Ans: The pyroxenes differ from the amphiboles in their composition in two respects. Although amphiboles are referred to as hydrous silicates, pyroxenes do not contain any essential water in any form in their structure. The second major difference in their chemical properties is the presence of the Asite in amphiboles which contains the large alkali elements, generally sodium and sometimes potassium. There is no presence of an equivalent site in pyroxenes to accommodate potassium. Due to the hydroxyl groups being present in amphiboles, there is a decrease in their thermal stability relative to the more refractory pyroxenes. Amphiboles decompose to form anhydrous minerals when kept at extreme temperatures.