process, this has very real implications for the efficient and proper manufacture of silica (Tamura et al. 1998).

Cristobalite Structure and Process Interactions

As noted above, the ultimate crystalline structure of the cristobalite formation in both natural and manufacturing processes is highly dependent on the temperature at which the crystals form. At temperatures in excess of 1470 degrees Celsius, stabilized beta-cristobalite with a more regular crystalline structure is formed, with the individual SiO4 tetrahedrons in a regular and essentially uniformly aligned (notwithstanding the shifting of the tetrahedrons due to the shared oxygen molecules of the crystalline structure) (Claeys 1998). This type of formation can be problematic in manufacturing processes, but only when temperatures of this degree must be sustained for long periods of time.

Far more of a problem in manufacturing processes is the formation of cristobalite at lower temperatures, especially during the formation and cooling of silica into various industrial and technological products (Kalpokaite-Dickuviene et al. 2009; Tamura et al. 1998; Wright & Leadbetter 1975). The less regular pattern and non-uniform alignment of the cristobalite crystalline structures that are more commonly found in nature and formed in manufacturing processes, identified as alpha-cristobalite, have proven to be a bigger problem in manufacturing processes. Not only does the alpha-cristobalite form at lower temperatures, but the less regular pattern of the crystals pose other problems for the manufacturing processes of many silica products, creating irregularities that are less easily countered and less easily predicted (Tamura et al. 1998; Wright & Leadbetter 1975). The cooler temperatures under which the alpha-cristobalite crystals form lead to a more varied timeline and structure than higher-temperature processes, complicating the cristobalite issue.