New Mexico Geological Society Annual Spring Meeting — Abstracts

The Cerro Cristo Rey uplift straddles the international border between the State of Chihuahua, Republic of Mexico, and Dona Ana County, New Mexico, U.S.A. The core of the uplift is a mid-Eocene, andesitic laccolith. The hornblende andesite, locally known as Muleros Andesite, primarily outcrops on the highest part of the uplift and deformed marine sediments of Cretaceous age outcrop on its flanks. Shaley and quartzose siltstone members of the marine formations are quarried by the EI Paso Brick Company and Ladrillera de Juarez, S.A., for the production of brick and tile. Various combinations of the shales and quartzose siltstone members help produce different color brick and tile on firing.

The constituent minerals of the shales and siltstones are silica, in the form of quartz, illite, montmorillonite, illitemontmorillonite kaolinite, calcite, minor amounts of feldspar and gypsum, minor to trace amounts of oxides of iron, titanium, sulfur, and carbon compounds. The modal amount of any of the above minerals is a manifest of the parent rock, environment of formation, and post-depositional processes.

On firing the shale/siltstone mixes to temperatures in the range of 900°C(1,652°F) -1,100°C (2,012°F), most of the minerals undergo polymorphic transformations, breakdowns, and react to form new minerals. Clay minerals usually breakdown and reorganize to mullite. The temperature at which mullite forms is determined by the type of clay minerals present whereas other constituents, which usually act as fluxes, result in glass.

The EI Paso Brick Company has been in production for approximately 100 years and there are still considerable reserves of the Mesilla Valley and Anapra Formations to sustain brick and tile production in the future.