what are two ways to identify quartz from feldspars. ive been stuck on this question for like an hour.
Well, in a way, it's because rocks are simply defined that way minerologically. As hot magma cools, it undergoes specific reactions. Bowen's reaction series describes the temperature dependent formation of minerals as magma cools. Rocks formed from magma are igneous rocks, and minerals crystallize as magma cools. The temperature of the magma and the rate of cooling determine which minerals are stable (i.e., which minerals can form) and the size of the mineral crystals formed (i.e., texture). The slower a magma cools, the larger crystals can grow. Just follow me on this. I don't mean it to be a lecture. There's felsic rock, with the highest content of silicon, a predominance of quartz, alkali feldspar and/or feldspathoids (the felsic minerals). These are going to be your granites and rhyolites. The mafic rocks have a lesser content of silicon relative to felsic rocks, and more mafic minerals of pyroxenes, olivines and calcic plagioclase. Basalts and gabbros. Ultramafic rock is extremely low in silicon, comprised of more than 90% of mafic minerals. You'll get a dunite. So it's more like felsic rocks occur over top of quartz/silica, not the other way around, and so forth. Look into Bowen's Reaction Series. There's a short Wikipedia entry but it isn't worth much; but it does have a diagram. The first link below explains it well, but doesn't have a diagram - use some tabs and just swap between them. It'll make sense.
