Trace Element Conundrum of Natural Quasicrystals

Simone Tommasini,Paul J Steinhardt,Luca Bindi,Paul D Asimow,Maurizio Petrelli

doi:10.1021/acsearthspacechem.1c00004

Abstract

We report laser ablation inductively coupled plasma mass spectrometry measurements of the trace element contents of the two naturally occurring quasicrystalline minerals, Al63Cu24Fe13 icosahedrite and Al71Ni24Fe5 decagonite, from their type locality in the Khatyrka meteorite. The isolated quasicrystal fragments were mounted separately from any matrix and are larger than the laser beam diameter. When the elements are sorted in order of volatility, a systematic and unique pattern emerges in both bulk natural quasicrystal specimens. They are highly depleted compared to primitive solar system materials (chondritic meteorites) in moderately refractory elements (those with 50% condensation temperatures near 1350–1300 K; V, Co, Mg, Cr) and significantly enriched in moderately volatile elements (those with 50% condensation temperatures between 1250 and 500 K; Sb, B, Ag, Sn, Bi). We compare the chondrite-normalized trace element patterns and ratios of the quasicrystals to those of scoriaceous cosmic spherules and other meteoritic components. The nonmonotonic shapes of the chondrite-normalized trace element patterns in both icosahedrite and decagonite are incompatible with a single condensation process from the gas of the solar nebula. Previous transmission electron microscopy studies show that the natural quasicrystals contain 3–5 vol % of silicate and oxide nanoparticle inclusions, which we consider to be the main host of the measured trace elements. On this basis, we construct a three-stage model for the formation of the quasicrystals and their inclusions: a high-temperature condensation stage and a low-temperature vapor-fractionation stage to make nanoparticles, followed by a third stage that leads to the formation of quasicrystals incorporating the two different types of nanoparticles and their incorporation into the CV chondrite parent body of the Khatyrka meteorite.

Highlights

Khatyrka is a peculiar CV-type meteorite that contains crystalline Cu−Al alloys.[1]
The trace element concentrations were determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) at the Department of Physics and Geology, University of Perugia, Italy.[18−20] The analyses were carried out with a Teledyne Photon Machine G2 laser ablation system coupled to a Thermo Fisher Scientific iCAP-Q quadrupole-based ICPMS
The temporal instability in the count rate that leads in some cases to large relative errors may be due to the fact that the trace elements are not uniformly distributed within the volume sampled by the laser beam (∼5 × 105 μm3)

Summary

Introduction

Khatyrka is a peculiar CV-type meteorite that contains crystalline Cu−Al alloys.[1] It contains quasicrystals (QCs), short for “quasiperiodic crystals”,2,3 materials well studied for the last 35 years in the laboratory but exceedingly rare in nature. QCs are built from quasiperiodic arrangements of atoms and exhibit rotational symmetries forbidden to ordinary crystals (e.g., fivefold). The origin of these unusual metallic alloys is enigmatic as they contain metallic aluminum, which forms under highly reducing conditions not normally found in nature. Previous and successive studies provided compelling evidence that the quasicrystals are natural and from a common meteoritic source, namely, (1) 14C-dating of material from undisturbed clay layers where some of the samples were collected yielded 6.7−8.0 ka BP;[1,5]

Methods

Results

Conclusion