8 ChemMatters | OC TOBER/NOVEMBER 2017
www.acs.org/chemmatters
By Brian Rohrig
considered minerals. But most of the
4,000 known minerals are chemical compounds. Refer to the chart on the next page (Table 1). Do any of these formulas
look familiar? They should look like the ionic formulas you have written when
learning about chemical nomenclature.
Pyrite and quartz are two common rocks composed of only one mineral, but most
rocks are solid mixtures of several minerals. In these rocks, the minerals can be in
layers or distributed in chunks throughout the rock. The aptly named rocky road
ice cream provides a good analogy—nuts and marshmallows are interspersed
throughout the ice cream just like various minerals can make up a rock.
Most minerals have a crystalline structure. A crystal is a solid in which
the atoms or molecules are arranged in a regular repeating pattern.
Crystals take time to form. The larger the crystal, the more time it took for it
to form. Glass, on the other hand, is an amorphous solid, meaning that the
atoms are not orderly. When a solid forms suddenly (by cooling quickly),
atoms do not have time to organize themselves into a repeating pattern.
Consider this analogy: Suppose your class entered an empty room and
each person was holding a chair. Upon entering the room, what would
happen if there was only one second to arrange the chairs into rows
before you all had to freeze? Chances are, a haphazard arrangement of
chairs would result. But if you had five minutes to arrange the chairs,
your class would have plenty of time to form nice, neat rows. Likewise,
the formation of atoms into a regular repeating pattern, or crystallization,
takes time. Amorphous solids, however, form quickly, but without any order.
The types of bonds that exist in a crystal depend on the types of elements
that it contains. Metals usually exhibit a crystalline structure, held together by
metallic bonds. A metallic bond is an attraction of the nucleus of one metal atom
to the electrons of another. The electrons are delocalized, as they are loosely
held by their nuclei. This sea of electrons is shared equally among neighboring
metal atoms. Metallic bonds can lead to intricate crystal structures. Bismuth, for
example, has a fascinating structure that can be grown in the lab, as shown on the
next page.
Diamond is probably the best-known example of a crystal; however, this crystal contains the same non-metallic atoms—it’s all carbon. Each carbon atom in
diamond is bonded to four other carbon atoms with a covalent bond. A covalent
bond is a chemical bond that involves the sharing of an electron pair between two
atoms. Diamond is the hardest material in the world—a lot of energy is needed to
separate the atoms in diamond because the covalent bonds are strong, and diamond contains many of them.
OU ARE SURROUNDED BY
ROCKS, BOTH INDOORS AND
OUT. Do you have granite or marble countertops in your kitchen?
Both are made from rocks. You
use rocks when you brush your teeth—
toothpaste gets its abrasiveness from
ground-up rocks, usually silica (from
sand) or mica. If you wash your face or
hands with an abrasive soap you make
use of pumice, a volcanic rock. The
planet we live on is essentially one giant
rock, and the rocks we stumble upon
are just little pieces that have broken
off from the giant mother rock we call
Earth. So what are rocks and where do
they come from?
Rocks and minerals
A rock is a mineral—a naturally occurring inorganic solid with a definite chemical composition
and ordered internal structure—or a mixture of
several types of minerals. Pure elements that are
mined, such as gold (Au) and copper (Cu) are S H
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