The stony meteorites represent the most heterogeneous class of
meteorites, ranging from primordial matter that remained more or less
unchanged for the last 4.5 billion years to highly evolved rocks from
other differentiated worlds, such as the Moon or the planet Mars.
"Stony-iron meteorites are less abundant than their stony and iron cousins are. Taken together, all pallasites and mesosiderites comprise a total known weight of about 10 tons, representing approximately 1.8% of the entire mass of all meteorites known. This low abundance is also reflected by their fall-ratio; when compared to the other major types of meteorites, stony-irons are exceptionally rare, representing just 1.5% of all witnessed falls."
Chondrites are more or less undifferentiated, primordial matter that has remained nearly unchanged for the last 4.5 billion years. These stony meteorites formed nearly simultaneously with the central star of our system, the Sun. It is thought that small droplets of olivine and pyroxene condensed and crystallized from the hot primordial solar nebula in form of small spheres that we nowadays call chondrules. This process of solidification and crystallization is not completely understood, and different scientists suggest different theories for chondrule formation. However, they all agree that those chondrules accreted with other material that condensed from the solar nebula forming a matrix, and of course, the larger parent bodies of those primitive meteorites; i.e. smaller and larger asteroids of chondritic nature.
In their chemical composition, chondrites resemble the Sun, depleted of the most volatile elements like hydrogen and helium. However, the distribution of elements has not been uniform in the original solar nebula - elemental composition varied as did the conditions under which the chondritic parent bodies formed. Different asteroids formed in various regions of the primordial solar nebula under different conditions. Those parent bodies were further subjected to different thermal and chemical processes as well as to impacts with other asteroids resulting in a variety of chondrites, which have been categorized into several clans, groups, and subgroups by modern meteoritics and cosmochemistry. We will elaborate on most of this clans and groups
on separate pages.
However, chondrites are not only differentiated into clans and groups reflecting chemical and isotopic relationships to each other, common parent bodies, or regions of formation in the primordial solar nebula. The chondrites of each clan and group are further subdivided according to petrologic viewpoints and are classified into petrologic types. Each type is designated with a number from 1 to 7 whereas type 3 builds the base line and describes a type of chondrite that has suffered little or any alteration by neither water nor any thermal metamorphism. The petrologic types mirror the degree of chemical equilibrium within the minerals of a chondrite. Petrologic types 1 to 3 represent highly unequilibrated chondrites due to a lack of thermal metamorphism while the types 4 to 7 are increasingly equilibrated due to extended thermal processes. For this reason researchers sometimes talk of unequilibrated / equilibrated chondrites when they are referring to certain petrologic types.
Unequilibrated chondrites of petrologic types 2 and 1 have been subjected to an increasing degree of aqueous alteration with the result that type 1 chondrites don't show any chondrules - they are virtually absent even though the meteorite is of chondritic composition and certainly contained chondrules in its early history. Type 2 chondrites exhibit only a sparse distribution of more or less aqueously altered chondrules. Both types are represented only by members of the carbonaceous chondrite clan.
Petrologic types 3 to 7 have been exposed to increasing thermal metamorphism that is reflected in an increasing alteration of the chondrules. Type 3 exhibits abundant, unaltered and distinct chondrules while the chondrules of petrologic types 4 to 6 are increasingly indistinct due to thermal metamorphism and recrystallization. In chondrites of petrologic type 7 we can witness the end of this process since chondrules are completely absent even though the meteorite has retained its chemical composition revealing its chondritic nature. Those type 7 chondrites can actually be regarded as true transitional specimens that form a link between chondrites and primitive achondrites.
It is important to say that the thermal metamorphism that creates petrologic types 4 to 7 doesn't involve any melting. Recrystallization takes place in the solid state, and there is only one exception to this rule. Some chondrites have suffered partial melting through impact events creating so-called impact melt breccias or IMBs. Our new find NWA 772, a L chondrite also known as "El Kachla", is a most beautiful example of such an impact melt breccia, another intermediate between chondrites and primitive achondrites.
The Different Clans and Groups of
The term "achondrite" is used to describe a stony meteorite without chondrules, and this lack of chondrules is the primary characteristic used to distinguish the two major stony groups, achondrites and chondrites. However, we have already stated that some chondrites don't contain any chondrules, and at least one type of achondrite contains distinct chondrules. These exceptions mark transitions from one class to another, and they remind us of the fact that chondrites represent primordial matter that remained relatively unchanged since the formation of our solar system. Within the process of accretion and differentiation of larger asteroids and planets, this primordial chondritic matter was melted and recrystallized to form achondrites. These evolved rocks are similar to terrestrial igneous basaltic or plutonic rocks.
The achondrites in our collections are samples of other differentiated worlds, and therefore represent a very heterogeneous class of meteorites. Most of them are primitive; that is, nearly chondritic in composition with an age similar to the primordial chondrites. These so-called primitive achondrites are the residues from partial melting that took place on small parent bodies having chondritic compositions. Following an initial heating phase, they were quickly cooled to become geologically inactive. In a later section, we will elaborate on the different types of primitive achondrites - also known as the PAC group.
Other, more evolved achondrites, have experienced a more extensive igneous processing including magmatic processes similar to geological activities encountered on Earth. Some of these achondrites are basalts, calcium-rich volcanic rocks that represent the upper crust of their parent bodies. Others are magnesium-rich plutonic rocks that formed in deeper regions of the crust and experienced prolonged thermal processing.
Several groups of evolved achondrites can be assigned to specific parent bodies. The meteorites of the HED group are believed to be samples of 4 Vesta, one of the largest asteroids in our solar system. Other basaltic achondrites, such as aubrites and angrites, are also considered to have an asteroidal origin, while a few can be assigned to larger parent bodies - the true planets and their moons.
The rare meteorites of the LUN group are genuine pieces of our own Moon - a fact that has been proven by comparisons to samples of Moon rocks that were returned to Earth by the Apollo missions during the late 60's and early 70's. The equally rare and most fascinating achondrites of the SNC group are believed to have their origin on our red neighbor, the planet Mars. These meteorites represent highly evolved rocks and resemble terrestrial rocks more than any of the other achondrites. All of these groups and subgroups will be discussed
on seperate pages. >>
The Different Clans and Groups of