TYPES OF METEORITES
Meteorites are composed of minerals, most of which also make up Earth's rocks, but they also contain minerals that have not been found on Earth.
According to their mineral composition, they are divided into three main types:
• stony meteorites: composed mainly of silicate minerals;
• stony-iron meteorites: contain almost equal amounts of metallic and silicate minerals;
• iron meteorites: composed almost entirely of metal.
Each group of meteorites can be divided into more classes depending on the minerals contained, characteristic structure, and chemical composition.
STONY METEORITES
The majority of meteorite finds are stony. They consist primarily of silicate minerals. There are two main types of stony meteorites: chondrites (some of the oldest materials in the Solar System) and achondrites (including meteorites from asteroids, Mars, and the Moon). Both chondrites and achondrites have subgroups, based on their mineral composition and structure.
A typical stony meteorite is black or brown on the outside. Inside, it is a stone containing silvery metal particles and/or rusty spots. It is covered on all sides with a thin glassy crust about 1 mm thick, darker compared to the interior.
Chondrites
Chondrites are some of the most primitive and unaltered rocks from the Solar System and are more than 4.5 billion years old. They are the material from which the Solar System formed and are little changed compared to rocks from larger planets, which have undergone geological activity.
Chondrite with shiny crust
Chondrites – cross-section
Chondrites are the most common meteorites, about 86% of all those discovered on Earth. They are named after the distinctive spherules scattered throughout the meteorite mass, called chondrules. A chondrule (from Greek: Χόνδρος - grain) is a small, spherical or oval object with a size from microscopic to that of a pea. These particles (chondrules), composed mainly of silicate minerals, are rapidly cooled droplets formed as a result of heating and crystallization in open space.
Chondrules most commonly found are the size of a millet grain
Cross-section of a radial chondrule, under microscope
Cross-section of a chondrule with lattice structure, under microscope
Chondrules are composed mainly of the minerals olivine and pyroxene - a composition identical to that of the main cementing mass of the meteorite. To date, chondrules have not been found in Earth rocks, so they are considered formations characteristic of meteorites, and their presence is one of the distinguishing features of stony meteorites.
Depending on the mineral composition and the source from which the meteorite came, different types of chondrites are differentiated.
Carbonaceous chondrites are rare and extremely interesting because they contain organic compounds, amino acids, and interstellar material. The hydrogen in them is about 2% of their weight.
Visually, they resemble charcoal and unfortunately tend to disintegrate quickly when exposed to Earth's climatic conditions.
Achondrites
Achondrites are igneous rocks and resemble basalts formed by processes within the Earth. Most achondrites are ancient rocks and are considered to represent material from the crust of asteroids. It is believed that some of them originate from the asteroid Vesta (HED meteorites).
A small portion of achondrites includes rocks similar to those brought back from the Moon by the Apollo and Luna programs. The origin of another group is associated with Mars.
Piece of Eucrite meteorite, NWA 15923. It was discovered in 2022 in the Sahara Desert by nomads, sellers of most North African meteorite finds. The largest stone of this find was 12 kilograms. Eucrites are achondrite stony meteorites of basaltic rock from the crust of Vesta, ejected during a collision with another asteroid. Eucrites consist mainly of calcium-poor pyroxene and calcium-rich plagioclase (anorthite mineral).
STONY-IRON METEORITES
Stony-iron meteorites formed from the inner crust of asteroids and consist of almost equal amounts of iron-nickel metals and silicate minerals. They are among the most beautiful meteorites. There are two different types of stony-iron meteorites: pallasites and mesosiderites.
Pallasites
Pallasites contain large, beautiful olive-green crystals of magnesium-iron silicate - olivine, completely enclosed in metal. They are thought to have formed in molten asteroids, similar to iron meteorites, where iron metal sinks toward the center of the cosmic body to form an iron core. It is believed that pallasites are samples from the boundary between the metal core and the silicate mantle around it.
