Geology Reference
In-Depth Information
I asked him to think about their stories through time. When did they first appear? What
processes led to their diversification? Have any beryllium minerals become “extinct”? No
one had ever tried to answer such questions. It's hard enough to catalog every mineral of
a given element, but to tease out when each species first appeared or disappeared is a mo-
numental task. For beryl, the commonest beryllium mineral (most prized in its deep green
variety, emerald), there are thousands of localities. It's a daunting challenge to track down
the oldest beryl.
After a year of toil, Ed Grew produced a landmark graph showing the cumulative num-
ber of beryllium minerals through time based on thousands of reported occurrences. As
expected, it took a very long time for the first beryl to appear—almost 1.5 billion years.
The element beryllium is present at only about two parts per million in Earth's crust, so it
takes time for hot fluids to select and concentrate that trace beryllium into an enriched flu-
id that can precipitate beryl crystals. For another billion years, only about twenty different
beryllium minerals appeared. By our fledgling theory, a big pulse of new minerals should
have occurred during the Great Oxidation Event, between 2.4 and 2.0 billion years ago,
but that's not what Ed found. Instead, the biggest increase occurred a bit later, more than
doubling the number of known species between about 1.7 and 1.8 billion years ago. That
span,rightatthebeginningoftheboringbillion,wasatimeofassemblyfortheColumbian
supercontinent. Perhaps beryllium was concentrated into new minerals during the intense
mountain-forming events associated with continental collisions.
EdGrewfollowedupwithanevenmoreimpressivesurveyofthe263knownboronmin-
erals. Tourmaline, most prized in its gorgeous semiprecious red-green variant, is found in
some of Earth's oldest rocks, but that was it for nearly half a billion years. In samples from
2.5 billion years ago, a measly twenty or so different boron species—less than 10 percent
of the modern total—are recognized. As with beryllium minerals, Ed observed a doubling
of the number of boron species in rocks of the boring billion era, this time in an interval
between about 2.1 and 1.7 billion years ago—an interval that brackets the formation of
the Columbian supercontinent. Again, this rapid increase in mineral diversity raises a lot
of questions—about when the postoxidation diversification actually took off, about the as-
sembly of supercontinents, and about the rise of mineralogical novelty during the boring
billion.
For our next foray into mineral evolution, we took on the ninety known minerals of the
scarce element mercury—a study that further complicates the picture. Like the much more
abundant element iron, mercury can occur in three chemical states—as an electron-rich
metal (the familiar silvery liquid of old thermometers), as well as in two different oxidized
forms.Accordingly,weanticipatedasharpincreaseinmercurymineraldiversityfollowing
the Great Oxidation Event, but the picture that emerged is rather different. As with the his-
toryofberylliumandboronminerals,theearliestmercurymineral—itscommonestore,the
