|MadSci Network: Earth Sciences|
Dear Vincent: Good question about quartzite and feldspar! However, sandstone is not sandstone, so to speak. Sandstone may be a mixture of quartz and feldspar, it may consist chiefly of rock fragments, it may even be 100 percent limestone fragments or small shells. Because quartz is hard and stable (at the Earth's surface), it tends to last longer than most other minerals in the weathering and erosion processes. Thus, sands that have been through a lot, such as dune sands or beach sands, tend to be extraordinarily rich in quartz. There are sands that are mostly quartz, and these become pure quartz sandstones and then the pure quartzites with which you are familiar. However, if sandstone that contains large quantities of other minerals is subjected to intense regional metamorphism, it cannot become a pure quartzite. Really dirty sandstones can become schists or, eventually, gneisses. Sandstones that contain modest amounts of minerals other than quartz can become impure quartzites. If you look closely at samples of quartzite, you will see that they are not all 100 percent quartz. Finally, as to the fate of feldspar, this is a mineral vulnerable to weathering. Feldspar becomes clay, and clay in a quartzite would become mica flakes. As an aside, it isn't really true that feldspar particles hold quartz particles together in a sandstone. Mineral cements, that may be quartz, feldspar, mica, clay, calcium carbonate, or other minerals, can cement particles to one another. Also, particles can be fused together in a process called pressure dissolution. The pressure is greatest where particles touch, and so these places tend to dissolve, forming sutured contacts. Anyway, the answer to your question is that if we are calling a rock a quartzite, then we are looking at a biased sample of the set "metamorphic products that were once sandstone." We have already selected a metamorphic product that once was relatively pure quartzose sandstone. A feldspar-rich sandstone doesn't become a quartzite unless the feldspars are removed by weathering or diagenesis before the application of enough heat and pressure to form quartzite. This discussion is simplified, because there are many possibilities out in the real world, but I hope it helps. Please let me know if you have further questions. David C. Kopaska-Merkel Geological Survey of Alabama PO Box 869999 Tuscaloosa AL 35486-6999 (205) 349-2852 FAX (205) 349-2861
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