New Mexico Mineral Symposium — Abstracts

The San Juan Peak area of the southern San Mateo Mountains is composed of various volcanic flows related to the Rio Grande rift. Some of these flows contain abundant lithophysae which contain a variety of microminerals. The San Mateo Mountains are west of I-25 in southern Socorro County. The collecting areas are readily reached from exit 115 on I-25, 35 miles south of Socorro. Roads turn from paved to graded to 4-wheel drive recommended, but good collecting is within easy reach. Good specimens can be found by working on boulders on hillsides and in dry washes.

The lithophysae are usually lined with quartz and sanidine, often as excellent crystals. Quartz can be found as its usual self, as tridymite, amethyst, or hyalite. Sanidine occurs as transparent bladed crystals, slightly cloudy, and often exhibiting a brilliant blue sheen. Other minerals found include hematite, pseudobrookite, bixbyite, titanite, fluorite, zircon, and pyroxene.

Another occurrence with similar mineralogy is accessed from Exit 100 on I-25. A roadcut on NM-1 about one mile north of the exit contains numerous lithophysae with similar mineralogy.

The San Mateo Mountains are an uplifted fault block within the Mogollon–Datil volcanic field. The San Juan Peak area is underlain by various rhyolitic flows and tuffs dated at 28 million years ago. Some units are spherolitic and have abundant and conspicuous crystal-lined lithophysae.