Crystalline Silica

In practical language, Crystalline Silica addresses what crystalline silica is, where it is found and used, and how it is identified. In addition, the book discusses the regulatory decisions yielding new interest in this ubiquitous substance and presents an overview of the techniques used to determine its presence and abundance. A list of selected readings and supplemental resources and a glossary of terms beyond the scope of this publication round out the text.

A concise assessment of the adverse effects on human health caused by exposure to quartz the most common form of crystalline silica. Quartz is a frequently occurring solid component of most natural mineral dusts. Human exposure occurs most often during occupational activities involving movement of earth disturbance of silica-containing products such as masonry and concrete or use or manufacture of silica-containing products. As respirable quartz dust particles can be inhaled and deposited in the lung the report gives particular attention to evidence of an increased risk of lung cancer in occupationally exposed workers. Most studies in laboratory animals have concentrated on adverse effects associated with long-term inhalation of particles. Effects observed include cellular proliferation nodule formation suppressed immune function and alveolar proteinosis. While exposure clearly induces pulmonary tumours in one species other species show less or no malignant tumour response. The evaluation of risks to human health draws on a large number of epidemiological studies of workers exposed to respirable quartz dust. Occupational exposure has been linked to an increased incidence of silicosis ung cancer and pulmonary tuberculosis. Studies have also documented statistically significant increases in cases of bronchitis emphysema chronic obstructive pulmonary disease autoimmune-related diseases including scleroderma rheumatoid arthritis and systemic lupus erythematosus and renal disease. In reviewing these findings the report underscores several uncertainties inherent to the study of respiratory diseases in occupational populations that complicate the assessment of risks associated with exposure to quartz dust. The need for improved methods of exposure assessment and data analysis is stressed.

In 1987, the International Agency for Research Against Cancer conducted a review of the health literature and concluded that crystalline silica was a probable human carcinogen. As a result of this finding, OSHA was required to regulate crystalline silica under its Hazard Communication Standard (HCS). The standard requires that all materials handled by OSHA-regulated facilities be labeled according to the requirements of HCS and that workers receive proper training on the handling of the material if the crystalline silica content equals or exceeds 0.1 weight percent (0.1%). MSHA currently is considering enacting its own HCS. This will be similar to OSHA's HCS. If the standard is enacted, most mineral producers will have to determine the respirable monitor filter and bulk crystalline silica contents of their ores and products so that workers and/or customers will know whether they are in compliance with the 0.1% HCS and/ or the OSHA permissible exposure level (PEL) of 50 micro-g for an 8-hour workday for respirable crystalline silica as determined from monitor samples. Two major concerns with the HCS are the widespread occurrence of crystalline silica in nature and the suitability of current technology for routinely determining crystalline silica concentrations at the 0.1% HCS level. Most ores are extracted from silica-bearing deposits, and silica is a common constituent of rocks and soils. OSHA's HCS will have the greatest impact on the producers of crushed stone, diatomite, dimension stone, gravel, industrial sand, perlite, pumice, pyrophyllite, sand, and talc because these materials frequently are shipped directly from the mill to the customer. MSHA's HCS would affect nearly all mineral producers. Those producers that have crystalline silica present in concentrations near the 0.1% cutoff point will have the most difficulty with the analysis. Crystalline silica can be quantified at the 0.1% level by X-ray difractometry in simple systems containing one, two, and possibly three minerals if (l) none of the accessory minerals has X-ray diffraction reflections that coincide or overlap with those of crystalline silica and (2) the standard has a particle size distribution and crystallinity similar to those of the sample. In some instances, it may not be possible to determine the crystalline silica content of a sample with any degree of certainty using the recommended regulatory protocol. In all cases, it is recommended that a qualified mineralogist identify the minerals in a sample prior to any regulatory analysis. Additionally, the uncertainty as to whether some silica polymorphs should be classified as crystalline or noncrystalline and the suitability of metastable high-temperature standards, such as cristobalite and tridymite, for regulatory analysis at ambient temperatures should be addressed further. This overview is written both to highlight these problems and to serve as a guide for analysts, regulators, and industry personnel who are involved in the crystalline silica issue. It also covers some of the difficulties and/or shortcomings in quantifying crystalline silica, such as the ubiquitous mineral quartz, in the workplace.

The exciting world of crystalline silicon is the source of the spectacular advancement of discrete electronic devices and solar cells. The exploitation of ever changing properties of crystalline silicon with dimensional transformation may indicate more innovative silicon based technologies in near future. For example, the discovery of nanocrystalline silicon has largely overcome the obstacles of using silicon as optoelectronic material. The further research and development is necessary to find out the treasures hidden within this material. The book presents different forms of silicon material, their preparation and properties. The modern techniques to study the surface and interface defect states, dislocations, and so on, in different crystalline forms have been highlighted in this book. This book presents basic and applied aspects of different crystalline forms of silicon in wide range of information from materials to devices.

Volume 29 of Reviews in Mineralogy provides an updated silica review which focuses on the most recent developments. This book describes the crystal structures and phase transitions of silica and its stuffed derivatives; bridges the relationship between the microstructural character of real silica minerals and the behavior of silica in the geological environment; covers Quantum mechanical considerations of the Si-O bond; shows how calculations based upon first-principles theory can explain and predict silica transitions at high temperatures and pressures; covers spectroscopic analyses of silica and how they reveal vibrational behaviors in response to variations in temperature, pressure, and composition and finally details the uses of silica for industrial purposes.