Question 19 Many communities use fluorine to treat their water supply and encourage consumers to use fluoridated toothpaste to reduce dental cavities. However, why does this fluoridation help? What mineral change occurs that reduces the possibility of cavities forming? The micas (biotite and muscovite) are similar, but they differ in one use. Question 20 Why would muscovite be a better electric insulator than biotite is? - biotite is too dense - biotite contains iron - biotite is too rigid - biotite is too brittle Feldspars comprise over half of the Earth's crust, so potassium feldspar and plagioclase feldspar are easily the most common minerals that you will encounter in the field. Question 21 Although we use feldspars as building stones, aggregates, and in ceramic materials, how do these minerals also contribute to our modern beverage industry? - they are used to filter and purify the water used in beverages - they produce the carbonation for carbonated beverages - they weather to form the aluminum used to make beverage cans - they are used to color and alter the taste of many beverages .