An Extra Ordinary Chondrite

Have you ever wondered how old our solar system is? How did the Sun and the planets form? Where did the material to create them come from? These are some pretty heady questions, but anyone with an inquisitive mind can ask them.

When a meteorite falls to the Earth, and they do everyday, it’s like a care package for researchers from the Solar System, loaded with all kinds of scientific clues about what’s out there. No need to defy gravity, spend billions of dollars, and risk the vacuum of space. Let gravity do the work for you!

A meteorite can theoretically be just about any naturally occurring solid object that the solar system has to offer that can make it through our atmosphere without totally burning up. In real life, meteorites are made up of a relatively limited number of minerals, aka rocks, some of which are very iron rich. Within this narrow band of mineral compositions, there are a considerable amount of different kinds of meteorites that make it to Earth. However some are more helpful than others when it comes to answering those lofty questions of origin we posed early.

Thanks to the study of certain isotopic ratios within Calcium Aluminum-rich inclusions (CAI’s) found in a “primitive” group of water and amino acid bearing chondritic meteorites known as Carbonaceous Chondrites, we understand the Solar System to be approximately 4.5 to 5 billion years old, depending on when you consider the Solar System to have have been born.

Researchers, and anyone with eyes to see, find tiny spherical shaped condensed glassy droplets known as “chondrules” in a wide range of chondritic meteorites whose’ chondrules and matrix have undergone varying degrees of equilibration and physical alteration from their original states when they formed many billions of years ago.

Glass forms when a crystalline substance is quenched, meaning it cools out of a hot liquid state so fast that it cannot form its natural crystalline structure, and instead becomes an amorphous glass. This implies both an environment with temperatures high enough to liquify the stoney material and then a subsequent cooling period so rapid it did not allow for complete recrystallization.

Most researchers suspect chondrules formed as the result of several different heating/cooling mechanisms including; shockwaves from nova events, unknown early Solar shock waves and heating dynamics, and possibly even electrical arcing effects (space lightning!) in the very early pre-solar highly ionized molecular cloud. These are just some of the proposed mechanisms for chondrule formation. However large precursor chondritic seed grains in some chondrules, among other things, bring into question current models for the timing for chondrule formation. It is seeming more and more likely that chondrule formation was not an isolated event, but was ongoing over a long period of time that may have even pre-dated the beginning phases of Solar System formation.

The nature and degree to which a meteorite’s respective chondrules and their surrounding matrix have been altered since they were formed, has a lot to do with how much it can tell us about the early days of the Solar System. Relatively unequilibrated ordinary and carbonaceous chondrites contain material from the very early and pre Solar System that scientists can study directly.    

Despite this, in the collecting of meteorites, a certain type of snobbery can sometimes find its way into a collector’s heart. The ordinary chondrite can often slowly lose favor over time as the collection matures and budgets increase, giving way to a collector’s desire for the more exotic “Achondrites” from Mars, the Moon, Vesta and places unknown. This seems even to be true for many of the researchers involved with meteorites, as often the best minds of meteoritics have opted to specialize in planetary achondrites. Who can fault them, these areas require intense study as well, and hey, its Mars and the Moon!

Many of these exceptionally rare and previously unobtainable achondrites have steadily been coming down in price over the years as more and more come out of the hot desserts.This has not helped the second class status of the unfortunately named “ordinary” chondrite. Exchange “ordinary” for a different word such as “carbonaceous” or even a letter “E” or “R” and suddenly the chondrites’ cool factor is back.

So, take note all ye achondrite snobs (just kidding!). Arguably one of the most important meteorite discoveries in over 75 years is an Ordinary Chondrite. The stone was sent by meteorite maverick Sean Tutorow of Eegooblago Meteorites, to meteorite researcher and chondrite expert Alan Rubin at UCLA’s Institute of Geophysics and Planetary Physics for analysis and classification.

