Arizona Geological Society

Peter Modreski will present "Pegmatites: Mineralogy, Gemstones, Economic Geology, and maybe not quite the same Giant-Crystal Rocks you always thought they were"

  • 03 May 2016
  • 6:00 PM - 9:00 PM
  • Sheraton, 5151 E Grant Rd. (& Rosemont), Tucson AZ 85712


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Pegmatites:  Mineralogy, Gemstones, Economic Geology, and maybe not quite the same Giant-Crystal Rocks you always thought they were

by Peter Modreski, U. S. Geological Survey 

Abstract:  Pegmatites are exceptionally coarse-grained igneous rocks, typically occurring as dikes or pods within igneous or metamorphic host rocks.  Some common definitions are that most grains are larger than 1 cm or 1 inch, but crystals can be huge, up to meters in size—feldspar crystals up to 35 feet in length have been described.  Granitic pegmatites are the most common, in which the typical minerals are quartz, microcline, albite, and micas, but pegmatites can occur in syenite and other types of igneous rock.  The classic description of pegmatites is that they form from unusually water-rich magma, which promotes the growth of large to “giant” crystals, and that they represent the last, highly fractionated portion of a granite pluton to crystallize.  Many “incompatible” chemical elements become concentrated in pegmatites, leading in some cases to economic concentrations of lithium (spodumene, lepidolite), beryllium (beryl), cesium (pollucite), tantalum and niobium (tantalite, microlite, columbite), tin, rare earths, uranium, and other metals.  The concentration of light elements (lithium, beryllium, boron, fluorine, phosphorus) in pegmatites, plus the water-enriched environment, sometimes results in the growth of gem minerals, especially tourmaline, beryl, and topaz.

Granitic pegmatites can be classified as simple (unzoned) pegmatites, zoned pegmatites which typically have a quartz core surrounded by feldspar-rich shells, and complex pegmatites in which late-stage aqueous fluids have altered the primary minerals to produce metasomatic zones enriched in lithium or rare metals.  Chemically, pegmatites can be classed as NYF (niobium-yttrium-fluorine) enriched, or LCT (lithium-cesium-tantalum) enriched.  The former are most often found in anorogenic or extensional environments and directly associated with granite plutons; the latter, at deeper levels in orogenic belts and emplaced within metamorphic host rocks.  The Pikes Peak batholith, Colorado, is a classic host of NYF-type pegmatites; examples of the LCT type include the Harding pegmatite, New Mexico and the White Picacho pegmatite district, Arizona. Famous pegmatite districts worldwide include San Diego County, CA; the Black Hills, SD; the White Mountains, NH; Minas Gerais, Brazil; Madagascar; and Pakistan-Afghanistan.

Although it was always tempting to conceive of the large or perfect crystals in pegmatites as forming by very slow cooling, recent experimental studies, especially the work of David London (Univ. of Oklahoma) and his students, have inclined toward the view that pegmatites may crystallize quite rapidly—in years or weeks (or days?) rather than in thousands or millions of years. As well, crystal growth in pegmatites is now being interpreted in terms of growth from a disequilibrium boundary layer enriched in incompatible elements. There is also much debate about which pegmatite magmas originate by partial melting, rather than fractional crystallization—that not all pegmatites have necessarily been derived from any “parent granite”.

Bio:  Dr. Peter J. Modreski has been a geochemist since 1979 with the U.S. Geological Survey, Lakewood, Colorado.  He has a B.A. (chemistry) from Rutgers College and an M.S. and Ph.D. from Penn State (geochemistry).  His research interests include mineralogy, gemstones, luminescence, Colorado geology, ore deposits, pegmatites, meteorites and impacts, alkaline igneous rocks, kimberlites, and volcanology, and he is the USGS geological resource specialist for abrasives, gemstones, quartz, beryllium, cesium, and rubidium. He is presently responsible for public and educational outreach at the USGS.  Pete was a co-author of Minerals of Colorado (1997) and he is a Consulting Editor of Rocks and Minerals magazine and a Department Associate with the Earth Sciences Department, Denver Museum of Nature and Science. 

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