Page 461 - Carrahers_Polymer_Chemistry,_Eighth_Edition

424 Carraher’s Polymer Chemistry

Si-O rings (three SiO tetrahedra) connected such that every six rings enclose a 12-membered Si-O
4
(six SiO tetrahedra) ring.
4
Quartz is found in several forms in all three major kinds of rocks—igneous, metamorphic, and
sedimentary. It is one of the hardest minerals known. Geologist often divide quartz into two main
groupings—course crystalline and cryptocrystalline quartz. Course crystalline quarts include six-
sided quartz crystals and massive granular clumps. Some colored varieties of coarse crystalline
quartz crystals, amethyst and citrine, are cut into gem stones. Others include pink (rose), purple, and
milky quartz, but most coarse crystalline quartz is colorless and transparent. Sandstone is a ready
example of granular quartz. Color is a result of the presence of small amounts of metal cations such
as calcium, iron, magnesium, and aluminum.
Cryptocrystalline forms contain microscopic quartz crystals and include the chalcedony group-
ing of rocks such as chert, agate, jasper, and fl int.
Quartz exhibits an important property that allows the piezoelectric effect. When pressure is
applied to a slice of quartz, it develops a net positive charge on one side of the quartz slice and a
negative charge on the other side. This phenomenon is the piezoelectric generation of a voltage dif-
ference across the two sides of the quartz crystal. Furthermore, the same effect is found when pres-

sure is applied not mechanically, but through application of an alternating electrical field with only
certain frequencies allowed to pass through the crystal. The frequencies allowed to pass vary with
the crystal shape and thickness. Such crystals are used in radios, televisions, and radar. This effect
also forms the basis for quarts watches and clocks. Voltage applied to a quartz crystal causes the
crystal to expand and contract at a set rate, producing vibrations. The vibrations are then translated
into a uniform measure of time.
While quartz crystals are suitable for the production of optical lenses, most lenses are man-
ufactured from synthetically produced quartz due to the scarcity of good-grade large quartz
crystals.
The feldspars are the most abundant minerals in the Earth’s crust, accounting for about 60%
of all igneous rocks. They are derivatives of silica, where about one-half or one-quarter of the sil-
icon atoms have been replaced by aluminum. Feldspar is used in the manufacture of certain types
of glass and pottery. Some feldspar crystals—such as moonstone (white perthilte), Amazon stone
(green microcline), and multicolored labradorite, are used as gem stones and in architectural deco-
rations. Some feldspar is used as a coating and filler in the production of paper.

Granite is a hard crystalline rock chiefly composed of quartz and feldspar. It is used in build-
ing bridges and building where great strength is needed. It is also employed in the construction of
monuments and gravestones since it can be polished giving a lasting luster and because of its ability
to withstand wear by the natural elements.
Sand is loose grains of minerals or rocks, larger than silt but smaller than gravel. Soil contains
mineral (often in the form of small sand granules) and organic matter.
Micas are also composed of silicon–oxygen tetrahedra. The anionic charge on the silicate sheet
is the result of the replacement of silicon by aluminum. Cations such as potassium are interspaced
between these negatively charged sheets. Some mica is used in construction and electrical engi-
neering applications. Synthetic mica is manufactured on a large scale for industrial consumption in

coatings, as fillers, and so forth. Micas are one of the many layered silicon–oxygen-intense materi-
als found in nature.

12.12 SILICON DIOXIDE IN ELECTRONIC CHIPS
Silicon dioxide plays a critical role in the electronics industry. The silicon used to produce silicon chips
is derived from silicon dioxide. Semipure silicon dioxide (to about 99%) is prepared from the reaction
of silicon dioxide with coke (a poor grade of graphite) using high temperature and an electronic arc.
SiO + C → Si + CO (12.19)
2 2

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