The compositions of the various feldspars all blend together smoothly. If the feldspars can be considered a single, variable mineral, then feldspar is the most common mineral on Earth. All feldspars have a hardness of 6 on the Mohs scale, so any glassy mineral that's slightly softer than quartz is very likely to be a feldspar. A thorough knowledge of the feldspars is what separates geologists from the rest of us.
Learn more about the feldspar minerals.
See the other feldspar minerals in the Feldspars Gallery.
Other Primary Minerals
Other Metamorphic Minerals
Other Hydrothermal Vein Minerals