Polished Sericho Pallasite Meteorite Slice, Kenya
9Brenham Pallasite Meteorite Slice, Kansas
Rare Gujba Meteorite Slice
Polished Pallasite Meteorite Slice - NWA 16799
The Fukang pallasite, which weighs over a ton, has earned the name "Queen of Pallasites" as the most beautiful meteorite ever found. It was discovered in Xinjiang Province in China in 2000 and has absolutely massive gem-quality crystals. These olivine inclusions vary in color from rich amber to bright green. The clarity and quality of the crystals are particularly remarkable, and when thinly sliced and illuminated, they produce an ethereal, stained glass effect that reveals the inner beauty of the meteorite.
The Fukang Pallasite
Mesosiderites
The name mesosiderites comes from the Greek words mesos ("middle" or "half"), and sideros ("iron") and means "half iron." They are stony-iron meteorites containing approximately equal parts of nickel-iron alloy and silicate minerals.
Mesosiderites are breccias - rocks composed of angular fragments of minerals or rocks cemented together with finer material. They are formed when debris from two asteroids collide, causing molten metal to mix with solid fragments of silicate rocks.
IRON METEORITES
These meteorites are composed of iron and nickel and represent only 5% of all meteorites found on Earth. A typical iron meteorite is black or rusty in appearance, and inside, dense silvery metal (FeNi) can be seen. They are recognizable due to their unique surface with large, shallow, oval indentations called regmaglypts. They have high density and are strongly attracted to magnets.
The Agoudal meteorite fell about 40,000 years ago and is probably responsible for creating two lakes in the Atlas Mountains of Morocco. The first documented pieces were collected in 2000, after which meteorite hunters searched the area of the lakes with metal detectors and discovered many more meteorites - on the surface or buried at a depth of several centimeters.
Campo del Cielo is a group of iron meteorites discovered in Argentina, first documented in 1576 by Spanish explorers. They fell to Earth about 4000-5000 years ago, scattering over an area of about 60 square kilometers.
When iron meteorites are cut and the surface is treated with acids, crossed bands known as Widmanstätten patterns can be seen. They form during very slow cooling in the asteroid's core and the different nickel content in the minerals taenite and kamacite. It is these beautiful structures, as well as the unique indentations, that make iron meteorites so sought after by collectors.
Plates of the Toluca iron meteorite from Mexico with beautiful Widmanstätten bands. Toluca is one of the oldest known meteorites on Earth. It fell 60,000 years ago in Southern Mexico, near the present-day city of Toluca. It was probably used by local peoples for craft tools and weapons.
TEKTITES
Tektites (from Greek tektos, molten) themselves are not meteorites, but natural glass objects up to several centimeters in size, resulting from the impacts of an asteroid or comet with Earth's surface.
The collision with Earth is the cause for the formation of this fascinating material - a rare greenish tektite. It was created as a result of the impact, which ejected molten material into the atmosphere, which after cooling fell back to Earth, scattering over a wide area in present-day Czech Republic. The glassy droplets that fell on the Earth's surface are called moldavite, and due to their aesthetic and precious qualities, are used in jewelry. Its unique origin has made moldavite a popular item for collectors and enthusiasts, as well as an object of scientific interest due to its unusual formation process. It is believed that this occurred as a result of a collision of an asteroid or comet with Earth about 14.8 million years ago, forming the Nördlinger Ries crater in present-day Germany.
Several tektite deposits are known in the world: in Australia, the Philippines, Tasmania Island, Java Island, Ivory Coast, Czech Republic - the only tektite deposit in Europe.
Libyan desert glass is a type of tektite formed by the heat and pressure of a meteorite impact with the sands of the Sahara Desert. The main component of this glass is the mineral lechatelierite. This desert glass is found in various shades of green and brown, depending on the inclusions during its formation. After thorough analysis, scientists discovered that Libyan desert glass formed during a meteor shower in the Sahara Desert nearly 29 million years ago.
* The material uses photos from www.meteorites.bg and www.fossilera.com
