¹Lang Zhang,^1,2Ai-Cheng Zhang,¹Shu-Zhou Wang
American Mineralogist 108, 1597-1611 Link to Article [http://www.minsocam.org/msa/ammin/toc/2023/Abstracts/AM108P1597.pdf]
¹State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China 2
²CAS Center for Excellence in Comparative Planetology, China
Copyright: The Mineralogical Society of America

Apatite is an important petrogenetic indicator in extraterrestrial materials. Here, we report the
mineralogical features of apatite and associated phases in three brachinites Northwest Africa (NWA)
4969, NWA 10637, and NWA 11756. Two types of apatite are observed: intergranular apatite and apatite
inclusion within chromite and silicate minerals. The intergranular chlorapatite is enclosed by or penetrated by irregular porous merrillite, indicating chlorapatite replacement by merrillite. The intergranular
chlorapatite is closely associated with a fine-grained pyroxene-troilite intergrowth along olivine grain
boundaries, which is a sulfidization product of olivine. High-Ca pyroxene is observed as a constituent
phase in the intergrowth for the first time. The apatite inclusions are either monomineralic or closely
associated with subhedral-euhedral pore-free merrillite. In NWA 4969, the apatite inclusions show a
large compositional variation from chlorapatite to fluorapatite and are systematically more F-rich than
intergranular apatite; while the apatite inclusions in NWA 10637 and NWA 11756 are chlorapatite. Most
of the two apatite types in brachinites contain oriented tiny or acicular chromite grains, suggesting the
exsolution of chromite from apatite. We propose that apatite replacement by merrillite, formation of
pyroxene-troilite intergrowth, and exsolution of chromite in apatite were caused by a shock-induced,
transient heating event (~930–1000 °C) on the brachinite parent body. This heating event resulted in
halogen devolatilization during replacement of the intergranular apatite by merrillite, which probably
disturbed the Mn-Cr isotopic system in brachinites as well. We also propose that the apatite inclusions
could be a residual precursor material of the brachinites.