Discovery of Buseckite, (Fe,Zn,Mn)S, a New Mineral in Zakłodzie, an Ungrouped Enstatite-rich Achondrite

Introduction: The Zakłodzie meteorite, a moderately weathered find discovered near the village Zakłodzie, Poland, is an ungrouped enstatite-rich achondrite ascribed to an impact melt [1] or internal melting within the parent body [2]. During our nanomineralogy investigation of the Zakłodzie meteorite, a new Fe-dominant monosulfide mineral, (Fe,Zn,Mn)S with the P63mc wurtzite-type structure was identified and named “buseckite”. We used electron probe microanalysis (EPMA), high-resolution scanning electron microscope (SEM), electron backscatter diffraction (EBSD), and micro-Raman to characterize its composition and structure. (Fe,Zn,Mn)S phases similar in composition to buseckite were previously found through EPMA of sulfides in Zakłodzie [3], Yilmia (EL6) [4], and Grein 002 (EL4/5) [5]. We report here the first confirmed (Fe,Zn,Mn)S mineral with a wurtzite-type structure and consider the origin of this phase and implications through its formation and survival for the evolution of the Zakłodzie meteorite. The mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 2011070). The name is in honor of Peter R. Buseck, a mineralogist at the Arizona State University, for his many contributions to mineralogy, meteorite research and transmission electron microscopy. Appearance, Chemistry and Crystallography: Buseckite occurs as scattered irregular to subhedral crystals (4 20 μm in dia.) along grain boundaries between two or more of enstatite, sodic feldspar [K-rich (An2.2Ab85.1Or12.7) to low-K, moderately calcic plagioclase (An20.9Ab76.6Or2.5)], quartz, tridymite, and troilite (Figs. 1-2). One buseckite grain contains euhedral laths of sinoite. Type buseckite is black and opaque with a mean chemical composition of (wt%) S 35.84, Fe 28.68, Zn 23.54, Mn 10.04, Mg 1.18, sum 99.28. The empirical formula, based on 2 atoms pfu, is: (Fe0.46Zn0.32Mn0.16Mg0.04)S1.01, yielding a simplified formula of (Fe,Zn,Mn)S. We observed no significant within grain or grain-to-grain differences in composition among buseckites. EBSD patterns of buseckite can only be indexed using the wurtzite structure and this yields a best fit based on unit cell data for synthetic (Zn0.558Fe0.442)S [6]. Buseckite has a hexagonal close packed structure, Space Group: P63mc, showing a = 3.8357 Å, c = 6.3002 Å, V = 80.27 Å and Z = 2. Occurrence and Associated Minerals: Buseckite occurs with enstatite, plagioclase, sinoite, troilite [Fe0.86Cr0.06Mn0.02Ti0.02)S], quartz, tridymite, cristobalite, low-Ni iron, martensitic iron, schreibersite, keilite, and graphite. One or both of enstatite and silica are in contact with each buseckite grain but alloys, which comprise roughly 20 volume % of the meteorite [2], are not. From EBSD and/or Raman, silica in Zakłodzie is mostly tridymite. Quartz is associated with sinoite; we observed one cristobalite grain next to tridymite. Origin and Significance: Zakłodzie is generally thought to be an impact melt of an enstatite-rich precursor (e.g., [1] but see [2]). Overall, the silicates imply peak temperatures above ~1500°C followed by rapid cooling. High temperature phase relations for the opaque assemblage can be treated, ignoring graphite and minor Mn-Zn-Ca sulfides, in terms of the Fe-Ni-S system. For 20 volume % alloy and 10 % sulfide, average densities of 8 and 4 g/cm, respectively, and assuming that the sulfide is dominated by troilite, we Fig. 1. SEM secondary electron (SE) image showing a buseckite grain with plagioclase and enstatite.