Feldspathoids are a group of silicate minerals that are similar to feldspars but contain less silica. They typically form in volcanic rocks and are characterized by their low silica content and high levels of alkali metals. Common feldspathoids include nepheline and sodalite, which are often found in alkaline igneous rocks. These minerals are important in geology as they can provide insights into the conditions under which certain rocks formed. Feldspathoids are less stable than feldspars and can alter into other minerals over time, making them significant in the study of mineral transformations and rock evolution.