Geology Reference
In-Depth Information
chemical bonding. Recall that atoms attach to each other when their fuzzy electron clouds
interactandshuffletoformmorestablearrangements—notablyatomswiththemagicnum-
bers of two or ten or eighteen electrons. For this kind of exchange to work, some atoms
have to give away electrons, while others have to accept them.
Oxygen is Earth's master electron acceptor. Every oxygen atom has eight positively
charged protons in its nucleus, which are electrically balanced by eight negatively charged
electrons. But oxygen is always looking for two extra electrons to make the magic number
of ten electrons. That incessant acquisitive urge makes oxygen one of the most reactive,
corrosive gases in nature. It's really rather nasty stuff.
Most of us think of oxygen, first and foremost, as an essential part of the atmosphere
(the 21 percent or so that keeps us alive). But that happy atmospheric development is a
relatively recent change in Earth history. For Earth's first two billion years at least, the at-
mospherewasutterlydevoidofoxygen.EventodayalmostallofEarth'soxygen—99.9999
percent of it—is locked into rocks and minerals. When you hike up a majestic, rugged
mountain or walk on a windswept, rocky outcrop, most of the atoms beneath your feet are
oxygen. When you lie on a sandy beach, almost two in three of the atoms that support your
weight are oxygen.
For oxygen to play this critical chemical role as electron acceptor, there also have to
be lots of atoms that can give away or share their electrons. The most prolific electron
donor is silicon, which accounts for almost one in every four atoms in Earth's crust and
mantle. Silicon has fourteen positively charged protons in its nucleus, which are nominally
balanced by fourteen negatively charged electrons. Silicon commonly gives up four elec-
trons to achieve the magic number of ten electrons, becoming a silicon ion with a positive
electrical charge. In Earth's rocky crust and mantle, those four relinquished electrons are
almost always gobbled up by two oxygen atoms, which become negatively charged ions.
Consequently, strong silicon-oxygen bonds are found in almost every rock, most notably
in quartz, or SiO
2
—a one-to-two mixture of silicon and oxygen atoms. Tough, translucent
quartz grains last a long time. They accumulate by the untold trillions along shorelines and
are by far the commonest mineral of beach sand. You've also probably seen the beauti-
ful, sharply faceted, transparent crystalline specimens of quartz sold in New Age stores as
“power crystals.” When you hold such a treasure in your hand, two-thirds of what you're
holding is oxygen.
Crystals with silicon-oxygen bonds, collectively called silicates, are Earth's most com-
mon minerals, with more than thirteen hundred different known species (and more added
almost every month). They are richly varied in their atomic structures and properties be-
causeoftheversatilityofthesilicon-oxygenconnection,whetherintheheartyweather-res-
istant structure of quartz and feldspar, or the cluster arrangements of gemmy green olivine
and red garnet (the semiprecious birthstones of August and January, respectively), or the
