Feldspars are a group of closely related minerals that together make up the majority of the Earth's crust. All of them have a hardness of 6 on the Mohs scale, so any glassy mineral that's softer than quartz and cannot be scratched with a knife is very likely to be a feldspar.
Feldspars lie along one of two solid-solution series, the plagioclase feldspars and the alkali or potassium feldspars (which include microcline). All of them are based on the silica group, consisting of silicon atoms surrounded by four oxygens. In the feldspars the silica groups form rigid three-dimensional interlocking frameworks.
This gallery starts with plagioclase, then shows alkali feldspar.
