Pegmatite

A pegmatite is a holocrystalline, intrusive igneous rock composed of interlocking phaneritic crystals usually larger than 2.5 cm in size (1 inch); such rocks are referred to as pegmatitic.

Most pegmatites are composed of quartz, feldspar and mica, having a similar basic composition as granite. Rarer intermediate composition and mafic pegmatites containing amphibole, Ca-plagioclase feldspar, pyroxene, feldspathoids and other unusual minerals are known, found in recrystallised zones and apophyses associated with large layered intrusions.

Crystal size is the most striking feature of pegmatites, with crystals usually over 5 cm in size. Individual crystals over 10 metres (33 ft) long have been found, and many of the world's largest crystals were found within pegmatites. These include, spodumene, microcline, beryl, and tourmaline.

Similarly, crystal texture and form within pegmatitic rock may be taken to extreme size and perfection. Feldspar within a pegmatite may display exaggerated and perfect twinning, exsolution lamellae, and when affected by hydrous crystallization, macroscale graphic texture is known, with feldspar and quartz intergrown. Perthite feldspar within a pegmatite often shows gigantic perthitic texture visible to the naked eye.

Mark Jacobson

Mark Jacobson (born 1948) is an American author and writer.

Early life

Jacobson graduated from the University of California, Berkeley, and achieved recognition in New York City while writing for the Village Voice in the 1970s, most particularly for a lurid account of life in the Chinatown Ghost Shadows gang.

Work

His books include his debut novel and cult classic Gojiro (1992), a Godzilla story; the autobiographical Jacobson family travel saga co-authored with daughter Rae Jacobson 12,000 Miles in the Nick of Time – a Semi-Dysfunctional Family Circumnavigates the Globe; the novel Everyone and No One; a collection of previously published pieces Teenage Hipster in the Modern World..., which includes the "Ghost Shadows" Village Voice articles; The KGB Bar Nonfiction Reader; the critically acclaimed but largely ignored compendium American Monsters (2004) co-edited with Jacobson's close friend Jack Newfield; the newly reissued compilation American Gangster containing the New York magazine piece "The Return of Superfly", the basis for a 2007 film starring Denzel Washington.