Have been receiving some emails this past weekend regarding a site of sorts discovered on the outskirts of Pinerolo (Italy) where my friends Dario Seglie of CeSMAP, Enrico Comba from the University of Turino and Daniel Ormezzano from the Natural History Museum (Turino) all live. There were bits left from a Roman building and then lower, greenstone axe remnants and some chert (flint) tools that date much earlier than the Roman ruins. We are interested in a greenstone artifact exhibit one day because the material is not only ancient both geologically or in human use, but seems to have attracted attention all over, in northern North America, Mexico, Central America and South American included. Like artifacts that are red in color, there is a likely a mythical or spiritual association with the color. So what is greenstone? Here is a short treatment on the subject I wrote: Greenstone: A geologists field term for any dark-green rock that owes its color to chlorite, actinolite, or epidote. Greenstones are generally altered or metamorphosed basic igneous rocks. Through plate tectonics, ocean bottom igneous rocks, known as ophiolite suite, can be obducted or shoved up onto the continent. Much if not most greenstone rock began as ophiolite suite, remnants from the ocean depths. Ophiolite suite is made up of peridotite, basalt or gabbro or their metamorphosed counterparts. Once on land, continuous exposure to weathering, especially rain, chemically alters basalts, olivenes or peridotite; a hydration process which eventually produces serpentine. Serpentine can be hard or soft, like soapstone, dependent on the original rock chemistry and amount of weathering. Peridotite, a dense mantle rock, is made up of olivene or dunite (olive-green in color) together with other minerals called pyroxenes. Pyroxenes are very high temperature, magnesium-rich minerals, green to black in color, depending on the amount of iron in each. Pyroxenes, with the addition of water, become amphiboles; namely hornblende, tremolite, actinolite and epidote. Epidote is pistachio-green in color; tremolite and actinolite range, in color, from light green to black (dependent, again, on the amount of iron present). The bronze color seen in many weathered igneous rocks is the result of the magnesium in pyroxenes breaking down in the atmosphere. Basalt, an igneous rock, begins as an extrusive (surface) eruption of magma (molten rock) from the earths mantle. Basalt is partially derived from melted peridotite. It is a dense rock which forms ocean crust or, on-land, is erupted from shield volcanoes. Once erupted, contact with water from the atmosphere or ocean crystallizes the magma/rock into basalt. Basalt contains augite (a pyroxene) and hi-temperature plagioclase feldspars, namely bytownite and labradorite: it is a fine-grained rock. On the ocean bottom, the green color of basalt comes from contact with chlorite in the sea water. Basalts erupted on land (extrusive eruption) can become greenstone through atmospheric weathering or metamorphism, an alteration caused by heat, pressure and fluids, generally at depth. Gabbro is the intrusive equivalent of basalt, erupting at depth. Chemically, gabbro and basalt are related. The major difference is the individual crystal size. Because gabbro erupts and crystallizes at depth, cooling slowly, gabbro crystals are much larger than basalt crystals. And then, here are some pictures of greenstone axes and artifacts. The first four are the fragments from Pinerolo. The axes shown are from here in Wisconsin. They are early, somewhere in the 5000+ years old range. The photo on red felt is of a small greenstone axe (at top) and gorget, both probably Hopewell in the 2500 years-old range. Next is a nice little greenstone uniface scraper, perhaps late Paleo. The last photo shows an Olmec ceremonial hand axe made from serpentine and a Mayan greenstone bead. Gotta love it!
Posted on: Mon, 05 May 2014 20:57:12 +0000
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