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Bill Metropolis, assistant curator at the Harvard University Geological and Mineralogical Museum, presented a lecture on “Mineralization of Pegmatites in San Diego County, California” and showed examples from the museum’s collection noon Friday in Van Wickle Hall room 108.
Free and open to the public, the presentation was sponsored by the geology department. Lunch was available at no charge to students and for $3 to faculty and staff.
Pegmatite is an igneous rock with extremely course grain size. It has the same base constituents as granite (quartz, feldspar, mica), except the crystals are larger in size. In basic granite, the rock forming minerals usually crystallize in sizes between 0.4-1 inch.
In pegmatites, the minerals can crystallize into larger sizes. It is not uncommon to find crystals over a meter in length in larger pegmatites, which may extend from five to 100 feet thick and 100 to 1000 feet in length. Crystals can grow to tremendous sizes, such as quartz crystals 17 feet long and eight feet in diameter, orthoclase crystals 33 feet by 33 feet, beryl 19 or more feet in length, tourmaline crystals 10 feet long, and mica sheets with up to 68 square feet of surface area.
The Mineralogical and Geological Museum dates back to 1891. Its teaching and display collection of minerals was transformed into a research collection with the bequest of A. F. Holden, a 1888 graduate of Harvard. The mineral collection ranks among the world’s finest due to its broad representation, wealth of rare species, large number of specimens described in the scientific literature, and the quality of its display specimens. The systematic mineral collection and displays of rough and cut gemstones are the principal exhibits in the mineral gallery.
The collections of rocks and ores, worldwide in scope, were acquired primarily through the fieldwork of faculty and students. The Smith meteorite collection, acquired in 1883, gave the museum international standing in the study o f meteorites. Taken together, these various collections constitute an impressive sampling of the earth’s crust.