Geology Reference
In-Depth Information
needlelike chain-silicate habits of some notorious forms of asbestos, or the thin flat sheets
of minerals like mica, once used as a cheap substitute for window glass.
Though less abundant than silicon, the elements calcium, magnesium, and aluminum all
play key structural roles in the most common silicate rocks throughout Earth's crust and
mantle. As positive ions, like their more bountiful silicon cousin, they occasionally bond
withoxygenalone,formingthecalciumoxidethatwerecognizeasthelawnchemicallime,
the rare compound magnesium oxide, and (when trace amounts of the rarer elements chro-
miumortitaniumareincorporatedintoaluminumoxide)theprizedgemstonesrubyorsap-
phire.
It is the sixth of the big six elements, iron, that is by far the most versatile. Each of the
other five—oxygen, silicon, aluminum, magnesium, and calcium—assumes one dominant
chemical personality. Oxygen almost always acts as an acceptor of two electrons, silicon
almost always acts as a donor of four electrons, aluminum as a donor of three electrons,
and magnesium and calcium as donors of two. But iron, element twenty-six, plays three
quite distinct chemical roles.
Iron's versatility is underscored by Earth's layered structure. About one in ten atoms in
Earth's oxygen-dominated crust and mantle is iron, whereas Earth's metallic core is more
than 90 percent iron. This sharp contrast stems from the fact that this element's twenty-six
electronsareaprettyfarcryfromeighteen,thenearestmagicnumber,makingironadonor
parexcellence. There'snowayironcangiveawayeightelectrons(nooneatomwillaccept
that many) so it has to make do with whatever acceptors happen to be present.
Sometimesironactsjustlikemagnesiumandgivesuptwoelectronstobecomea+2ion.
Iron in this divalent state lends a distinctive greenish or bluish color to many minerals and
other chemicals. The characteristic green color of the gemstone peridot (an iron-bearing
olivine) and the bluish-green color of oxygen-starved blood in your veins are telltale signs
of divalent iron. In this guise, iron bonds to oxygen in a one-to-one ratio. And because
magnesium and iron atoms are similar in size, these elements often substitute freely for
eachotherincommonmineralsofEarth'scrustandmantle.SomeofEarth'smostabundant
minerals, including olivine, garnet, pyroxene, and mica, have variants that display pretty
much any magnesium-to-iron ratio, from colorless versions with 100 percent magnesium
to dark-hued varieties with 100 percent divalent iron.
Iron is not restricted to the +2 state, however. In the presence of lots of electron accept-
ors,itreadilygivesupathirdelectrontobecomea+3ion.Thistrivalent formofironlends
a characteristic brick-red color to its host. Red rust, red soils, red bricks, and oxygen-rich
red blood owe their vivid hues to trivalent iron. Like aluminum, which also adopts the +3
state, trivalent iron bonds in a two-to-three ratio with oxygen to form Fe
2
O
3
—a common
mineral named hematite, for its bloodred color. Just as magnesium often proxies for iron's
divalent form, aluminum commonly replaces iron's trivalent variant. The minerals garnet,
