Iron, IIE, silicated Most Common type of iron meteorite, composed mainly of taenite and kamacite and named for the octahedral (eight-sided) shape of the kamacite crystals. When sliced, polished and etched with an acid such as nitric acid, they display a characteristic Widmanstätten pattern. Spaces between larger kamacite and taenite plates are often Click on Term to Read More
(HH Chondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More related)
27° 50′ S., 150° 20′ E. A mass of about 265 kg was found by Mr. Frank Timms on open shrub farmland near Miles, in Queensland, Australia. About 100 kg was exported to the USA by M. Killgore and submitted for analysis. Miles has been classified as a group IIE iron belonging to the ‘fractionated IIE’ grouping, as distinct from the ‘normal IIE’, ‘IIE-An’, and ‘Modifying term used to describe meteorites that are mineralogically and/or chemically unique and defy classification into the group or sub-group they most closely resemble. Some examples include Ungrouped Achondrite (achondrite-ung), Ungrouped Chondrite (chondrite-ung), Ungrouped Iron (iron-ung), and Ungrouped Carbonaceous (C-ung). Click on Term to Read More iron’ categories distinguished by Wasson and Wang (1986). Researchers divided the eight known (at the time) silicate-bearing IIE irons into five groups based on The most abundant group of minerals in Earth's crust, the structure of silicates are dominated by the silica tetrahedron, SiO44-, with metal ions occurring between tetrahedra). The mesodesmic bonds of the silicon tetrahedron allow extensive polymerization and silicates are classified according to the amount of linking that occurs between the Fragment of foreign (xeno-) material enclosed within the primary matrix of a rock or meteorite. Click on Term to Read More types: 1) chondritic clasts (Netschaëvo), 2) partially melted but undifferentiated clasts (Techado), 3) completely melted clasts with a loss of Element that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn Click on Term to Read More and sulfide (Watson 001), 4) plagioclase–orthopyroxene–clinopyroxene basaltic partial melts (Miles and Weekeroo Station), and 5) plagioclase–clinopyroxene partial melts (Colomera, Kodaikanal, and Elga) et al., 1998). Other researchers (e.g., Ruzicka, 2014) have followed a more simplified classification Definable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with for the ten silicate-bearing IIE irons known at the time, recognizing only two subgroups—unfractionated (Netschaëvo, Techado, and Watson 001) and fractionated (Miles, Weekeroo Station, Colomera, Kodaikanal, Elga, Tarahumara, and NWA 5608). As a result of their classification of the silicated IIE Mont Dieu, Van Roosbroek et al. (2015) suggest that a five-stage division from most primitive to most differentiated is most useful as follows: 1) Mont Dieu and Netschaëvo; 2) Techado; 3) Watson; 4) Miles and Weekeroo Station; 5) Kodaikanal, Colomera, and Elga. On the other hand, an investigation of silicated IIE irons by McDermott et al. (2015) led them to propose a classification scheme that comprises only four categories, from most primitive to most differentiated as follows: primitive chondritic (Netschaëvo, Mont Dieu, Garhi Yasin, Techado) → evolved chondritic (Watson 001) → differentiated with high opx (>10 vol%; Weekeroo Station, Miles, Tarahumara) → differentiated with low opx (<10 vol%; Kodaikanal, Colomera, and Elga).
Schematic diagram of an impact melt pool origin for IIE irons
Diagram credit: McDermott et al., 45th LPSC #1910 (2014) Utilizing precise Hf–W chronometry in a study of nine IIE irons, Fisher-Gödde et al. (2016) ascertained ε182W values which attest to the occurrence of three separate metal–silicate segregation events on the parent body:
- 3.7–5.3 m.y. after Sub-millimeter to centimeter-sized amorphous objects found typically in carbonaceous chondrites and ranging in color from white to greyish white and even light pink. CAIs have occasionally been found in ordinary chondrites, such as the L3.00 chondrite, NWA 8276 (Sara Russell, 2016). CAIs are also known as refractory inclusions since they Click on Term to Read More (Colomera, Barranca Blanca, Arlington, Mont Dieu)
- 10–15 m.y. after CAIs (Weekeroo Station, Watson, Miles, Kodaikanal)
- ~27 m.y. after CAIs (Tarahumara)
In addition, on a CRE-corrected coupled ε100Ru vs. ε92Mo diagram, these nine IIE irons plot with ordinary chondrites indicating a probable genetic relationship; i.e., IIE irons formed through impact-generated melting on an Work in Progress Ordinary chondrites (OCs) are the largest meteorite clan, comprising approximately 87% of the global collection and 78% of all falls (Meteoritical Society database 2018)1. Meteorites & the Early Solar System: page 581 section 6.1 OC of type 5 or 6 with an apparent shock stage of S1, Click on Term to Read More parent body during several impact events over an extended period of time. In addition, the IVA irons plot with the IIE irons and ordinary chondrites, and all of these groups likely originated in a similar reservoir (see diagram below).
Diagram credit: Fisher-Gödde et al., 47th LPSC, #2704 (2016) In contrast to the ‘nonmagmatic’ IAB complex irons, the IIE precursor material contained a lower abundance of volatiles such as S and C, and consequently the melting temperatures were higher, resulting in silicates with nonchondritic compositions (Wasson and Wang, 1986). The metal–sulfide-rich H6 meteorite Y-791093 contains both chondritic and metal–sulfide components which are texturally, mineralogically, and compositionally similar to members of the H-chondrite group. It might be transitional between the H chondrites like Rose City and the primitive IIE irons with silicate inclusions like Netshaëvo (Ikeda et al., 1997). Similar to Miles, Y-791093 lacks a Thomson (Widmanstätten) structure and probably formed at a shallow depth rather than in a core.
click on photo for a magnified view Diagrams credit: Dey et al., 50th LPSC, #2977 (2019)
Prior interest in asteroid 6 Hebe as the source of the H chondrites has lost some favor after hydrocode model data revealed inconsistencies between expected and observed CRE ages based on the scenario of direct injection into resonances. Studies by Rubin and Bottke (2009) on this subject have led them to conclude that family-forming events resulting in large Small rocky or metallic object in orbit around the Sun (or another star). reservoirs having homogenous compositions, and which are located near dynamical resonances such as the Jupiter 3:1 mean motion resonance at 2.50 AU, are the most likely source of the most prevalent falls such as H chondrites and HED achondrites. See further details on the Abbott page. The specimen of Miles shown above is a 95 g partial slice showing abundant globular silicate inclusions.
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