New Mexico Mineral Symposium — Abstracts

Among the many descriptions to be found of aragonite, none is more challenging than that given by Frederick H. Pough (1960, pl. 22) in his book, A Field Guide to Rocks and Minerals, where he describes a variety of aragonite from New Mexico as "a pseudomorph after a trilling." Stuart A. Northrop (1959, p. 117) mentioned in his book, Minerals of New Mexico, "early settlers used them as sad-iron rests." The specimens from Puerto de Luna can be reddish brown, cream, or washed-out green; with full six-sided or partial (rotten) forms; displaying roses, light-brown posts, and six-sided posts, with six-sided micro rosettes all over. The specimens range from 1/8 inch to 4 inches in height and from 6 to 8 inches in width with varying weights.

I first went to the sites in 1959 while visiting my parents in Santa Rosa, New Mexico. They were managers of the Sun n' Sand Motel. Mom, a rockbound, took advantage of the opportunities offered to get out in the area. Mom and Dad would have a few specimens at the motel desk for guests to admire or "swap for" as rockhounds do, and most of the specimens were accepted as aragonite based on Pough's (1960) description. Only one negative comment was ever made, and of course remembered, when Mom was accused of going out and making them! Initially, through my parents' influence, I developed many questions like:

• Are these pseudomorphs calcite after aragonite, dolomite, and/or gypsum because they are found in lengthy gypsum beds?
• Why are the specimens all perfect or near perfect at one site (southeast), and why are they rotten in the same formation to the northwest?
• Is this an "evaporite deposit," and if so, is it similar to that of the Great Salt Plains at Jet, Oklahoma?
• Are the specimens old or new in terms of historical geology? Are they still growing?
• What is the chemistry and crystallography (floating atoms, exchange, and changing crystal shape from orthorhombic to hexagonal)?
• If a pseudohexagonal twin seems to pierce another pseudohexagonal twin at an angle in the same specimen, is this a true penetration twin?

Location—Southeast of Santa Rosa, Guadalupe County, New Mexico, near the historic town site of Puerto de Luna on the Pecos River. North and west of the Pecos River bridge, a layered formation is littered with clear plates of selenite; all specimens are rotten. Whole specimens are found weathering out of gypsum red beds approximately 1 mi to the south and east in an always-dry, trenched, soft floodplain deposit (10 ft deep by 30-50 ft wide). The exposure of material extends for at least 1 mi. The collecting of specimens is done on private land, and due to the severe fire danger in recent years on both grasslands and in forests, it is doubtful if permission could be obtained to cross this area.

Geology—Triassic sandstone and siltstone consisting of multicolored sandstone, conglomerate, locally with limestone pebbles of the Santa Rosa Formation, capping reddish-brown and tan sandstone with gypsum and dolomite in the lower part of the Grayburg and Queen Formations. Advancing and receding warm seas still present in Permian through Cretaceous time formed shallow lagoons in southeastern New Mexico. Thin limestone, evaporites, and red beds were deposited. Connections of the lagoons became so severely restricted that when the waters evaporated, a great thickness of gypsum and salt was left.

Seawater contains 0.14% calcium sulfate, 0.01% calcium carbonate, 0.32% magnesium chloride, and others. As the seawater becomes trapped, evaporation sets in. As the volume of water is reduced, dissolved salts become more concentrated. If the volume of water is reduced by evaporation to approximately half, salinity is doubled, with calcium carbonate and iron oxide being precipitated. As the original volume of seawater is reduced by evaporation to approximately 20%, gypsum is deposited. It is estimated that 25 mi of seawater would be required to yield 100 ft of gypsum.

The evaporating basin must have been repeatedly replenished during a very long period of evaporation under fairly constant climatic conditions of aridity and rapid evaporation. The great salt, gypsum, and anhydrite deposits of the world were formed during the Silurian, about 400 m.y. ago and the Permian about 200 m.y. ago. The salt deposits of New Mexico are Permian; hence, it is believed that formation of beds ranged from the Permian to the Triassic. Within the Upper Triassic Santa Rosa Formation are bands of dark, reddish-brown mudstone. Between these are the layers of gypsum. As reported by Hurlbut (1970), "the sequence of deposition of oceanic salts is as follows: first, calcium carbonate and iron oxide; next, gypsum and anhydrite; then, sodium chloride; and finally, salts of potassium and magnesium." The aragonite pseudomorphs are weathering out of the vertical banks at all levels and down on and into the floor of the dry creek bed.

