The name feldspar derives from the German Feldspat, a compound of the words Feld, "field", and Spat, "a rock that does not contain ore".The change from Spat to -spar was influenced by the English word spar, a synonym for "mineral".In a related article on geologic ages (Ages), we presented a chart with the various geologic eras and their ages.
The alternate spelling, felspar, has largely fallen out of use. Compositions of major elements in common feldspars can be expressed in terms of three endmembers: Potassium-Feldspar (K-spar) endmember KAl Si or more properly plagioclase feldspar.
As we pointed out in these two articles, radiometric dates are based on known rates of radioactivity, a phenomenon that is rooted in fundamental laws of physics and follows simple mathematical formulas.
Dating schemes based on rates of radioactivity have been refined and scrutinized for several decades.
As a result, there is a changing ratio of carbon-14 to the more atomically stable carbon-12 involves actually counting individual carbon-14 atoms.
This allows the dating of much older and smaller samples but at a far higher cost.