Basalt is a common and widespread dark colored igneous rock. It forms as a result of the cooling of lava flows or small intrusions; it is typically black and is composed primarily of plagioclase feldspar and pyroxene, and sometimes also olivine. The grain size is fairly small due to rapid cooling. Gas bubbles trapped in the cooling lava may be preserved as voids, known as vesicles, within the hardened rock. Sometimes fluids moving through the cooled rock deposit so-called secondary minerals, such as silica, calcite, chlorite and zeolites, within the vesicles. These mineral-filled cavities are known as amygdales (amygdules), and the rock is termed amygdaloidal.