Orthoclase feldspar (also called potassium feldspar) crystals in a granite sample were analyzed and found to contain 1500 atoms of parent element 40K and 4500 atoms of daughter 40 Ar. The half-life of this isotope system is 1.3 billion years. How many atoms of the parent element were present when the orthoclase crystals first formed? QUESTION 21 Orthoclase feldspar (also called potassium feldspar) crystals in a granite sample were analyzed and found to contain 1500 atoms of parent element 40K and 4500 atoms of daughter 40 Ar. What percentage of the parent atoms % (just type in the number) remains? QUESTION 22 Orthoclase feldspar (also called potassium feldspar) crystals in a granite sample were analyzed and found to contain 1500 atoms of parent element 40K and 4500 atoms of daughter 40 Ar. The half-life of this isotope system is 1.3 billion years. How many half-lives have elapsed?