Things that you cannot see in a meteorite

People contact me frequently saying that their rock must be a meteorite because they can “see” diamonds, lonsdaleite, carbonados, stishovite,  tridymite, maskelynite, nickel, gold, iridium, platinum, other rare phases, and even short half-life isotopes. You cannot see or identify any of these things by sight in a meteorite, even under moderate magnification. Experienced collectors do use these features to identify meteorites. Why? Because they can’t see them.

Mineral phases

Diamonds, lonsdaleite, carbonados, stishovite, tridymite, and occur in some meteorites but they all occur as microscopically small grains that can only be seen with aid of a microscope or other expensive instruments like a scanning electron microscope (SEM). More importantly, even if you could see them it is impossible to identify a diamond, lonsdaleite, carbonado, silica polymorph, maskelynite, or rare metal by sight. Again, expensive instruments are required. Just because it is shiny or black does not mean that it is a diamond or carbonado. There are many other kinds of shiny and black minerals that are seen in earth rocks. See also lonsdaleite diamonds.

Rare chemical elements

Certain chemical elements are much more abundant in meteorites (especially in chondrites, i.e., 93% of stony meteorites) than in typical rocks of Earth’s crust. Geochemists call these elements siderophile (iron loving) because in meteorites they occur mainly in iron-nickel metal (but also in some sulfide minerals). Siderophile elements include nickel, cobalt, and 8 highly siderophile elements. The HSEs, in decreasing order of abundance, are platinum, ruthenium, palladium, osmium, iridium, rhodium, gold, and rhenium. Note that all of these elements occur in concentrations of less than 2 ppm (<2000 ppb) in ordinary chondrites (Table). Concentrations are even lower in other types of stony meteorites. Even in meteorites, all the HSEs are in concentrations much too low to be detected by standard techniques used to analyze rocks, that is, they are below the detection limits (sometimes stated as LOD – limit of detection). Special and expensive tests are required to accurately measure HSEs in meteorites. The low abundance means that you cannot see highly siderophile elements in a meteorite. You might be able to see grains of Fe-Ni metal, but without chemical analysis you cannot conclude that the metal contains any of the highly siderophile elements. Note that the relative abundance of the highly siderophile elements is very similar in all ordinary chondrites, e.g., the ratio of Ir to Au (Ir/Au) is typically 2.5 to 3.5.

As I note elsewhere, hand-held X-ray fluorescence (XRF) analyzers often report the presence of highly siderophile elements in stony meteorites but such reports are always erroneous in that the analyzers cannot actually “see” any element that occurs at such low concentrations.