Hornblende (dark amphibole). Photographed by John Hebberger, Jr.
THE LAST ONE PERCENT
While it is a deceptively simplistic observation, humans and chimpanzees share about 98.8% of the same DNA ( DNA: Comparing Humans and Chimps ). Now, we can all agree that chimpanzees are pretty cool in their own right, swinging around in trees the way they do and all that. But even though we all act like apes from time to time, who among us would prefer being chimpanzee to being human? Not to disrespect the chimpanzee, but everything that makes us human is a product of that last one percent. Our character, our insights, our achievements, our freedom to embrace or reject a multitude of virtues and vices… it is all a product of how that last one percent of a simple soup with just four amino acids crystalized into organized sequences of genetic code. That, right there, is a concept probably not comprehendible to a chimp but surely worth a GJH campfire to the rest of us. Be it by design or just a miraculous fiat of chance, humans are extraordinary in this universe that is generally so hostile to life.
It is the same way with pegmatites. When a large blob of felsic (silica-rich side of the rock spectrum) melt intrudes into older basement rock at great depth, temperature, and pressure (the same conditions necessary to effect change in humans), certain things begin to happen in a predictable fashion. First, the quartz and feldspars – basically the most abundant minerals on earth – begin to crystalize out of the molten soup. These reactions do not require water, which is also part of the original melt. Consequently, as the common elements crystalize into what we will loosely refer to as granite, the steadily dwindling fraction of molten rock is comprised of increasingly high concentrations of water and dissolved exotics like boron, beryllium, lithium, chlorine, aluminium, and titanium. This is the magic fraction – basically the last one percent of the original soup. It has much more of the interesting stuff that differentiates a boring granite from a vibrant pegmatite. Cooling and pressure crack the new body of granite and the last remaining dollops of the soup press in along the fractures and intrusion boundaries and crystalize into dikes, veins, and sills of pegmatite.
Nothing bad about granite, mind you. Cool stuff, granite is. But pegmatites are certainly the icing on the petrologic cake. These always beautiful assemblages of large crystals – some of which contain gem stones and other valuable minerals – are the subject of the next public lecture. Drop in this Tuesday, (June 7th) at the Teton County Library Auditorium at 6:00 pm to learn more from Mark Jacobson, a retired earth scientist from the Chevron Corporation.