Re: What happened to the Feldspar in Quartzite during the metomorphic process?

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