Fluorapatite Breakdown and Fluor-edenite Formation in the Pristine Winonaite Calate 047 from Chile's Atacama Desert

Pavel Plechov , Ksenia Konovalova, Viktor Gekimyants, Vasily Shcherbakov

We report the discovery and characterization of Calate 047, a new amphibole-bearing winonaite found in Chile's Atacama Desert. This meteorite exhibits a fine-grained granoblastic texture with equigranular silicates (70-350 μm) and extremely magnesian compositions (olivine Fa1.6-1.8, low-Ca pyroxene Wo1.46-1.7Fs0.44-1.17, high-Ca pyroxene Wo45.26-45.81Fs0.70-0.93, plagioclase An17.7-18.8). Calate 047 contains fluor-edenite amphibole (grains up to 250 μm) that lack poikilitic textures observed in previously described amphibole-bearing winonaites. Raman spectroscopy confirms the dominance of fluorine in the W-site, with characteristic peaks at 676 cm⁻¹ and multiple bands between 1000-1109 cm⁻¹. Oxygen fugacity calculated from olivine composition indicates extremely reduced conditions (IW-2.7 to IW-2.8). This study establishes fluor-edenite as a systematic mineral phase in highly reduced winonaites, resolving the paradox of fluorine-rich amphibole formation in apatite-poor meteorites. We demonstrate that fluorapatite decomposition below IW-2.3 releases fluorine during reduction to schreibersite, providing the essential component for amphibole crystallization through solid-state metamorphic reactions. The absence of intermediate phosphates (merrillite, chladniite) in Calate 047 confirms direct transformation of apatite to schreibersite under extreme reduction.
As the third documented case of amphibole in winonaites, this finding provides compelling evidence for volatile-mediated metasomatism in the IAB-winonaite parent body. The correlation between amphibole occurrence and oxygen fugacity below IW-2.3 establishes a new mineralogical indicator for the most reducing conditions recorded in solar system materials, constraining redox evolution models for early planetary bodies.