Fayalitic olivine in matrix of the Krymka LL3.1 chondrite: Vapor‐solid growth in the solar nebula

Abstract

(This paper is dedicated to the memory of Paul Pellas) ) is a ubiquitous component in the matrix of Krymka (LL3.1) as well as in other highly unequilibrated chondrites (ordinary and carbonaceous). In Krymka, the fayalitic olivine has an unusual anisotropic platy morphology that occurs in at least five types of textural settings that can be characterized as: (1) isolated platelets, (2) clusters of platelets, (3) euhedral to subhedral crystals, (4) overgrowths of platelets on forsteritic olivine, and (5) fluffy (porous) aggregates. From transmission electron microscope (TEM) investigation, the direction of elongation of the platy olivine overgrowths on forsteritic olivine substrates is along the c axis and in most cases it corresponds with the c axis of the substrate olivine, which suggests that the fayalitic olivine grew in this unusual morphology and is not a replacement product of preexisting material. The fayalitic olivine in the matrix of Krymka is compositionally similar to olivine with platy morphology in the matrix of some CV3 chondrites and both have similar Fe/Mn ratios, but important morphological differences indicate that their relationship needs to be explored further. Textural and compositional data indicate that the fayalitic olivine in the matrix of Krymka, as well as in some other unequilibrated ordinary chondrites, formed prior to final lithification of the meteorite and probably prior to parent body accretion. We find that formation of the fayalitic olivine by vapor-solid growth provides the best explanation for our observations and data and is the only feasible mechanism for the formation of fayalitic olivine in the matrix of Krymka. We propose that the fayalitic olivine formed by vaporization and recondensation of olivine rich-dust, during a period of enhanced dust/gas ratio in the nebula.

Fayalitic olivine in CV3 chondrite matrix and dark inclusions: A nebular origin

“…They occur as single crystals, in clusters, and as overgrowths on forsterite substrates (Figs. Ig and 2h;Weisberg et al, 1997b). Similar lath-shaped olivine has also been found in the matrix of some CK chondrites (Scott et al, 1997).…”

“…The long axes of the platy crystals are orthogonal to the surface of the substrate forsteritic olivine crystals. Investigation by TEM of the platy olivine overgrowths on the forsteritic olivine substrates in the Krymka matrix reveals that the direction of elongation of the platy olivines is along the c axis and, in most cases, this corresponds with the c axis of the substrate forsterite (Weisberg et al, 1997b). In many cases, the boundary between the overgrowth and substrate is marked by a submicrometer-sized layer of chromite and pentlandite (Fig.…”

“…In the highly unequilibrated Krymka (LL3.1) ordinary chondrite, fayalitic olivine overgrowths occur as stacks of parallel-oriented platelets or laths, forming as epitaxial overgrowths on isolated forsteritic olivine fragments in the matrix (e.g., Fig. Ig;Weisberg et al, 1997b). The long axes of the platy crystals are orthogonal to the surface of the substrate forsteritic olivine crystals.…”

Mg-Rich Silicate Crystals in Comet Hale–Bopp: ISM Relics or Solar Nebula Condensates?

“…Pure-Mg silicate crystals (Fo 98 , En 98 ), however, are a rare component (∼1% by mass) of unequilibrated ordinary chondrites and carbonaceous chondrites (Bradley et al 1999). Often chondritic (Fo 50 ) and Fe-rich (∼Fo 0 ) olivine materials occur in meteorites and are associated with reheating events, where the Fe migrates back into the Mg-rich crystal lattice or forms an Fe-rich rim (Weisberg et al 1997, Krot et al 1997. Much of the silicate crystals in the meteoritic record have been altered, have chondritic composition, and thus have lost the record of their condensation from hot nebular gasses.…”

Steps toward interstellar silicate mineralogy

“…Fayalite has not been observed in circumstellar shells so far. However, fayalite has been found in the matrix of a chondritic meteorite indicating a vapour growth prior to parent body accretion (Weisberg et al 1997).…”