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Introduction
This article, the first in a series of four on granite working, deals with granite as a material, an industry, and a product and begins the description of the granite quarrying process. The second article will complete the account of granite quarrying. The final two articles concern the process of finishing granite and will conclude with a discussion of power sources, toolmaking, patents, granite workers, labor unions, and safety and health issues. The four articles will appear in consecutive issues of The Chronicle.
Granite Composition
Granite is a very common stone found world-wide. Granite has been commercially quarried in the United States, Canada, Scotland, Finland, Italy, Ukraine, India, China, and Africa. Granite is found throughout the United States and has been commercially exploited in every New England state (Figure 1). Granite is composed primarily of quartz and feldspar with smaller amounts of mica. Quartz contributes to granite's strength, hardness, and luster, and acts as a cement binding all the elements together. Feldspar, granite's principal ingredient, occurs in a number of forms, mostly sodium/aluminum/silicon-rich plagioclase and potassium-rich microcline. In addition to contributing to strength and hardness, feldspar primarily determines granite's color, resistance to discoloration and decay, and ability to receive a polish. Mica (mostly white muscovite and black biotite) is present in much smaller amounts. The relative amounts of white and black mica are an important factor in both the color and commercial value of the granite. If white mica predominates, the granite will be light-colored, and if the black predominates, the granite will be dark, often approaching black. If the white and black occur in roughly equal amounts, the granite will be speckled. Since mica does not polish well nor does it retain its luster, excessive amounts of mica decrease the commercial value of granite. As an example, granite of the Barre, Vermont, area contains 31.7 percent plagioclase feldspar, 26.4 percent microcline feldspar, 23.3 percent quartz, 6.4 percent muscovite mica, 4.5 percent biotite mica, 2.7 percent orthoclase, 2.1 percent chlorite, and 1.6 percent calcite.
Granite is an igneous rock, formed by high heat and pressure from molten rock called magma. Deep magma over time forced its way up through fissures and cracks toward the surface where it cooled into columns of granite called plutons....