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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.
Forms
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. |
