From quasiperiodicity to periodicity in Nature: Proxicosahedrite, Al52Cu30Fe11Si7, the first natural approximant of an icosahedral quasicrystal from a micrometeorite

Abstract

The discovery of naturally occurring quasicrystals has profoundly expanded the boundaries of mineralogy, demonstrating that atomic arrangements once thought to be restricted to synthetic materials can form in extraterrestrial environments. Here we describe proxicosahedrite, ideally Al52Cu30Fe11Si7, a new mineral species (IMA 2023-077c) occurring in a micrometeorite (FB-A1) recovered from Mt. Gariglione, Calabria, southern Italy. Proxicosahedrite occurs as subhedral grains 1–2 μm in size intimately associated with stolperite, khatyrkite and Cu-bearing metallic aluminum within a vesicular Al–Cu–Fe-bearing microspherule approximately 500 μm across. Associated phases include forsterite, spinel, magnetite, taenite, awaruite, heazlewoodite, metallic iron and silicate glass. Electron-microprobe analyses yield the average composition Al 33.51(0.75), Cu 46.50(1.30), Fe 14.33(0.65) and Si 4.59(0.60) wt.%, corresponding to the empirical formula Al51.9Cu30.6Fe10.7Si6.8 and the simplified formula Al52Cu30Fe11Si7 based on 100 atoms. The mineral is cubic, space group Pm–3, with a = 12.3443(4) Å, V = 1881.04(11) Å3 and Z = 1. Because of the extremely small crystal size, structural characterization was achieved through focused-ion-beam extraction, transmission electron microscopy, three-dimensional electron diffraction and dynamical structure refinement. The resulting structure corresponds to the 1/1 cubic approximant of the Al–Cu–Fe–Si icosahedral quasicrystal and is equivalent to the synthetic phase Al55Si7Cu25.5Fe12.5.

Secondary-ion mass spectrometry measurements of associated olivine grains reveal oxygen-isotope compositions that plot along the carbonaceous-chondrite anhydrous-mineral line and outside the terrestrial fractionation field, confirming the extraterrestrial origin of the host micrometeorite. Textural relationships, isotopic signatures and crystallographic data collectively exclude anthropogenic contamination and demonstrate the natural origin of the mineral.

Proxicosahedrite represents the first natural crystalline approximant of an icosahedral quasicrystal and the third natural quasicrystal approximant recognized to date. Its occurrence provides further evidence that highly reduced metallic assemblages formed under extreme conditions in the early Solar System and reveals that both quasiperiodic and periodic manifestations of the Al–Cu–Fe–Si structural family occur naturally. The discovery significantly broadens the known diversity of intermetallic minerals and establishes a direct crystallographic bridge between conventional crystals and natural quasicrystals.