Geology Reference
In-Depth Information
amphibole, and mica display every imaginable aluminum-to-iron ratio, with the iron-rich
varieties presenting as red rather than green.
Sowiththeextremelyusefultrickofswitchingbackandforthfromthe+2tothe+3state
(we'll come back to this remarkable ability in a couple billion years, when life first comes
onthe scene), ironinits divalent andtrivalent guises acts like the other members ofthe big
six. But wait—iron has one more critical role to play in Earth: it can rather easily form a
metal.
All the types of chemical bonds introduced so far involve an exchange of electrons, res-
ulting in ions. Silicon, aluminum, magnesium, calcium, and iron giveth electrons; oxygen
taketh them away. Consequently, these linkages are called ionic bonds. However, metals
adopt a very different bonding strategy. In a metal, each atom gives up one or more elec-
trons, to become positively charged. But those disenfranchised electrons hang around in
the metal in a kind of sticky, negatively charged sea, which holds all the positively charged
atoms together like regimented arrays of little BBs in molasses. Iron metal is a vast collec-
tion of iron atoms that collectively share such delocalized electrons.
The consequences of this communal behavior are profound. For one thing, all those
sharedelectronsarefreetomovearound,sometalsmakeexcellentconductorsofelectricity
(electricity beingnothingmorethanthecontrolledflowofelectrons).Bycontrast,inionic-
ally bonded materials made of oxygen plus magnesium or silicon, every electron is locked
into place so tightly that electricity can't possibly flow. Another consequence of metallic
bonding is that such materials tend to bend rather than break. The electron sea surrounding
the atoms can be folded and twisted without losing its collective strength, quite unlike the
behavior of most brittle rocks and minerals.
The perceptive reader will have noted that iron is not alone in performing this metal-
forming trick. Aluminum metal cans, foil, and household wiring are commonplace; mag-
nesium metal alloys are a mainstay of high-tech racing cars and other toys; and the semi-
metalsiliconliesattheheartofeveryelectronicgadget(henceSiliconValley).Butmetallic
aluminum, magnesium, and silicon are modern marvels of the chemical industry. It takes
hugechunksofenergytoripthosestubbornelementsawayfromoxygen,andtheirmetallic
states almost never form in nature.
Iron is less committed to oxygen and more fickle in its chemical bonding partners. Un-
like silicon, aluminum, magnesium, or calcium, it is perfectly happy to link to other elec-
tron acceptors, notably sulfur—iron sulfide is the shiny mineral pyrite, or fool's gold. Un-
likethoseotherelements,ironreadilyformsadensemetalthatsinkstothecenterofplanets
and forms their massive cores.
Molten Earth
