Minerals, Meteorites, and Metallography
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HISTORICAL NOTE
Minerals, Meteorites, and Metallography On May 6, 1885, Henry Clifton Sorby gave an invited lecture before the Iron and Steel Institute in London about the metallographic studies of ferrous metals he had completed 20 years earlier. Though he presented no new results, the speech electrified the assembled metallurgists. As Sorby’s biographer Norman Higham wrote in A Very Scientific Gentleman, this meeting led to “the modern development of metallography, a development of such outstanding importance that its neglect by metallurgists from 1864 to 1885 can only be wondered at.” Sorby was born May 10, 1826 in Sheffield, England. In grammar school, he won an arithmetic contest whose prize was a book entitled Readings in Science, which consequently determined his career path. Though he would not join his father’s successful cutlery business, income from it would provide him with four years of private tutoring in science and mathematics. Eventually, it would finance his lifelong ventures as an independent amateur scientist, and allow him to take up or abandon scientific topics as he pleased. In the early 1800s, geology was a hot topic. It had only recently begun to be approached systematically instead of speculatively. Debates raged about how rocks were formed. Sorby noted from his readings on the subject that the analytical techniques being applied to rocks could not possibly answer this question. At the time, scientists were grinding rocks into powder and dissolving them in acid to determine their chemical composition, but these techniques revealed nothing about their mineral structure and how they were formed. Around this time, others were examining thin slices of fossilized wood, as well as teeth and bones, under a microscope. Soon Sorby was applying the technique to rocks. He would take a thin chip of rock and polish it on one side before gluing it to a glass slide. Then he would grind and polish the opposite face of the sample until it was as thin as 1/1000th of an inch. Viewing samples under a microscope using transmitted light, he could resolve particles measuring 1/2000th of an inch in diameter. Furthermore, he used polarized light to identify calcite, quartz, and chalcedony in some samples. Despite this modest success, attempts
were made to discourage him from such investigations. Most notable was the admonition of geologist Daniel Sharpe, who was trying to discover why slate cleaved into planes, that “One must not look at mountains through a microscope.” Sorby silenced Sharpe in 1851 when his observations of slate led to a definitive explanation for its planar cleavage. His foray into metallurgy came through his correspondence with R.P. Greg, who had published an atlas of meteorites in 1861. Soon, he was examining meteorites under his microscope. Because their high iron content prohibited the transmission of light, he had to examine his samples using reflected light. “It was a natural thing that I should be led from the study of the microscopic structure of rocks to that of meteorites,” Sorby wro
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