Mineralogy—Sedimentary, deposition from water movement. The loose sediment is cemented into coherent solid rock by any of several processes—one of which is crystallization. Sandstone, shale, and carbonate rock constitute the most abundant rock.

Carbonates are formed when carbon dioxide contained in water combines with oxides of calcium and magnesium. Many sedimentary rocks are nonclastic—formed by intergrowth of crystals. Nonelastic rocks can be formed from material directly precipitated. Again, we have our carbonate friends including calcite crystallizing in the hexagonal crystal system, aragonites of the orthorhombic system, and dolomite also in the hexagonal system. After calcium carbonate has accumulated, it becomes recrystallized or otherwise consolidated into indurated rock (a process of the hardening of a rock material by the application of heat or pressure or by the introduction of a cementing material).

The aragonite pseudomorphs at Puerto de Luna are definitely trillings as defined by Pough (1960). For aragonite to form it must be in a proper environment not only for its development but also for it to maintain its stability. The growth is not always steady. The slightest impurity may cause uneven growth resulting in "weird" crystal shapes with perhaps over development of some crystal faces and partial formation of others (rotten?). The formed aragonite is altered when some quirk in the environment changes—when it loses its chemical composition to a combination of factors (temperature and pressure, loss of water, loss or gain of chemical elements) or when its chemical composition changes but its crystal structure remains the same. Then, it is known as pseudomorph. The pseudomorph at Santa Rosa is one whose chemical composition has changed twice. Virgil Lueth analyzed five specimens by X-ray diffraction to determine whether they are calcite, dolomite, or gypsum, with all specimens retaining the pseudohexagonal form of aragonite. In the common table of pseudomorphs relating to this paper three examples are given:

Replacing Mineral after Replaced Mineral
Calcite Aragonite, celestite
Gypsum Anhydrite, aragonite
Dolomite Aragonite

Off the Yeso-Fort Sumner Highway (U.S.-60) to the north, Pecos "diamonds" may be found that are definitely quartz dolomite paramorphs. Did the dolomite extend into the Santa Rosa Formation? Yes, according to Kelley (1972).

Crystallography—In a review of aragonite, Sinkankas (1964) states that an orthorhombic crystal is uncommon. Sinkankas further describes the crystals as short to long, prismatic along the c-axis, in six-sided prisms, nearly hexagonal in cross sections, and terminated by a pair of faces making a wedge. Practically all crystals are twinned along the plane m {101}, forming sixling prisms, nearly hexagonal in cross sections.

Pough (1960) describes the crystal shape as trillings (three intergrown individuals), common, and looking like short hexagonal prisms or hexagonal plates. Re-entrant angle visible in fresh, sharp crystals in the center of each apparent prism of these pseudohexagons and striations on the apparent base disclose the three individuals.

Sinkankas (1964) also discusses "cyclic and penetration twins," which produce misleading shapes resulting in a determination of a crystal shape in one form when actually it is in another. In an example, he shows an aragonite twin with the impression that it is a hexagonal crystal being viewed. However, on examination small re-entrants were seen. Because a penetration twin is one in which two or more complete crystals seem to cross through each other with each crystal having a common center, as in staurolite, it is believed that a parallel layer of growth on top of an existing circular form or at right angles to perpendicular (Santa Rosa pseudomorph posts) would make these a penetration twin. However, perhaps the temperature-pressure equation during the forming led to crystals cementing themselves to each other with the outer layer being gypsum covering the original joining line.
Are the crystals still growing? I believe they are just "weathering out."

Further reading
Ford, W. E., and Dana, E. S., 1898 and 1922, Compound or twin crystals; in A textbook of mineralogy: John Wiley & Sons, Inc.
Levin, H. L., 1978, The Earth through time: W.B. Saunders Co., pp. 335, 374, 377.
Sanborn, W. B., 1976, Oddities of the mineral world: Litton Educational Publishing. "Pseudomorphs".
 

References:

1. Hurlbut, C. S., Jr., 1970, Minerals and man: Random House, pp. 118, 121, 122.
2. Kelley, V. C., 1972, Geology of the Fort Sumner sheet, New Mexico: New Mexico Bureau of Mines and Mineral Resources, Bulletin 98, 55 pp., 2 sheets, scale approx. 1:190,000.
3. Northrop, S. A., 1959, Minerals of New Mexico: University of New Mexico Press, 665 pp.
4. Pough, F. H., 1960, A field guide to rocks and minerals: Houghton Mifflin Company, 317 pp.
5. Sinkankas, J., 1964, Mineralogy for amateurs: Van Nostrand Reinhold Company, p. 371.