A different kind of magma
The island of Pantelleria, located in the Strait of Sicily, is built from a rare type of volcanic rock called peralkaline rhyolite. Specifically, it is the type locality for "pantellerite," a variety of rhyolite so distinctive it was named after the island. This magma is peralkaline, meaning it has a higher concentration of sodium and potassium relative to aluminum. This specific chemistry lowers the magma's viscosity, making it flow more easily than typical high-silica rhyolites.
This unusual magma is the product of extreme fractional crystallization. It begins as basaltic magma deep beneath the island. As this parent magma cools in a chamber at a low pressure of around 0.1 gigapascals, minerals like plagioclase and olivine crystallize and settle out. This process, removing specific elements from the melt, continues until about 95% of the original magma has solidified. The small amount of remaining liquid becomes exceptionally enriched in silica (68-73%), sodium, iron, and volatile elements like chlorine, forming the unique pantellerite composition. This process results in the formation of uncommon minerals, such as the sodium-rich amphibole cossyrite, which is also named after Pantelleria's ancient name, Cossyra.