A detailed mineralogical, petrographic, and geochemical study of the highly reduced chondrite, Acfer 370

Abstract

Among the many ungrouped meteorites, Acfer 370, NWA 7135, and El Médano 301—probably along with the chondritic inclusion in Cumberland Falls and ALHA 78113—represent a homogeneous grouplet of strongly reduced forsterite‐rich chondrites characterized by common textural, chemical, mineralogical, and isotopic features. All of these meteorites are much more reduced than OCs, with a low iron content in olivine and low‐Ca pyroxene. In particular, Acfer 370 is a type 4 chondrite that has olivine and low‐Ca pyroxene compositional ranges of Fa 5.2–5.8 and Fs 9.4–33.4, respectively. The dominant phase is low‐Ca pyroxene (36.3 vol%), followed by Fe‐Ni metal (16.3 vol%) and olivine (15.5 vol%); nevertheless, considering the Fe‐oxyhydroxide (due to terrestrial weathering), the original metal content was around 29.6 vol%. Finally, the mean oxygen isotopic composition Δ17O = +0.68‰ along with the occurrence of a silica phase, troilite, Ni‐rich phosphides, chromite, and oldhamite confirms that these ungrouped meteorites have been affected by strong reduction and are different from any other group recognized so far.

Forsteritic olivine in EH (enstatite) chondrite meteorites: A record of nebular, metamorphic, and crystal‐lattice diffusion effects

“…Of the minor chondrite groups with three‐oxygen isotope ratios close to EH chondrites, forsteritic olivine is found in low‐FeO (or “HH”) chondrites (Pourkhorsandi et al., 2017; Pratesi et al., 2019), Kakangari (K)‐type chondrites (Barosch et al., 2020; Nagashima et al., 2015), and forsterite (F)‐type chondritic clasts found in the Cumberland Falls aubrite (Neal & Lipschutz, 1981; Figure 7). One can also add winonaites to this list (although we will discuss it in more detail in the next section), as these primitive achondrites likely represent metamorphosed chondrites (Tomkins et al., 2020).…”

“…Rather than the production of FeO-bearing olivine, decreasing temperatures drive a switch in condensation style from CaO-to MnO-rich forsterite (Ebel et al, 2012;Sugiura 3 and 4. Data for other chondrites and achondrites from various sources (Barosch et al, 2020;Benedix et al, 1998Benedix et al, , 2000Benedix et al, , 2005Berlin, 2009;Bunch et al, 1970;Chizmadia et al, 2002;Fogel, 2005;Huang et al, 1996;Jones, 1994;Jones & Scott, 1989;Kimura et al, 1993;King et al, 1981;Libourel & Chaussidon, 2011;Matsunami et al, 1993;Mikouchi et al, 2011;Nagashima et al, 2015;Neal & Lipschutz, 1981;Okada et al, 1988;Pourkhorsandi et al, 2017;Pratesi et al, 2019;Rosenshein et al, 2006;Rubin et al, 2002;Rubin & Wasson, 1986;Simon et al, 1994;Steele, 1986Steele, , 1990Sugiura et al, 2009;Takeda et al, 2000;Watters & Prinz, 1979;Weisberg et al, 2004). (Color figure can be viewed at wileyonlinelibrary.com) et al, 2009).…”

Mineralogical, petrological and planetological heritage. The (Italian) story so far

“…However Italian meteorite collections have not only a historical significance, but they are also an important source of extra-terrestrial material for research purposes. Cataloguing meteorite specimens in naturalistic collections along with their relative archival documents, museum catalogues and inventories can lead to a better characterization of previously analysed samples and to the potential discovery of meteorites unknown to the scientific community (Trevisani 2011;Moggi Cecchi et al 2015, 2019aPratesi et al 2019;Llorca et al 2020;Marocchi et al 2020). Furthermore, as rightly suggested by Dorfman (2012), meteorite collections hold many levels of intangible values (Ahmed 2006).…”

An evolutionary system of mineralogy. Part III: Primary chondrule mineralogy (4566 to 4561 Ma)

“…Oldhamite (CaS): PC oldhamite is a common accessory primary chondrule phase in enstatite chondrites, where it occurs as small grains in metal-sulfide nodules with other reduced phases, including alabandite, caswellsilverite, daubréelite, and niningerite (Mason 1966;Crozaz and Lundberg 1995;Fogel 1997;Nakamura-Messenger et al 2012;Weyrauch et al 2018). Calcium sulfide has also been reported as crystals up to 200-micrometers diameter within a metal matrix in the ungrouped, highly reduced meteorite Acfer 370 (Pratesi et al 2019). Oldhamite in EH3…”