Long before the Earth and other planets existed, the solar system consisted of a protosun and an immense, dense cloud of gases and silicate/metal dust particles. This preplanetary system is called the protosolar nebula (PSN). Over time the dust grains began to stick together (accreted) to form fluffy dust balls much like "dust bunnies" that accumulate under your bed. Turbulent high temperature events during the collapse of the PSN melted these dust balls into molten drops that quickly cooled (quenched) into glass and tiny minerals. Henry Clifton Sorby, a 19th century English geologist, recognized that these millimeter-sized spherical objects resembled "fiery drops of rain" and called them chondrules (from the Greek word, chondros or grains) and meteorites that contain them, chondrites.
The PSN was a very dynamic place. Temperatures fluctuated from so high that dust evaporated to so low that more dust condensed from the nebula. This temperature cycling was accompanied by the accretion of chondrules, condensates, and unmelted dust into small bodies (meters to a few kilometers in diameter) called planetesimals. The PSN was crammed full of these swirling planetesimals that eventually collided with each other and accreted to form bigger bodies, asteroids and planets.
Internal heating of the these accreted bodies ultimately melted the accreted chondrules However, some planetoids and asteroids were too small to have much internal heating and their component chondrules escaped destruction. Inherent chemical and mineralogical differences resulted in three distinct groups of chondritic parent bodies, which are the sources of ordinary chondrites, enstatite chondrites and carbonaceous chondrites. The last type contains carbon and organic compounds that drifted into the PSN from interstellar regions, principally giant molecular clouds. Most of the carbon in your body and your front lawn came from places that even the Starship Enterprise will not encounter.
As parent bodies continued to grow in size, internal temperatures significantly increased from radioactive decay, increased mass, and energy deposited by impacts. The heat buildup in small chondritic bodies (less than 100 kilometers in diameter) was sufficient to metamorphose some of the primitive chondritic materials. Glass crystallized into new minerals and existing minerals recrystallized into larger grain sizes. These processes resulted in chemical equilibration. Much like the transformation that results when baking the raw ingredients of a cake, a new stable form of material was produced from earlier components. Meteorites with metamorphosed chondrites are called ordinary chondrites (OC) and those that escaped metamorphism and remained pristine are referred to as unequilibrated ordinary chondrites (UOC).
Schematic diagram of planetary differentiation. Dense metal sinks inward; whereas, lighter material (rock and molten magma) rises.
In largest parent bodies, temperatures rose above the melting points of the accreted minerals; they melted and assumed features typical of terrestrial igneous activity, e.g., volcanic lava flows. Meteorites that formed from igneous processes are called achondrites, meaning without chondrules. If a parent body grew to a sufficient size, then dense metal from large scale melting separated out and sank towards the center or core of the body. This process, called differentiation produced Earth's large nickel-iron core. Iron meteorites are products of such differentiation; stony iron meteorites, pallasites, represent an incomplete separation of silicates and metal. Mesosiderites are also mixtures of metal and silicates, but probably formed by the recombination of metal and silicate fragments after collisional impacts. As you may have gathered, meteorite classification can be complex and difficult. Meteorites are divided into divisions (e. g., chondrites), classes (e. g., ordinary), and groups (e. g., H, L, LL).
All the meteorites mentioned above come from the Main Asteroid Belt that occupies the space between Mars and Jupiter, and from Earth-crossing asteroids whose orbits extend inside Earth's orbit. In the early years of the solar system formation, collisions among objects in space, e. g., dust particles, planetesimals, asteroids, planetoids and protoplanets, were very frequent. Evidence of these collisions is visible today on the Moon's surface, which shows thousands of ancient impacts that left craters ranging from centimeters to hundreds of kilometers in size. Even today, ejected materials from recent impacts on asteroids, Mars, and the Moon continue to rain down on the Earth as meteorites. Because of this constant supply of extraterrestrial samples, meteorites are called the "poor man's space probes." We don't need to retrieve them from their parent bodies; we just wait and let them "fall into our laps."
Meteorites, especially the primitive chondrites, continue to give us insight into the early history of the solar system. From the Apollo lunar sample missions and remote sensing ventures to asteroids and Mars, we have obtained some first hand information about planetary compositions and geological events. Impacts still occur on all planetary surfaces and the ejected material in the form of meteorites continues to provide us with unique information about our heavenly neighbors.
Like snowflakes, no two chondrules are exactly alike. Chondrules are classified into several groups based on texture and chemical-mineral characteristics; the most common are shown below with petrographic examples.
In barred olivine (BO) chondrules, olivine crystallized in bar-like form where parallel plates of olivine are in optical continuity, usually with a spherical shell of olivine ("armored"). See Figures 1-4.
Cryptocrystalline (C) chondrules contain pyroxene grains too small to be seen under a microscope and often show extinction domains (portions where the grains are optically similar); see Figures 5 and 6.
Granular olivine-pyroxene (GOP) chondrules contain clusters of olivine and pyroxene, commonly without crystal faces or as small grains of olivine included in a single larger pyroxene grain, see Figs. 7-8.
Porphyritic chondrules have crystals of olivine (PO), pyroxene (PP) or both minerals (POP) surrounded by a glassy matrix, which is petrologically referred to as mesostasis. See Figures 9-15.
Radial pyroxene (RP) chondrules have fan-shaped arrays of pyroxene radiating from a point on the chondrule surface (excentroidal). See Figures 16 and 17.
Although rare, relict grains in chondrules are those that survived the high temperature phase that initially melted most primitive chondrules and dust. Present chondrules crystallized from this melting episode. They typically have a dusty appearance, e.g., tiny, dark inclusions, like those in the central violet grain shown in Figure 18.