Crystalline silica is an indispensable part of both the natural and the technological worlds. We all come in contact with it daily and have all our lives. it has been called one of the building blocks of our planet. Although it is a mainstay of modern technology, it is neither modern nor manufactured. it was known to the ancients, and its uses are still being expanded today.

Crystalline silica exists in seven different forms or polymorphs, four of which are extremely rare. The three major forms, quartz, cristobalite, and tridymite, are stable at different temperatures. Within the three major forms, there are subdivisions. Geologists distinguish, for example, between alpha and beta quartz, noting that at 573 EC, quartz changes from one form to the other. Each of these subdivisions is stable under different thermal conditions. Foundry processes, the burning of waste materials, and other manufacturing procedures can create the kinds of conditions necessary for quartz to change form. In nature, quartz in its alpha, or low, form is most common, although both lightning strikes and meteorite impacts can change alpha quartz into keatite or coesite. Alpha quartz is abundant, found on every continent in large
quantities. In fact, alpha quartz is so abundant and the other polymorphs of crystalline silica are so rare, some writers use the specific term quartz in place of the more general term crystalline silica.

Natural Occurrence
All soils contain at least trace amounts of crystalline silica in the form of quartz. It may have been part of the rock that weathered to form the soil, it may have been transported, or it may have crystallized from an amorphous (that is, a noncrystalline) silica that formed during the weathering process. Quartz is also the major component of sand and of dust in the air. Quartz is present in igneous rocks-but only those that contain excess silica. As magma cools, olivine, pyroxenes, amphiboles, feldspars, and micas form first. These minerals (all silicates) need silica to form, because silicates are made from silicon, oxygen, and a metal, usually one of the six most common metals. Quartz forms only if sufficient silicon and oxygen are left over after these silicates have formed. Nature's odds are stacked in quartz's favor, however. The fact that quartz is the second most common mineral in the world (feldspar is most common) indicates that plenty of silicon and oxygen were left over during the cooling process to allow ample quantities of quartz to form. In fact, the average quartz content of igneous rocks is 12%.
In geologic history, igneous rocks originated from magma, the material carried to the surface from the Earth's molten core. The other two types of rocks are sedimentary and metamorphic. Quartz is abundant in all three types of rock. It is one of Earth's primary building blocks. The rock cycle describes the relationship between the three types of rock. Igneous rocks reflect activity (heat and pressure) beneath Earth's crust; metamorphic rocks reflect activity both beneath the crust and within and at the surface; and sedimentary rocks reflect conditions (wind, water, and ice) at the Earth's surface. Over geologic time, sedimentary rocks maybe altered by heat and/or pressure to create metamorphic or igneous rocks. All rocks may be eroded to make sediments that, in turn, harden (lithify) into sedimentary rocks. Thus, the history of the Earth’s
crust, the lithosphere, is one of continuous change. During these changes, quartz endures. It is one of Earth's harder minerals, so it resists erosion, and it is soluble in very few chemicals, so it is seldom dissolved.
In Igneous Rocks. Crystalline silica is present in igneous rocks that contain excess silica. It is a common component of granite, rhyolite, quartz diorite, quartz monzonite, and andesite, to name a few. Crystalline silica as quartz also may be present in deposits of hardened, or consolidated, volcanic ash, known as volcanic tuffs. When magma spews from a volcano, it drops in temperature so rapidly that the ash is usually glassy, a noncrystalline state. The 1980 eruption of Mount St. Helens is a perfect example of this process. If the silica crystallizes before the molten rock leaves the volcano then the quartz is imbedded in a glassy matrix. Volcanic glasses do crystallize over time, so a complex mixture of finely crystalline quartz and silicates eventually replaces the volcanic glass. Cristobalite and tridymite, the rarer forms of crystalline silica, may also be present in volcanic tuffs.
In Sedimentary Rocks. Crystalline silica in the form of quartz is an extremely common component of sedimentary rocks. Sedimentary rocks form when minerals released during weathering or by chemical precipitation accumulate in a basin and are consolidated. Quartz, which is extremely resistant to physical and chemical breakdown by the weathering process, stays intact chemically even when fragmented and dispersed by erosion, wind, or other weathering processes. Quartz is present in a variety of sedimentary rock types, ranging from sandstones to conglomerates, in trace to major amounts.
In Metamorphic Rocks. Metamorphic rocks, which form through heat or pressure, also contain crystalline silica as quartz. New textures may be created in the rock (for example, lineation's or increased crystal sizes), and new minerals may be formed during metamorphism. Quartz may be present in the original rock, it may crystallize from silica-bearing fluids that entered the rock during metamorphism, or it may form as part of the metamorphic transformation.




    •  Forms
    •  Natural Occurrence