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New Mexico Mineral Symposium — Abstracts

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Crystallized Mineral Depositsof the San Pedro River Basin, Arizona

Barbara L. Muntyan

The gypsum (selenite) deposits near the hamlet of St. David in Cochise County, Arizona, have been known to mineral collectors for more than fifty years. The collecting areas are located on the west side of the San Pedro river valley. This valley is a graben: a fault-block downed dropped valley, possibly associated with Basin and Range faulting. The river itself flows north out of Mexico and is one of the last free-flowing rivers in Arizona. Although it is a quiet stream for most of the year, the San Pedro can turn into a roaring torrent during the summer monsoons. The landscape in the collecting areas is high chaparral: sparse grasses, a few prickly pears, creosote bushes and occasional mesquite trees.

Pleistocene Ice Age erosion is found on many of the benches; some of them contain vertebrate fossils. The gypsum deposits cover a wide area. Although most collectors only know the area near St. David, crystals can be found in an area approximately 200 square miles.

Gypsum is hydrated Calcium Sulfate: CaSO4.2H20. A monoclinic mineral, it is soft (only 2 on the Mohs Scale), and has a highly developed cleavage parallel to the C axis of the crystals. Gypsum crystals result from evaporation of saline waters. It is also soluble in water, which means that material exposed on the surface is etched, dull, and broken along cleavage planes.t field guides and most collectors are familiar only with small desert roses perhaps a half inch to 1 inch in diameter, and indeed there are literally thousands—probably millions—of them found on the surface or near-surface throughout the collecting areas. Most of the area is Arizona State Trust Land. Access to the familiar collecting sites is from the Apache Powder Road; a former dynamite manufacturing operation to supply dynamite to the mines in Bisbee, the plant’s ten-story tower is a prominent landmark visible from much of the collecting spots.

The best crystals are found in situ in a number of washes that run down to the San Pedro River to the east. On the correct horizons in some of these washes, roses can reach 8 inches in diameter. In addition, depending on the wash, fine monoclinic crystals up to 6 inches in length, fishtail twins, hoppered crystals, and crystals with movable bubbles have been found. Crystal color can be tan, red-brown, white, cream or clear, transparent selenite crystals.

Because of the potential for flash flooding during the summer monsoons, many of the washes are scoured or change channels. In addition, the ground is rock-hard bentonite and other clays, which are softened by the monsoons. Thus, optimum-collecting time is after the rains, but only after the trails dry out a bit to allow motor travel. Four-wheel drive and high clearance is definitely required.

Map by W. W. Besse
pp. 10-11

35th Annual New Mexico Mineral Symposium
November 8-9, 2014, Socorro, NM