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March-April 2013

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Connoisseur's Choice: Diopside, Merelani, Arusha Region, Tanzania

The origin of basalt is one of the more intriguing problems that have been a part of geology since it began as a science. There was a time when basalt was considered a sedimentary rock, a concept that was championed by Abraham Gottlieb Werner (1749–1817), the influential professor of mining and mineralogy at the Bergakademie in Freiberg, Germany. There were opponents to this idea; chief among them was James Hutton (1726–1797) who argued that basalt was of igneous origin. The sedimentary versus igneous debate raged until 1816 when one of the key discoveries that basalt is indeed of igneous origin was made by Pierre-Louis-Antoine Cordier (1777–1861). Cordier (for whom the mineral cordierite is named) examined minerals in crushed samples of rocks of definite volcanic origin and compared these to minerals in crushed samples of basalts. This examination entailed looking at the physical characteristics and the fusibilities of various minerals that make up these rocks. Some of the minerals he examined included olivine, mica, ilmenite, feldspar, amphibole, and pyroxene (note that at this time distinguishing between the different species within mineral groups was but a work in progress). His conclusion was that the recent igneous rocks were almost indistinguishable from basalt and that they were all of igneous origin. In his study Cordier did not propose to change the name pyroxene, which is derived from the Greek pyro (fire) and xenos (stranger). The name had been applied by Haüy to greenish crystals found in lavas, even though he considered these crystals to have been accidently placed in the lava (Young 2003). Pyroxenes are one of the most important groups of rock-forming minerals, and they are conveniently divided into two subgroups based on their crystal systems. Pyroxenes with monoclinic crystals are clinopyroxenes, and those with orthorhombic crystals are orthopyroxenes. In general, pyroxenes are, as Nesse (2012, p. 211) says, “drab and commonplace.” But, as with any generalization, there are exceptions, and, for pyroxenes, diopside is an exception (spodumene is another).

Paul W. Pohwat is the collection manager (minerals) in the Division of Mineralogy at the National Museum of Natural History (Smithsonian Institution).

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