A large fragmented stone weighing 4,085 g was found in Morocco during the Fall of 2004. The stone was analyzed and classified at Northern Arizona University (T. Bunch and J. Wittke), and NWA 2635 was initially considered to be a highly equilibrated and recrystallized H chondrite lacking any relict chondrules. The olivine Fa (18.9) and pyroxene Fs (16.8) values are consistent with the H chondrite group.
Based on the completed O-isotope analyses, it is evident that the ratios plot on an oxygen three-isotope diagram slightly outside of the main field of H chondrites while still overlapping the fields that resolve the H chondriteIIE iron parent body. Further studies will be needed to examine the possibility that the O-isotope ratios might have been shifted due to open system aqueous flows on the H chondrite parent body. Since NWA 2635 is a recrystallized, texturally evolved chondrite with elemental ratios and an O-isotopic composition showing affinities to the H chondrite group, it is appropriately assigned to the newly proposed group of metachondrites (Irving et al., 2005).
Northwest Africa 2635 is unshocked (S1, peak shock pressure <5 GPa), and has experienced moderate terrestrial weathering (W2). The numerous small vugs that occur throughout this meteorite may be indicative of a very high-temperature environment during formation, consistent with greater depth within the asteroid. Despite some petrographic differences, e.g., differences in grain size, differences in O-isotopic compositions, and the absence of clinopyroxene in NWA 2635, there exist very close similarities between NWA 2635 (ACHO-ung) and the paired metachondrites NWA 2353 (ACHO-ung) and NWA 3145 (PAC), and in all likelihood they constitute a pairing group. Furthermore, there is a strong possibility that the H7 chondrite NWA 2835 also belongs to this pairing group (Irving and Kuehner, UWS; Bunch, NAU). The specimen of NWA 2635 shown above is a 6.45 g partial slice.
Diagram credit: Greenwood et al., Chemie der ErdeGeochemistry, vol. 77, p. 24 (2017)
'Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies'
(open access: http://dx.doi.org/10.1016/j.chemer.2016.09.005)