CHARACTERIZATION OF AL HUWAYSAH 010 UNGROUPED ACHONDRITE

The existence of ungrouped meteorites reflects the complexity in placing these specimens in a correct interpretative framework. The issue is not only a taxonomic problem but also genetic. The problem is even more relevant when it comes to achondrites. These meteorites, in fact, experience a series of very complex processes ranging from partial differentiation to complete fusion. It is therefore very important to focus our attention on ungrouped achondrites. In this work, we have investigated the meteorite Al Huwaysah 010. This meteorite has been studied, using Scanning Electron Microscopy, Electron Probe Microanalysis, Reflectance Spectroscopy, micro-X-ray diffraction, and Laser- Ablation inductively coupled plasma Mass Spectrometry, to obtain new constraints about this ungrouped achondrite and try to associate it with a parent body family. Al Huwaysah 010 is characterized by high olivine abundance (80 vol%), high-Ca pyroxene (6 vol%), low-Ca orthopyroxene (4 vol%), plagioclase (3 vol%), chromite (3 vol%), and other than weathering products (4 vol%) including iron oxides and oxyhydroxides, Ca-sulphates and Ca-Mg carbonates. It is worth mentioning the presence of a tiny plate of graphite along with a possible occurrence of oldhamite. Mineral phases show compositionally homogenous olivine with Fo83.3 and a low Fe/Mn ratio of 29.3, whereas the high-Ca pyroxene, occurring as subhedral and anhedral crystals, has an augitic composition (En45.5Fs10.8Wo43.7) and a Mg# of 80.2. The orthopyroxene occurs only in a fine-intergrowth with a composition of En85.5Fs13.9Wo0.7 and a Mg#85.6. Both pyroxenes are homogenous without any compositional zonation. The plagioclase has a homogenous composition of An48. Other phases are spinel, chromite, and accessories like iron grains (Ni-free or Cr-bearing Fe- Ni alloy), troilite, chlorapatite, pentlandite (as inclusion in chromite), and graphite. Moreover, iron oxides, iron oxyhydroxides, calcium sulfate (gypsum), and dolomite are present in the sample, likely resulting from significant terrestrial weathering of the meteorite. Olivine-chromite intercrystalline and pyroxene-based intracrystalline geothermometers yielded temperatures of 864 ± 42°C and 520 ± 30°C respectively. This work also presents the reflectance spectra obtained on Al Huwaysah 010 ungrouped achondrites in the range of VNIR and Mid-IR. The spectral properties provide evidence of the mafic mineralogy, and the low VNIR spectral reflectance is consistent with the presence of opaque phases like chromite, fine graphite, and metals. Although we can recognize the presence of olivine from VNIR spectra, comparing the spectral parameters to potential parent bodies characterized by olivine-pyroxene mineralogies it falls within the field of the SIII type below the olivine- orthopyroxene line. The obtained characterization allows us to suggest for this ungrouped achondrite a classification as the most reduced of the brachinites.