Nearly all meteorites consist of extraterrestrial nickel and iron. Meteorite classification predominantly depends on the quantity of iron within a specimen; those devoid of this mineral are incredibly scarce. Meteorites categorically fall into three primary groups: irons, stones, and stony-irons, although numerous subcategories exist.
Stones are the most common group of meteorites. They were once part of the outer crust of a planet or asteroid. When hunting for meteorites in the field, identifying stone meteorites can be a challenging task as they often resemble typical terrestrial rocks. Upon their initial fall, freshly fallen stone meteorites display a distinctive black crust that forms as the meteorite's surface burns during its passage through the atmosphere. Another distinguishing quality of stone meteorites is their propensity to contain significant iron, allowing them to firmly adhere to powerful magnets.
Some stone meteorites possess slight, vibrant, grain-like inclusions called chondrules. The minute particles originated in the solar nebula, preceding the formation of our planet and the solar system, thus making them the oldest known matter available for scientific study. Meteorites containing chondrules are categorized as chondrites. In contrast, meteorites lacking chondrites are called achondrites. These meteorites include volcanic rocks from space formed due to igneous activity within their parent bodies, erasing all traces of ancient chondrules through melting and recrystallization. Achondrites typically contain minimal or no extraterrestrial iron, rendering them more challenging to detect than other meteorites. Nevertheless, the specimens often boast a remarkable glossy fusion crust that bears a striking resemblance to enamel paint.
Around a hundred lunar meteorites (lunaites) and roughly thirty Martian meteorites (SNCs) have been discovered on Earth, all falling into the achondrite category. Impacts on the Moon's and Mars's surfaces caused other meteorites to propel fragments into space, and some of these fragments eventually landed on Earth. Regarding value, lunar and Martian specimens rank among the most precious meteorites, often commanding prices of up to $1,000 per gram on the collectors' market, making them significantly more valuable than their weight in gold.
Iron meteorites are relatively less common than stone meteorites, characterized by their density, considerable weight, and unique shapes, resulting from their journey through the Earth's atmosphere, which often involves melting. Once integral components of a vanished planet or a sizable asteroid, iron meteorites are believed to have originated within the Asteroid Belt between Mars and Jupiter. Iron meteorites are highly magnetic and possess a density far more significant than most earthly rocks. Their iron content generally ranges between 90 to 95 percent. Iron meteorites also contain nickel and trace elements.
Structurally, iron meteorites are classified based on two critical iron-nickel alloys: kamacite and taenite. The alloys form an intricate interlocking crystalline structure recognized as the Widmanstätten Pattern, revealed after cutting the meteorites into slabs, polishing them, and etching them with a mild nitric acid solution. The lattice arrangement presents a remarkable display, uncovering the celestial history embedded within these iron meteorites. The bandwidths of kamacite crystals help divide iron meteorites into various structural classes. Specimens showing narrow bands are called fine octahedrites, whereas those with wider bands are called coarse octahedrites.
Among the three primary types of meteorites, stony-irons are the scarcest, accounting for less than two percent of all known meteorites. They consist of nearly equal parts of nickel-iron and stone, categorized into pallasites and mesosiderites. These stony-irons are believed to have originated at the core/mantle boundary of their celestial parent bodies. Pallasites are composed of a nickel-iron matrix filled with olivine crystals. They sometimes feature olivine crystals of extraordinary purity and an emerald-green color, known as the precious gemstone peridot. Pallasites are noted for their stunning appearance when the crystal slabs are cut and polished, exuding an otherworldly beauty.
Mesosiderites represent the smaller group of stony-iron meteorites. The meteorites incorporate nickel-iron and silicates and often exhibit a visually appealing high-contrast silver and black matrix when cut and polished. The seemingly haphazard mix of inclusions gives rise to distinctive and striking features. Mesosiderite meteorites are exceedingly rare. Among the thousands of officially cataloged meteorites, fewer than one hundred are classified as mesosiderites.
