Nonetheless, crystalline silica can represent a large fraction of the respirable dust present in agricultural settings because quartz is so abundant in most soils. A wide array of other rock- and soil-forming minerals, including silicate and nonsilicate minerals may also be present. Silica is typically only one of many constituents of mineral dust. Tripoli, flint, chert, jasper, chalcedony, agate, onyx, and silica flour are examples. Microcrystalline varieties of silica also include small grains of this material, possibly combined with amorphous silica. Other than alpha quartz, all of these forms are thermodynamically metastable at earth surface temperatures and pressures and will slowly convert to alpha quartz given enough time (millions of years). Coesite and stishovite are two rare polymorphs formed at high pressure (e.g., meteorite impact craters and some ultra-high-pressure metamorphic rocks). Tridymite and cristobalite are other fairly common forms of crystalline silica and are generally found in rocks formed at high temperature (e.g., volcanic rocks). Exceptions to the generally widespread occurrence of quartz are the highly weathered soils of tropical regions, where even the resistant quartz has been lost through weathering processes.
Quartz is so prevalent that the term quartz is often used in place of crystalline silica. That quartz is among the most abundant minerals in many, if not most, soils is a reflection of its chemical stability and resistance to weathering. Alpha quartz is the most common form, and is found in large quantities in many rocks and soils worldwide. Quartz is present as alpha and beta (high temperature) forms. Quartz is the most common crystalline form of silica encountered in nature. Pinkerton, in Encyclopedia of Toxicology (Third Edition), 2014 Uses and Occurrence in Nature The presence of silica, extensive fibrosis, scarcity of mitotic figures, and poorly developed vasculature all serve to distinguish these lesions from fibrous histiocytomas and undifferentiated pleomorphic sarcomas. The collagen varies from delicate interstitial or perivascular fibers in the early stages to broad bands and finally mats or large nodules. A striking feature of the lesion is the large amount of fibrosis. Numerous silica crystals can be identified under polarized light. PAS-positive, diastase-resistant bodies may be present in the histiocytes and probably represent large phagolysosomes, organelles involved in the intracellular storage of silica. Although usually well differentiated, the histiocytes occasionally display moderate pleomorphism. They consist of sheets of histiocytes with a clear or amphophilic cytoplasm. Grossly, the lesions are poorly defined, gray-yellow masses with a gritty consistency on cutting. Typically, they occur many years after the injection of silica, so the causal relation of the injection is minimized or overlooked. 158 Clinically, these lesions present as slowly enlarging tumorous masses, usually in the inguinal region or abdominal wall. It seems principally to be an iatrogenic disease secondary to the now-obsolete injection therapy for hernias. Goldblum MD, in Enzinger and Weiss's Soft Tissue Tumors, 2020 Silica ReactionĪlthough the usual response to silica in soft tissue is a localized foreign body reaction, exuberant reactions to the material simulate a fibrohistiocytic neoplasm ( Figs.
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