Quartz, Calcite, Magnetite and Hematite in Meteorites

Although for convenience I sometimes state here that the minerals quartzcalcitemagnetite and hematite 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) and Rubin and Chi (2017) that have been observed in some meteorites. However, all these minerals are minor to rare in any type of freshly fallen meteorite and do not occur naturally at all in the most common types of meteorites.

Quartz

There is no type of meteorite in which quartz (silica, SiO2) would be evident without the aid of a 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 (tridymitecristobalite, 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. If you can scratch glass with a sharp edge or point of the rock, then it is not a meteorite.

Calcite

Calcite (calcium carbonate, CaCO3) is one of a long list (60+, Rubin, 1997a) of terrestrial “secondary” (alteration) minerals that are ubiquitous in meteorites from hot deserts. The longer a meteorite resides on Earth, the the more altered it becomes from interaction with the atmosphere and terrestrial fluids. Calcite is observed mainly in fractures, veins, vesicles, and vugs. “Primary” calcite (formed on the parent body) occurs in a few kinds of meteorites: martian meteorites, some chondrites, and enstatite meteorites, but it is microscopic.

I am frequently sent photos of rocks that appear to be chunks of limestone, a rock consisting mainly of calcite. No limestones have been found as meteorites. Limestones are easy to identify because if you put a few drops of moderately strong acid (e.g., the hydrochloric or muriatic acid used to clean mortar off bricks after tuckpointing) on the rock, the drop rock “fizzes” (bubbles) as carbon dioxide is released.

Iron Oxides

Magnetite (Fe2+Fe3+2O4) and hematite (Fe3+2O3) 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.

“Although hematite blueberries have not been reported in martian meteorites, grains of hematite do occur” (Rubin and Chi, 2017). Hematite does not occur naturally in other meteorites, but occurs in many meteorites, e.g., iron finds (Buchwaldt, 1977), 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.

Rocks composed mainly of iron oxides, essentially iron ores, are among the most common rocks mistaken for meteorites. Such rocks catch people’s attention because they (1) are much denser (4.5-5.3 g/cm3) than most other common rocks (typically, 2.6-3.0 g/cm3), (2) often occur as concretions with strange shapes, and (3) attract magnets if they contain magnetite. 

The standard field test for hematite is the streak test. If a rock makes a reddish of grayish streak, it is not a meteorite.

References

Buchwald V. P. (1977) The mineralogy of iron meteorites. Philosophical Transactions of the Royal Society A, 286, 453-491.

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

Rubin A. E. (1997a) Mineralogy of meteorite groups. Meteoritics & Planetary Science, 32, 231-147.

Rubin A. E. (1997b) Mineralogy of meteorite groups: An update. Meteoritics & Planetary Science, 32, 733-734.

Rubin A. E. and Ma C. (2017) Meteoritic minerals and their origins. Chemie der Erde – Geochemistry, 79, 325-385.