granite - teaching student specimens of typical granite UNIT OF 5 SPECIMENS
Granite is an intrusive igneous rock composed primarily of feldspar, quartz and mica. At least 35%of the feldspar is white to pink orthoclase, and quartz is at least 20% of the rock by volume. The mica in this granite is biotite. During the Cretaceous it intruded the Jurassic granodiorite of the Sierra Nevada batholith and forms a light colored mass on its eastern slope at Sand Canyon in Inyo County, California. Its extrusive equivalents are rhyolite, pumice and tuff.
Granites range in color from white like this one to pink, with the orthoclase feldspar giving it the characteristic shade. The granodiorite of the batholith is much darker, with dacite its extrusive equivalent. True granites are not common. Elementary and secondary students would be correct in calling anything that looks like granite "granitic," and should recognize that an accurate identification is done by experienced geologists and by specialists in petrography. Granitic rocks are a large family and "granitic" is more than acceptable. Students in a geology course who can distinguish between quartz and potassium/sodium feldspars and who can determine the percentages of each in a hand specimen, should be able to say this is granite.
Photos show typical boulder-pile "tor" weathering and a large granite boulder exfoliating. Fresh granite was exposed along an access road during construction of the Los Angeles Aqueduct in the early 1900s. The large boulders are deeply weathered and are much lighter in appearance than the surrounding granodiorite. The third photo shows a granodiorite boulder with a prominent dike running through it. Notice how much darker this rock is than the granite. It's the intrusive equivalent of dacite and close in composition to andesite, intermediate between granite and basalt.
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