Meteorites of northeastern New Mexico
David E. Lange and Klaus Keil
Meteorites are naturally occurring, solid objects which reach the earth from space. Man has known about meteorites for millennia, and classical Chinese and ancient Greek and Latin literature have recorded the fall of stones from the sky. The oldest fall from which material is definitely known is the Ensisheim meteorite which fell on the 16th of November in 1492 near Ensisheim, Alsace, France. Until the early 1800's scientists remained skeptical about stones that fell from the sky. Then on April 26, 1803, a fireball was observed over France and a shower of several thousand stones fell near L'Aigle. Biot (1803) described this fall and established that meteorites do indeed fall from the sky.
Brown (1961) has estimated that about 500 meteorites arrive on the earth each year of which about five are recovered (about one fall per year is recovered in the U.S.). Meteorites are named after their place of fall or find (closest village or well known landmark). Meteorites vary in size, with the smallest weighing 0.0003 gm (Sikhote Alin, USSR) and the largest weighing about 60 metric tons (Hoba, Southwest Africa). Meteorites frequently break apart during entry into the earth's atmosphere, producing meteorite showers. The largest known meteorite shower in terms of the number of specimens occurred near Pultusk, Poland, in 1868, from which about 100,000 stones were recovered.
Meteorites are divided into four broad classes: achondrites, chondrites, stony-irons and irons. Chondrites are the most abundant meteorites, making up 86% of the known meteorite falls, and stony-irons and irons the least abundant with 1.3 and 3.7%, respectively (Table 1). The irons and stony-irons, however, make up over 50% of the finds, because they are easier to recognize and are more resistant to terrestrial weathering.
The achondrites (Figs. 1, 2) are generally divided into eight make up about 80% of the meteorite falls. Van Schmus and Wood (1967) have subdivided H, L, and LL group chrondrites into six subgroups based on petrographic parameters, such as nature of chondrule-matrix boundaries, presence or absence of igneous glass and feldspar, and degree of homogeneity of mafic silicates. They implied that the most highly recrystallized chondrites (type 6) have formed by metamorphism of the less recrystallized types (5, 4, 3, etc.).
The stony-irons are divided into four groups on the basis of mineralogy and chemistry, namely pallasites, mesosiderites, siderophyre and lodranite. The last two groups have only one member each. The pallasites are coarse-grained with centimeter-sized olivine crystals in a nickel-iron matrix. The mesosiderites are breccias with silicates similar to those of polymict orthopyroxene-pigeonite-plagioclase achondrites, but contain considerable metal.
Most iron meteorites, chondrites and stony-irons contain two nickel-iron phases, kamacite and taenite. Kamacite usually has 5 to 7% Ni, and taenite, from 20 to 50% Ni. Iron meteorites (Fig. 5) are composed entirely of kamacite crystals. Octahedrites (6-18% Ni) are composed of kamacite plates which parallel the faces of an octahedron with taenite rimming the kamacite. This structure is known as the Widmanstätten structure and is revealed by etching. Nickel-rich ataxites (>12% Ni) have a fine-grained intergrowth of kamacite and taenite. Recently, the iron meteorites have been divided into 15 groups on the basis of their trace element composition (Ni, Ga, Ge and Ir) (Wasson, 1974). This division is supported by textural and mineralogical data (for example, the occurrence of troilite, schreibersite, daubreelite, cohenite, graphite and silicates).
- Lange, David E.; Keil, Klaus, 1976, Meteorites of northeastern New Mexico, in: Vermejo Park, Ewing, Rodney C.; Kues, Barry S., New Mexico Geological Society, Guidebook, 27th Field Conference, pp. 293-299.