Other Evolved Asteroidal Achondrites
Apart from the members of the HED group, several other evolved achondrites obviously formed on partially differentiated asteroids. Representing the only samples of their parent bodies, they are of major scientific importance, adding fundamental insights to our knowledge of the differentiation processes and igneous activities on other worlds. Most of them can be assigned to two distinct, genetically unrelated groups, the angrites and the aubrites, which will be discussed below.
Still, some evolved asteroidal achondrites don't fit neatly into existing classes, most likely representing unique samples of their parent bodies. A most intriguing example of such an ungrouped achondrite is the meteorite NWA 011, a small stone that was recovered from the Sahara desert in 1999. Originally, it was identified as a eucrite, but subsequent chemical and isotopic studies have shown that it represents a new type of basaltic achondrite, genetically unrelated to the other members of the HED group. Hence, NWA 011 is believed to represent a distinct parent body, an asteroid equally evolved and of a similar size as the HED parent body, Vesta.
Personally, we believe that many other "eucrites" and "diogenites" in our collections will turn out to be pieces of previously unsampled, differentiated worlds after thorough re-examination. For many years, the unique SNC member ALH 84001 was thought to be a diogenite before it was recognized as a sample of the planet Mars. The same holds true for many other achondrites that are now known to be of lunar or Martian origin. Considering this, we believe it is virtually certain that many new types of evolved asteroidal achondrites are just waiting to be discovered in the wealth of new meteorites recently coming from the hot deserts of Africa and Asia.
The achondrites of this group are named for their type specimen, Angra dos Reis, a meteorite that fell in Rio de Janeiro, Brazil, in early 1869. Angra dos Reis is a unique igneous rock that is largely composed of the rare pyroxene fassaite, an aluminium-titanium diopside. During the last two decades, a handful of similar fassaite-rich meteorites have been recovered, leading to the establishment of the angrite group, presently comprising six members.
The angrites are primarily composed of varying amounts of fassaitic pyroxene, anorthitic plagioclase, minor olivine, kirschsteinite, along with other accessory minerals and phases. They are basaltic rocks with cumulate textures, often containing porous areas and abundant round vesicles with diameters up to 2.5 cm. These vesicles have been interpreted as remnants of gas-bubbles that formed prior to the crystallization of the rock. However, current research suggests that the vesicles originally were solid spheres that have been exsolved in subsequent stages of rock-formation. Both theories are consistent with a magmatic origin of the angrites, making them the most ancient igneous rocks known. They show crystallization ages of ~ 4.55 billion years, which suggests their formation occurred in the early days of the unfolding solar system. The angrites are thought to have formed on one of the earliest differentiated asteroids from the igneous processing of CAI-rich chondritic matter, similar to carbonaceous chondrites of the CI or CM group.
By comparing the reflectance spectra of the angrites to that of several main belt asteroids, two analogs were identified - 289 Nenetta, and 3819 Robinson. Further research will determine whether one of these asteroids actually represents the angrite parent body. Only two angrites are currently available to the private collector; these are Sahara 99555, a single stone that was found in the Sahara desert in 1999, D'Orbigny, an Argentinian find from 1979 that wasn't recognized as a meteorite until
1998 and NWA 1670 with a total weight of only 30gr.... >>
The aubrites are named for Aubres, a small meteorite that fell near Nyons, France, in 1836. Consisting primarily of the orthopyroxene enstatite, they are also called enstatite achondrites. Aubrites represent evolved rocks of igneous origin and are not to be confused with primitive enstatite-rich achondrites such as Zaklodzie or Itqiy. Due to their typical light-colored fusion crusts, their white interiors, and their fragile compositions, most aubrites in our collections are witnessed falls or finds from the blue-ice fields of Antarctica. Excluding all probable pairings, only 16 distinct members are known. Strangely, not a single aubrite has been recovered from the hot deserts of Africa or Asia thus far.
As stated above, the aubrites consist primarily of large white crystals of the nearly iron-free and magnesium-rich orthopyroxene, enstatite. They also contain small but varying amounts of olivine, nickel-iron metal, troilite, and a variety of exotic accessory minerals, indicating a magmatic formation under extremely reducing conditions. The aubrite magmas most probably formed from an enstatite chondrite source, suggesting a close relationship to the members of the E group. Most aubrites are heavily brecciated, attesting to a violent history for their parent body, which resulted in the xenolithic inclusions found in some members, e.g. ALH 78113, and Cumberland Falls. Both of these contain inclusions of forsterite-rich chondritic composition that hasn't been sampled from any other type of meteorite. This indicates that the aubrite parent body collided with an asteroid of "F-chondritic" composition shortly after its formation, resulting in heavy brecciation and abundant xenolithic inclusions.
Comparisons of the aubrite spectra to the spectra of asteroids have revealed striking similarities between the aubrites and the main belt asteroid 44 Nysa, as well as to other members of the Hungaria family, also known as "the children of Nysa". A small asteroid within this family that has a near-Earth orbit, asteroid 3103, is suspected of being the actual parent body of the aubrites. This object has a diameter of only ~ 1.5 km and hasn't yet been given a name to accompany its numerical designation. Should it perhaps be named "Aubres"?