Most Popular Mineral on Earth’s Crust
Puzzle 17 of Vienna Hard Pack
Question is: Most Popular Mineral on Earth’s Crust
and the answer is: FELDSPAR
Word Meaning:
FELDSPAR — (mineralogy) Any of a large group of rock-forming aluminosilicate minerals containing potassium, sodium, or calcium that, together, make up about 60% of the Earth's outer crust.
Definitions from Wiktionary.