| Iron-nickel in Meteorites |
Iron-nickel is technically a group of scientifically classified minerals. Most mineral collectors do not make a distinction between the individually-named minerals in this group, and refer to specimens as Iron or Iron-Nickel.
Most Iron-nickel is of extraterrestrial origin, in the form of meteorites. Hundreds of meteors fall toward earth every day, but most of them burn up in the atmosphere. The few that make it to earth usually land in water, and the few that land on the ground are generally small.
All known meteorites fall into one of three main categories, they are: Stony, Iron and Stony Iron. Meteorites - regardless of type - will exhibit something called fusion crust. Fusion crust is formed when a meteor enters the Earths atmosphere at a high rate of speed. The meteor quickly heats up as it encounters cooler air, and extreme pressure. It heats so dramatically that the exterior begins to glow and then burn. The glowing, molten exterior flows and forms a thin eggshell like crust. Unfortunately for meteorite hunters, this crust is somewhat fragile and is one of the first identifying characteristics of a meteorite to weather away. Consequently, most meteorites found do not display fusion crust.
Stony meteorites called “Ordinary Chondrites” make up about 80-90 percent of all falls, and will be attracted to a magnet since they contain iron. A cut cross section of an Ordinary Chondrites will show tiny flecks of Iron-nickel sprinkled throughout the extraterrestrial matrix which is comprised mostly of chondrules. Chondrules are small millimeter sized spherical grains that were once molten and came together in space through gravitational accretion.
It is important to note that some very rare stony meteorites have no Iron-nickel. Lunar and Martian meteorites are two types that will not adhere to a strong magnet because of the absence of Iron-nickel in their makeup. These types of meteorites are rare because they look very much like common Earth rocks, and only the most skilled meteorite hunters are able to identify them in the field. In fact, lunar meteorites have only been found in places such as Antarctica and Oman where there are few terrestrial stones, so spotting a rare meteorite is made possible.
Iron meteorites are literally made up of about 95 percent iron and nickel. This type of meteorite is considered rare since only about 6 percent of all witnessed falls are of this type. An iron meteorite is what most people envision when thinking about the “look” of a meteorite. Iron forms tend to be powerfully sculpted specimens due to atmospheric ablation. Some have deep thumbprints and scoops which are beautiful features in the eyes of a meteorite enthusiast.
Iron meteorites have a hidden beauty too. When an iron meteorite has been cut, polished and then etched with a weak acid, intersecting holographic line patterns called witmanstatten patterns will appear on the smooth surface.
Stony Iron meteorites are typically composed of about 50 percent Iron-nickel and 50 percent stony material. These meteorites include both mesosiderites and pallasites. Of the two types it is the pallasite that is the more aesthetically pleasing. A pallasite is made up of beautiful Olivine crystals surrounded by brilliant shiny Iron-nickel. When a pallasite meteorite is cut thin enough –about 4 mm - its true beauty is revealed. Held up to a light source the meteorite becomes translucent and rays of amber, green and orange dance across the surface of the specimen.
There are some natural deposits of terrestrial Iron-nickel, but they are rare and limited. Iron-nickel (terrestrial or extraterrestrial) develops a coating of rust if washed or if kept in a humid area. If a specimen must be washed with water, it should be thoroughly dried.
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