In chemistry, a catalyst is any substance that speeds up a reaction without being consumed. Zeolites are crystalline solids consisting of silicon, aluminum, and oxygen that form a framework with cavities and channels inside that can hold cations, water, and/or tiny molecules. Molecular sieves are a term that is frequently used to describe them. Various of them are found in nature as minerals and are widely mined in many regions of the world, where they are used in industry and medicine. Zeolites are microporous minerals that are used in a variety of catalytic applications in industry. The acidity and structure of zeolites are inextricably linked. Both catalytically active Bronsted and Lewis acid sites can be found in zeolites.