New Mexico Mineral Symposium — Abstracts

Meteorites are the oldest known examples of solar system material available for study on Earth, with formation ages of around 4.6 billion years. They represent rocks from a variety of planetary bodies within our solar system including asteroids, comets, the Moon, and Mars. Thus, with the exception of the Moon, they provide us with specimens of bodies that have not been sampled by space missions, manned or robotic. The vast majority of meteorites come from within the asteroid belt, a region of small rocky planetessimals located between the orbits of Mars and Jupiter. Since their formation they have largely escaped the wide-scale geologic processes that have affected Earth, the Moon, and Mars. As such they are extremely important for providing insights into the early geological history of the solar system and subsequent evolution of small planetary bodies.

Three broad groups of meteorites can be recognized: stones, characterized by the presence of common terrestrial silicate minerals, such as olivine and pyroxene; stony irons, consisting of a mixture of silicates and iron-nickel metal; and irons, consisting almost entirely of iron-nickel metal. The stones can be further divided into two groups called the chondrites and achondrites. Chondrites are characterized by the presence of abundant, millimeter-sized silicate spherules called chondrules, while the achondrites are igneous rocks with affinities to terrestrial mafic to ultramafic lavas. The mineralogy of these meteorites is of special importance in determining their origins and conditions of formation.

The arid climate of New Mexico makes the state particularly suitable for the preservation and recovery of meteorites after they have fallen. Approximately 160 meteorites have been recovered in New Mexico, many from the eastern plains region, particularly in Roosevelt County, in the southeastern part of the state. This area alone has yielded close to 200 meteorite samples corresponding to some 80 separate meteorite falls. Of the meteorites discovered to date in New Mexico, most are stones (~85%), the majority being of a type called the ordinary chondrites, the most common group worldwide. Only one carbonaceous chondrite, similar to the famous Allende meteorite that fell in northern Mexico in 1969, has been recovered in the state. The remainder are irons (~11%) and stony irons (2.5%). Of the stony irons, two pallasitic meteorites, Acomita and Dora, are especially fine examples with centimeter-sized olivine crystals set in an iron-nickel metal matrix. More than 140 minerals occur in meteorites. Many occur on Earth, but a significant number are found uniquely in meteorites. The main constituents of stony meteorites are olivine and pyroxene, but inclusions containing Ca- and Al-bearing minerals such as hibonite, gehlenite, corundum, spinel, fassaite, mellite, and perovskite also occur, especially in the carbonaceous chondrites. Iron-nickel metal is also present attesting to the reducing conditions under which these meteorites formed. Iron meteorites and pallasites contain a number of unusual carbides, phosphates, phosphides, and sulfides, such as cohenite ((Fe,Ni)₃C), haxonite (Fe₂3C₆), and schreibersite ((Fe,Ni)₃P). The most unusual meteoritic minerals occur in a group of chondrites and achondrites called the enstatite meteorites, which are not represented among New Mexico meteorites. These meteorites are dominated by the pyroxene, enstatite (MgSiO3), but contain numerous exotic sulfides, nitrides, chlorides, silicides, and phosphides. These include oldhamite (CaS), caswellsilverite (NaCrS₂), heideite (Fe,Cr)_1+x(Ti,Fe)₂S₄, djerfisherite (K₃CuFe₁₂S₁₄), osbornite (TiN), lawrencite (Fe,Ni)Cl₂, sinoite (Si₂N₂O), and perryite (Ni,Fe)₅(Si,P)₂. The minerals and mineral assemblages found in meteorites attest to the wide range of physical conditions (gas pressure, temperature, etc.) that were present during the early stages of the formation of our solar system.