^1,2Julian Alfing,¹Markus Patzek,¹Addi Bischoff
Geochemistry (Chemie der Erde) Link top Article [https://doi.org/10.1016/j.chemer.2019.08.004]
¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm Str. 10, D-48149, Münster, Germany
²Institut für Mineralogie, Westfälische Wilhelms-Universität Münster, Corrensstr. 24, D-48149, Münster, Germany
Copyright Elsevier

For the bulk rocks of CI chondrites, various values are given for the modal abundance of matrix (95–100 vol%) and the accompanying mineral constituents. Here, we have determined the modal abundance of phases >5 μm in the CI chondrites Orgueil, Ivuna, Alais, and Tonk. Considering this cut-off grain-size to distinguish between matrix and coarse-grained constituents, then, we find the modal abundance of the minor phases magnetite, pyrrhotite, carbonate, olivine, and pyroxene to be 6 vol% in total, and these phases are embedded within the fine-grained, phyllosilicate-rich matrix, making up 94 vol%. The values vary slightly from meteorite to meteorite. Considering all four chondrites, the most abundant phase is – by far – magnetite (4.3 vol%) followed by pyrrhotite (∼1.1 vol%). All four CI chondrites are complex breccias, and their degree of brecciation decreases in the sequence: Orgueil > Ivuna > Alais ∼ Tonk. Because these meteorites contain clasts with highly variable modal abundances, we therefore also studied individual clasts with high abundances of specific coarse-grained phases. In this respect, in Orgueil we found a fragment with a 21.5 vol% of magnetite as well as a clast having 31.8 vol% phosphate. In Ivuna, we detected an individual clast with a 21.5 vol% of carbonates. Thus, since the CI composition is used as a geochemical standard for comparison, one also should keep in mind that sufficiently large sample masses are required to reveal a homogeneous CI composition. Small aliquots with one dominating lithology may significantly deviate from the suggested standard CI composition.

¹Kurt Liffman
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.08.009]
¹Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia
Copyright Elsevier

Type 3 chondritic meteorites often contain chondrules and refractory inclusions that are coated with accretionary, fine-grained rims (FGRs). FGRs are of low porosity, were subject to centrally directed pressure, may contain high temperature products like microchondrules and there is a linear relationship between the rim thickness and the radius of the enclosed object.

FGRs are thought to have formed by the gentle adhesion of dust onto the central object with the subsequent compression of this fluffy rim within the parent body. However, this model does not explain the low porosity, micro-chondrules and centralized pressure. This model also has difficulties explaining the linear relationship between rim thickness and object size including the existence of a non-zero constant in that linear relationship.

We propose that FGRs formed by the relatively high-speed interaction between dust and the object, where high initial impact speed produced abrasion and, possibly, microchondrules. FGR formation occurred over a range of lower speeds aided by vacuum adhesion of fragments from the impacting dust particles. This model naturally produces the rim thickness linear relationship with non-zero constant, low porosity and centrally directed pressure. We call this process kinetic dust aggregation (KDA), which is another name for the aerosol deposition processes used in industry. KDA may be a tentative, part explanation of how dust aggregation occurs in protostellar disks on the pathway from dust to planets.

^1,2Fabrizio Campanale,²Enrico Mugnaioli,¹ Luigi Folco,²Mauro Gemmi,³Martin R.Lee,³Luke Daly,⁴Billy P.Glass
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.08.014]
¹Dipartimento di Scienze della Terra, Università di Pisa, V. S. Maria 53, 56126 Pisa, Italy
²Center for Nanotechnology Innovation@NEST, Istituto Italiano di Tecnologia (IIT), Piazza San Silvestro 12, 56127 Pisa, Italy
³Department of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK
⁴Department of Geosciences, University of Delaware, Newark, DE, USA
Copyright Elsevier

Coesite, a high-pressure silica polymorph, is a diagnostic indicator of impact cratering in quartz-bearing target rocks. The formation mechanism of coesite during hypervelocity impacts has been debated since its discovery in impact rocks in the 1960s. Electron diffraction analysis coupled with scanning electron microscopy and Raman spectroscopy of shocked silica grains from the Australasian tektite/microtektite strewn field reveals fine-grained intergrowths of coesite plus quartz bearing planar deformation features (PDFs). Quartz and euhedral microcrystalline coesite are in direct contact, showing a recurrent pseudo iso-orientation, with the [11¯1]* vector of quartz near parallel to the [010]* vector of coesite. Moreover, discontinuous planar features in coesite domains are in textural continuity with PDFs in adjacent quartz relicts. These observations indicate that quartz transforms to coesite after PDF formation and through a solid-state martensitic-like process involving a relative structural shift of {1¯011}
quartz planes, which would eventually turn into coesite (010) planes. This process further explains the structural relation observed between the characteristic (010) twinning and disorder of impact-formed coesite, and the 101¯1 PDF family in quartz. If this mechanism is the main way in which coesite forms in impacts, a re-evaluation of peak shock pressure estimates in quartz-bearing target rocks is required because coesite has been previously considered to form by rapid crystallization from silica melt or diaplectic glass during shock unloading at 30-60 GPa.