The Acatlán Ignimbrite is a zoned rhyolite-andesite ignimbrite that was erupted during the Quaternary, covering an area of about 300 square kilometers just southwest of Guadalajara, Mexico. It is part of the Acatlán Volcanic Field, which consists of numerous lava flows of basalt and andesite, cinder cones of andesite, and many domes of rhyolite and dacite. The andesitic portion of the ignimbrite is made of black, glassy pumice that is denser and has larger vesicles than the pink to gray rhyolitic pumice. The andesite pumice is aphyric. This study determined the phase relations of the andesitic pumice composition at one atmosphere pressure in order to see what this composition would have looked like if it had been erupted as a lava flow. Experiments were conducted in a vertical-tube gas-mixing furnace. Oxygen fugacity was controlled to the quartz-fayalite-magnetite buffer using a mixture of CO2 and H2 gases.