The Mineralogical Society of Southern California

A garnet collected as a teenager in Arizona’s Aquarius Mountains carries a nanometers-thin surface film producing metallic green iridescence with no precedent in the mineralogical literature. Worn edges show ordinary black almandine beneath, confirming the color is a coating, not a new species. X-ray fluorescence reveals that coated and uncoated garnets from the same outcrop — visually identical in hand specimen — are chemically distinct populations: the coated garnets carry elevated P, S, Ca, Sc, Cr, and Zn with a high K/Rb ratio pointing to a mafic-influenced volcanic fluid pulse, while the uncoated garnets record a more evolved, felsic source. Raman spectroscopy, laser-induced breakdown spectroscopy, SEM imaging, and EDS elemental mapping then resolve the coating’s internal structure: a layer of hematite (Fe₂O₃) directly on the garnet surface, overlain by nitrogen- and phosphorus-bearing organic carbon composed of packed, sub-micron, coccoid-bacteria-like structures (C:N ≈ 4.9:1). The evidence supports a model in which iron-oxidizing bacteria selectively colonized garnets whose trace-element chemistry made lattice Fe²⁺ more accessible as an electron donor, converting it to hematite and leaving fossilized biomass behind — turning a 20-million-year-old vapor-phase garnet into an unexpected archive of desert microbial activity.

 

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