Quartz and white feldspar are two of the most common minerals found in rocks. They can be difficult to tell apart, but there are a few key differences to look for.
So, if the mineral has cleavage and sparkles, it may be feldspar. If no sparkles, no matter how you rotate it in the light, it may be quartz.
Feldspar grows (when allowed to) into rectangular crystals, and then usually breaks the same way. This breakage pattern is called cleavage, and feldspar has two directions of cleavage that meet at close to a 90 degree angle (just like a stairway). If you see little sparkles when you look at the rock, the flashes may be the light reflecting off the mirror-like cleavage surfaces.
|What Is the Difference Between Quartz and White Feldspar in Rocks? Feldspar Red Circles, Quartz yellow circles.|
Quartz has no cleavage (it breaks with a conchoidal fracture - just like obsidian, or the curved fractures that you can often see in the windshield of your car after a rock hits it), so usually no mirror-like flashes of light. The bad news is that sometimes the curved conchoidal surfaces can flash as well, but this usually happens with the clear varieties of quartz (which you already know isn't feldspar anyway).
How to tell quartz from white feldspar in rocks
If you are looking at a rock and you are unsure whether you are seeing quartz or white feldspar, you can try the following:
Look for cleavage. If the mineral has cleavage, it is most likely feldspar. If it does not have cleavage, it is most likely quartz.
Try to scratch the mineral with a piece of glass. If the mineral can be scratched, it is most likely feldspar. If it cannot be scratched, it is most likely quartz.
Look at the mineral's color and appearance. If the mineral is glassy and clear or white, it is most likely quartz. If the mineral is dull and white, pink, gray, or yellow, it is most likely feldspar.
How to Classify Igneous Rocks Into (Ultramafic, Mafic, Intermediate and Felsic)?