The high-pressure behavior of the heavier group 15 elements As, Sb, and Bi was investigated by means of ab initio density functional calculations employing pseudopotentials and a plane wave basis set. The high-pressure structural sequence of these elements is distinguished by the occurrence of the Bi-III structure, which is a complex, incommensurately modulated, host-guest structure. We approximated this structure by a supercell which reproduced the experimentally established pressure stability ranges of the host-guest structure for the different elements extremely well. With pressure we find an increasing admixture of d states (s-d hybridization) in the occupied levels of the electronic structure of As, Sb, and Bi. However, the s-d mixing remains at a low level. Thus, the emergence of a complex intermediate pressure structure cannot be explained by a pressure-induced altered valence state for these elements. Instead, it is argued that the Bi-III structure is a consequence of a delicate interplay between the electrostatic and the band energy contribution to the total energy. In the intermediate pressure range of heavier group 15 elements, both important parts of the total energy account equally for structural stability.

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We present microstructural investigations of Laves phases AB

Rinmanite, ideally Zn 2 Sb 2 Mg 2 Fe 4 O 14 (OH) 2 , is a new mineral species from the Garpenberg Norra Zn-Pb mine, Hedemora, Dalarna, in south-central Sweden, where it occurs in a skarn assemblage associated with tremolite, manganocummingtonite, talc, franklinite, barite and svabite. Rinmanite crystals are prismatic, up to 0.5 mm in length, with good {100} cleavage. The VHN 100 is in the range 841-907. D calc = 5.13(1) g•cm-3. The mineral is black (translucent dark red in thin splinters) with a submetallic luster. The mineral is moderately anisotropic and optically uniaxial (-). Reflectance values measured in air are 13.5-12.1% (= 470 nm), 12.9-11.8% (546 nm), 12.6-11.7 (589 nm) and 12.2-11.3% (650 nm). Electron-microprobe analyses of rinmanite (wt.%) gave MgO 8.97, Al 2 O 3 0.82, MnO 2.47, Fe 2 O 3 34.33, ZnO 14.24, Sb 2 O 5 36.31, H 2 O 1.99 (calculated), sum 99.13, yielding the empirical formula (Zn 1.58 Mn 0.31 Mg 0.06) ⌺1.95 Sb 2.03 [Mg 1.95 Fe 3.88 Al 0.15 ] ⌺5.98 O 14.01 (OH) 1.99. Rinmanite is hexagonal, space group P6 3 mc, with a 5.9889(4), c 9.353(1) Å, V 290.53(5) Å 3 and Z = 1. A structure refinement from single-crystal MoK␣ X-raydiffraction data (R = 3.1% for 199 reflections) shows that rinmanite is isostructural with nolanite. The octahedral M(1) and M(2) sites are occupied by (Mg + Fe) and Sb, respectively, whereas Zn is located at the 4-coordinated M(3) site. The strongest eight lines in the powder-diffraction pattern [d in Å (I obs)(hkl)] are: 5.19(20)(100), 3.47(34)(102), 2.99(43)(110), 2.67(44)(103), 2.522(100)(112), 1.660(28)(213), 1.517(33)(205) and 1.497(54)(220). Mössbauer spectroscopy confirms that rinmanite contains Fe 3+ at octahedral sites and is in a paramagnetic state down to at least 77 K. Rinmanite is believed to have formed during prograde metamorphism of an initially lowS , siliceous carbonate protolith enriched in Zn, Sb and Fe, at relatively high P(H 2 O). The new mineral species is named after Sven Rinman (1720-1792), of Bergskollegium.