The analysis came back with a classification of LL 3.00 Ordinary Chondrite and it was eventually given the official name Northwest Africa 8576. What’s so special about that, you might ask? Indeed there are a relatively large amount of LL  (very low metal) ordinary chondrites represented in our world’s scientific and private collections. However it's the numbers after the LL that have researchers and collectors in the know so excited. The 3.00 classification indicates little or no equilibration, making this ordinary chondrite not so ordinary.

Determining the difference between an LL 3.05 and an LL 3.00 is not an easy task. According to Dr. Rubin, using optical microscopy he determined no chromium had equilibrated from the meteorite’s iron-oxide rich olivine grains to produce telltale chromite in exsolution. Sighting a 2005 paper by Grossman, J. N. & Brearley, A. J., Dr. Rubin states in the official Meteoritical Bulletin write-up that, ¨this implies it is outside the 3.05-3.15 range.¨  Dr. Rubin further indicates that, “The meteorite cannot be type 3.2 or higher since it has such high Cr [chromium] in the ferroan olivine and a low standard deviation similar to Semarkona (LL3.00)¨.

For those new to the finer points of meteorite classification, let’s run through a brief explanation of what the first set of letters and numbers represent within the naming conventions of an ordinary chondrites’ classification. When interpreting an ordinary chondrites’ classification, one starts with a letter designation indicating the amount of metal in the chondrite. H indicates high metal content, L indicates low metal content, and LL indicates very low metal content. These more generalized metal content oriented designations, have evolved into clear geochemical classifications that are further parsed down into the level of equilibration. The numbers that come after the letter(s) represent the level of this chemical equilibration and alteration a chondrites’ constituent chondrules and finer matrix have undergone.

When a relatively unequilibrated chondrite falls in the range from 3.00 to 3.9 it is considered a ¨Type 3¨ Ordinary Chondrite. It is the lesser equilibrated Type 3 chondrites that both scientists and collectors seek out. An ordinary chondrite with a designation of 3.05 or say 3.2 is relatively unequilibrated, where one with a designation of 5 or 6 is very equilibrated. When a meteorite’s state of equilibration exceeds the qualities of a Type 3 and moves into the 4, 5 and 6 ranges, scientists no longer use the smaller "subtype" increments to describe them. You are unlikely to ever see a chondrite with a classification of LL 4.2 or H 5.5 for example, but you will see many 3.05 through 3.9 legacy classifications. As of current guidelines, going forward from 2015, for new classifications only chondrites displaying equilibration from 3.00 through 3.25 will be given subtype designations. Anything over 3.25 or without enough data to support the claim of lower subtype, will simply be classified as LL3, or L3, or H3, without the subtype being indicated, as it either isn't significant or wasn't sufficiently established.

Researchers have catalogued a great many ordinary chondrites with many degrees of equilibration, but the relatively unequilibrated Type 3 specimens with low subtypes are much more rare. In fact, the only other LL ordinary chondrite exhibiting type 3.00 characteristics similar to NWA 8576 was found in Semarkona, India in 1940. The Semarkona meteorite was the sole example of an LL 3.00 chondrite for nearly 75 years. One might assume researchers have significant examples to construct the models that describe the various levels of alteration and equilibration of the ordinary chondrite because we generally understand there to be massive amounts of ordinary chondrites collected over the past few centuries. However, of all of that tonnage, only these two meteorites with a combined mass of ~450 grams has been classified as LL 3.00.  A 74 year lag between the discoveries of these two extraordinary ordinary chondrites attest to just what a big event in meteoritics the discovery of another LL 3.00 really is.

So, welcome NWA 8576! Semarkona can use the company. Undoubtedly several academic papers are already in the works with NWA 8576 front and center. What new discoveries or theories you might spawn from the minds of this and the next generations of researchers, we will just have to wait and see. Until then, welcome to Earth.

UPDATE: A recently discovered 53.8 gram specimen, NWA 12692, has been classified as an LL 3.00, bringing the specimen count up to 3! The TKW and availability for research purposes still remains low. Congratulations, Z. Kerensky!