Calcium is sometimes a constituent of naturally occurring amphiboles. Amphiboles and pyroxenes are common accessory minerals. Similar trends are observed for amphiboles, clinopyroxenes, olivines, and others. Biotite, amphiboles and pyroxenes are common dark accessory minerals. It commonly occurs in association with orthoclase, sanidine, labradorite, olivine, leucite, amphiboles and other pyroxenes. The amphiboles in the rocks formed at depths of . Closely allied to this group are the hornblendites, consisting essentially of hornblende and other amphiboles. Common mineral associates in igneous rocks include olivine, pyroxenes, amphiboles and magnetite. Cummingtonite shares few compositional similarities with alkali amphiboles such as arfvedsonite, glaucophane-riebeckite. Biotite, sodium-rich amphiboles and pyroxenes along with iron-rich olivine are common minor minerals. It is named after amphiboles that form under such circumstances. Some important rock-forming silicates include the feldspars, quartz, olivines, pyroxenes, amphiboles, garnets, and micas. Feldspar, amphiboles, pyroxenes, and volcanic glass are the principle donors of clay minerals. Amphiboles are the primary constituent of amphibolites. With high alkalis, soda-bearing pyroxenes and amphiboles may be present.