Howardite (HED achondrite)
Fell, April 1942; 357 g

After a thunderous midmorning detonation, a single crusted stone and a few fragments were found. The bulk of this Howardite meteorite (>90%) consists of pyroxenes and plagioclase. The pyroxenes are predominantly orthopyroxene and pigeonite in roughly 2:1 proportions, although traces of augite are also reported. The plagioclase is predominantly bytownite, but anorthite is also found. Other silicate phases include minor tridymite, olivine, and K-containing maskelynite glass. Opaques include chromite, spinel, troilite, chalcopyrite, copper, and several varieties of Fe-rich metal. The above description for Bununu is quite similar to those of many other Howardites. One important difference is that Bununu was observed by two masters of petrographic microscopy — Brian Howard Mason and Paul Ramdohr — so that the mineralogical inventory for tiny Bununu's many minor phases appears to be a little more complete than that of several larger Howardites. However, new techniques are being devised for identifying microscopic phases and further mineralogical information about such phases may be forthcoming (if not already reported elsewhere).

Howardites are predominately brecciated mixtures of Eucrite and Diogenite materials and are thus part of the Howardite-Eucrite-Diogenite (HED) clan of achondritic or differentiated meteorites. It is believed that the constituents of most HED meteorites were formed, mingled and ejected from the asteroid 4 Vesta — the Eucrites formed near the surface or at moderate depths, the Diogenites formed in the Vesta mantle, and the Howardites were assembled before or during the more violent impacts which eventually would dispersed them all into the interplanetary void. Indeed, distinct and well-defined 40Ar-39Ar ages for different mineralogical phases point to a prolonged period of strong impacts during Bununu's first 3-4 hundred million years. The recent DAWN mission to Vesta seems to have confirmed this general scenario, but recent high resolution oxygen-isotope studies have also made it perhaps even clearer that not all HED meteorites were formed on the same parent bodies. We apparently still have work to do before we can definitively assign specific meteorites such as Bununu to their asteroidal parent bodies.

Other Regions, Features and Areas containing this locality

References