Native Silicon and Fe-silicides from the Apollo 16 Lunar Regolith: Extreme Reduction, Metal-silicate Immiscibility, and Shock Melting

Silicon in metallic form – dissolved in Fe-Ni metal, as silicide, or as Si 0 – is rare in solar system materials because its formation requires extremely reducing temperatures or oxidation states or both. Fe-Ni metal in carbonaceous chondrites can contain small proportions of Si [1], as does metal in shocked enstatite chondrites and aubrites [2]. Iron silicides are known from ureilite meteorites [3,4], where extreme reduction and shock heating are both significant. Silicides are also reported from the Stardust mission comet-return samples, both as indigenous phases [5] and as products secondary to dust capture [6]. Iron silicides have recently been reported in fragments of lunar regolith, and are inferred to represent deposits from vapor generated in impact events [7]. These lunar silicides include Fe 3 Si (hapkeite), Fe 2 Si, and FeSi [7]. Here, we report phases even richer in silicon – native silicon and iron silicides (with Fe:Si ratios < 1) – in anorthositic fragments from the Apollo 16 re-golith. Textures suggest that these materials formed as the result of silicate-metal immiscibility driven by high temperature shock melting. Such silicon-rich phases are known in terrestrial samples, but are extremely rare. The best-documented occurrence is in a fulgurite [8], where extremely reducing conditions arose from the combination of organic carbon (from soil and plant roots) and the intense heating of a lightning strike; Native silicon coexisting with Fe-silicides (FeSi, Fe 3 Si 7) was identified. Fe-silicides are found in other fulgurites [9]. Other reports of native silicon include volcanic sublimates [10], inclusions in moissanite from kimberlite [11], inclusions in terrestrial Fe-Ni metal [12] (likely to be silicides) and detrital grains in heavy sands [13] (possibly human artifacts). Samples and Methods. During a search for zircon in the >20µm fraction of Apollo 16 regolith sample 61501,22, two grains (A6-8 and A6-7) were found to contain native silicon. Chemical analyses for A6-8 were obtained at the University of Wisconsin by electron microprobe (Cameca SX51). SEM, BSE, forescatter and EBSD analyses were obtained using a Hitachi 3400N. Petrography: Although lunar grain A6-8 is relatively small (~90 µm diameter) it displays great complexity (Fig. 1). An EPMA traverse (A-B) across the major features of this grain (numbered #1-#6 in Fig 1A) is shown in Figure 2. The sample is cut by a Si-rich vein-like feature (#3) that is bright in BSE. In the upper portion of Fig. 1A, the relatively uniform grey phase …