Yassir A. Abdu

The crystal structures of orthopyroxene (En_86.3Fs_8.6Wo_5.1, space group Pbca) and pigeonite (En_81.7Fs_8.8Wo_9.5, space group P2₁/c) from the Almahata Sitta ureilite (fragment#051) have been refined to R1 indices of 3.10% and 2.53%, respectively, using single‐crystal X‐ray diffraction data. The unit formulas were calculated from electron microprobe analysis, and the occupancies at the M1 and M2 sites were refined for both pyroxenes from the single‐crystal diffraction data. The results indicate a rather disordered intracrystalline Fe²⁺‐Mg cation distribution over the M1 and M2 sites, with a closure temperature of 726(±55)°C for orthopyroxene and 704(±110)°C for pigeonite, suggesting fast cooling of these pyroxenes. The Mössbauer spectrum of the Fe‐Ni metal particles of Almahata Sitta ureilite (fragment#051) is dominated by two overlapping magnetic sextets that are assigned to Fe atoms in Si‐bearing kamacite, and arise from two different nearest‐neighbor configurations of Fe* (=Fe+Ni) and Si atoms in the bcc structure of kamacite; (8F*, 0Si) and (7Fe*, 1Si). In addition, the spectrum shows weak absorption peaks that are attributed to the presence of small amounts of cohenite [(Fe,Ni)₃C], schreibersite [(Fe,Ni)₃P], and an Fe‐oxide/hydroxide phase. The fast cooling of pyroxene to the closure temperature (after equilibration at ~1200°C) and the incorporation of Si in kamacite can be interpreted as due to a shock event that took place on the meteorite parent body, consistent with the proposed formation history of ureilites parent body where a fast cooling has occurred at a later stage of its formation.

Four Ca-rich clinopyroxene samples (3 terrestrial clinopyroxenes and a druse clinopyroxene from the D’Orbigny meteorite) have been studied by near-infrared (NIR) spectroscopy and X-ray structure refinement in order to address the issue of anomalous type-B NIR spectra seen in some terrestrial and extraterrestrial Ca-rich clinopyroxenes. The D’Orbigny druse clinopyroxene shows a typical type-B NIR spectrum, with two absorption bands located at ~ 1.1 and 2.3 μm. Based on previous Mössbauer and X-ray structure refinement results on synthetic clinopyroxenes, the 1.1 μm band is assigned to Fe²⁺ at the M1 site in diopside and high-pigeonite (i.e. pigeonite with C2/c symmetry), and the 2.3 μm band to Fe²⁺ at the M2 site in high-pigeonite. A comparison of NIR spectral type and X-ray structure refinement of the studied samples indicates that the average structure refinement of all type-B clinopyroxenes show a residual electron density peak in the difference-Fourier synthesis, corresponding to the split M2’ site which is mainly occupied by Fe²⁺. The results present a new interpretation for type-B NIR spectra of Ca-rich clinopyroxenes.

Folvikite, Sb5+Mn3+(Mg,Mn2+)(10)O-8(BO3)(4), is a new oxyborate mineral from the Kitteln mine, Varmland, Sweden, where it occurs as a primary skarn mineral embedded in calcite. It forms striated prismatic crystals up to 0.3 mm, and is black to dark reddish-brown with submetallic lustre and a reddish-brown streak. It is brittle, has a Mohs hardness of 6, and the calculated density is 4.14 g/cm(3). Folvikite is biaxial with indeterminate optic sign due to pervasive twinning. The optic axial angle is 68.9(4)degrees. Refractive indices were not measured; the calculated mean refractive index is 1.85. Strong pleochroism was observed in plane-polarized light: AB = brown (intermediate), OB = dark brown (maximum) and ON = honey brown (minimum). Folvikite is monoclinic, space group P2, a = 5.3767(10), b = 6.2108(10), c = 10.9389(18) angstrom, beta = 94.399(9)degrees, V = 364.22(16) angstrom(3) and Z = 1. Chemical analysis by electron microprobe gave Sb2O5 18.15, MgO 24.11, MnO 29.73, Mn2O3 11.62, Al2O3 0.27, Fe2O3 0.45, B2O3 15.27, sum 99.60 wt.%. The B2O3 content was assigned as B = 4 apfu and the Mn2O3 / (MnO + Mn2O3) ratio was determined from the crystal structure. The empirical formula was normalized on the basis of 20 anions pfu: (Sb1.025+Mn1.343+Al0.05Fe0.053+Mg5.46Mn3.822+square(0.26))(Sigma 12)O-8(BO3)(4). A simplified formula may be written as Sb5+Mn3+(Mg,Mn2+)(10)O-8(BO3)(4) with Z = 1. The crystal structure was solved by direct methods and refined to an R-1 index of 4.1%. Folvikite is a member of the (3 angstrom) zigzag wallpaper-borate structures in which chains of edge-sharing octahedra extend along the c axis and are cross-linked by BO3 groups. There are five X sites partly occupied by Mn2+ > Mg, one octahedrally coordinated M-site occupied by Sb5+ > Mg, two M sites occupied by Mg >= Mn > Sb5+, two M sites occupied by Mn3+ > Mn2+, two M sites occupied by Mg > Mn2+, and one M-site occupied by Mg > square; plus two [3]-coordinated B sites occupied by B. As with the other zigzag borates, the polyhedra are arranged in F-walls, C-walls and S-columns.