Hidden metallic iron in amorphous silicate dust?

Insights from condensation experiments and mid-infrared spectroscopy

¹ Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan
² Department of Environmental Science and Technology Science, Osaka Sangyo University, Osaka 574-8530, Japan

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Received: 16 June 2025
Accepted: 26 November 2025

Abstract

Context. Amorphous silicate dust is a major component in the interstellar and circumstellar dust formed in the outflow of asymptotic giant branch (AGB) stars. Although iron depletion is observed in the interstellar medium, the exact form and fraction of iron in solids remains a matter of debate. In particular, it is unclear whether the amorphous silicate dust around AGB stars contains metallic iron.

Aims. We aimed to provide optical constants of amorphous silicate nanoparticles and examine the effects of metallic iron on their spectral features to better constrain the dust properties. We did this by producing amorphous silicate nanoparticles with and without metallic cores.

Methods. We performed condensation experiments using an induction thermal plasma system to produce dust analogs of the CI chondritic composition in Mg–Ca–Na–Al–Si–Fe–Ni–O and Mg–Ca–Na–Al–Si–O systems. We measured the absorbance and reflectance of the samples, observed the structure of the products, and determined the optical constants.

Results. Two types of amorphous silicate nanoparticles (ϕ10−200 nm) with nearly CI chondritic compositions were produced: one contained kamacite (Fe_0.9Ni_0.1) cores with a diameter ratio ranging from 0 to 0.87 (average 0.50), and the other was iron-free homogeneous amorphous silicate. The amorphous silicates of the CI chondritic composition with various-sized metallic cores may be prevalent in circumstellar and interstellar dust.