METEORITE OR METEORWRONG?

quartz, calcite, magnetite, hematite, & micas


Although for convenience we sometimes state here that the minerals quartz, calcite, magnetite, hematite, and micas do not occur in meteorites, these statements are not entirely true. Each of these minerals are among the many minerals listed by Rubin (1997a,b) that have been observed in some meteorites. However, all these minerals are minor to rare in any type of meteorite and don't occur at all in the most common types of meteorites.

There is no type of meteorite in which quartz would be evident without the aid of a petrographic microscope and a petrographic thin section. It takes skilled petrographers considerable effort to find and identify silica minerals in meteorites. With regard to quartz, Rubin (1997a) mentions it only in the discussion of enstatite chondrites, eucrites, and basaltic shergottites, all rare types of meteorites. Enstatite chondrites and eucrites contain minor amounts (a few percent, at most) of free silica (tridymite, cristobalite, and quartz). Less than 1% of all known meteorites are enstatite chondrites and <1% are eucrites. In the basaltic shergottites (~0.05% of meteorites, but quartz has only been reported from a few), it is an accessory mineral (<1% of volume). (See e.g., Shergotty in the Mars Meteorite Compendium.) Some other achondrites contain trace amounts of the silica polymorphs, tridymite and cristobalite. If you can see quartz with the naked eye, then the rock is not a meteorite. The standard field test for quartz is the scratch test

Magnetite and hematite are oxides of iron. Magnetite occurs as a trace to minor mineral in several kinds of meteorites. Rubin (1997a) states that it is the "principal oxide phase in the CK chondrites," a rare type of meteorite (0.3%). Geiger and Bischoff (1995) found that the modal abundance of magnetite ranged from 1% to 8% in the 19 CK chondrites that they studied. Thus, a large CK chondrite with 8% magnetite might deflect a compass needle.

Hematite does not occur in meteoroids, but occurs in some meteorites as a terrestrial weathering product. Iron rust is mainly hematite. Most meteorites contain iron metal. That metal will begin to rust soon after the meteorite falls. Any meteorite that looks rusty or has reddish staining probably contains some hematite, but a freshly fallen meteorites will not contain hematite. Not all hematite is rusty colored; some is gray. The standard field test for hematite is the streak test.

Rubin (1997a) states that "Carbonates occur as veins and aggregates in CI chondrites; the aggregates probably formed from fragmented carbonate veins. The principal phases are ferroan magnesite, siderite, dolomite, and pure calcium carbonate (vaterite and/or calcite);" references are given. As of December 1999, there were only 5 CI chondrites known among more than 22,000 meteorites cataloged by Grady (2000). Rubin (1997a) also states that "Phyllosilicates [e.g., micas] occur in the matrix and the interiors of chondrules... Associated with the phyllosilicates are Ca-carbonates and magnetite." Again, however, these mineral occur in very small quantities. Some meteorites may contain minor calcite as a result of weathering on Earth. This calcite might be evident in veins upon hand inspection.

References:

Geiger T. and Bischoff A. (1995) Formation of opaque minerals in CK chondrites. Planet Space Sci. 43, 485-498.

Grady M. M. (2000) Catalogue of Meteorites, With special reference to those represented in the collection of the Natural History Museum, Fifth Edition, Cambridge University Press, Cambridge, 689 pp and CD-ROM.

Rubin A. E. (1997a) Mineralogy of meteorite groups. Meteor. Planet. Sci. 32, 231-247.

Rubin A. E. (1997b) Mineralogy of meteorite groups: An update. Meteor. Planet. Sci. 32, 733-734.


www.catchafallingstar.com
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Prepared by:

Randy L. Korotev


Department of Earth and Planetary Sciences
Washington University in St. Louis


Please don't contact me about the meteorite you think you’ve found until you read this and this.

e-mailkorotev@wustl.edu