Mackinawite structure

Consistent with this, Vaughan and Ridout (56) have proposed extensive delocalization of d electrons within mackinawite layers and electronic band structure calculations indicate that mackinawite is a metallic conductor with a conduction band of mainly d character (57). The Fe-Fe metallic bonding in mackinawite results in a Fe oxidation state somewhat less than +11, so a non-stoichiometric formula is necessary for electroneutrality. Certain studies indicate that mackinawite non-stoichiometry is due to periodic sulfur vacancies in the crystalline lattice, resulting in the formula FeSi.x, where x 0.025 (52), while others report evidence for the presence of excess Fe atoms in the mackinawite crystalline lattice, resulting in the non-stoichiometric formula Fei+xS, where x = 0.04-0.07 (38). A more recent investigation of mackinawite structure (55) has not resolved the question of sulfur vacancies versus excess iron atoms. 

Square metal nets are found in the tetragonal modifications of FeQ (Q=S, Se, Te) the Fe-Fe distances in the mackinawite modification of FeS are only 2.60 A.230 There are also metallic phases T1M2Q2 (M=Co, Cu Q=S, Se) and TlFe2Se2 that all adopt the ThCr2Si2 structure type, where the TM atoms form flat square nets with short M-M distances (2.65-2.75 A).232... 

Figure 10.2 Crystal structures of (a, b, c) natural iron and (d) zinc sulphides (a) greigite, (b) mackinawite, (c) smythite, and (d) sphalerite...

Lennie AR, Redfem SAT, Schofield PF, Vaughan DJ. Synthesis and Rietveld crystal structure refinement of mackinawite, tetragonal FeS. Miner Mag 1995 59 677-83. 

Fig. 7a-d. Idealised lepresentations of various layer types observed in non-commensurate layer structures (a) octahedral layer, MX2, (111) B1 type (b) galena-like half octahedral (square pyramidal) layer, MX, (100) B1 type (c) the valleriite-like tetrahedral layer, MX, (111) anti-C 1 type (d) the mackinawite-like tetrahedral layer, MX, (100) anti-Cl type... 

In phase 2 the mackinawite-like sulphide layer (with a chess-board pattern of partly-occupied sites) is said to alternate with an open hydrate/hydroxide layer which has additional water molecules between the octahedra. There is room for some doubt about the structural details of this latter layer set, but the compound does have an SC structure though perhaps without any clear modulation in the semicommensurate direction. Drits (private communication, 1979) reports that the electron diffraction patterns of some... 

The sulphides of iron include Fe3S4 (spinel structure), Fe7Sg (pyrrhotite), FeS (troilite), and FeS2 (pyrites and marcasite). Ferrous sulphide rarely has the Fe S ratio precisely equal to unity, though stoichiometric FeS can be prepared. A microcrystalline form prepared by precipitation has been examined by X-ray powder photography and also by electron diffraction. This form (mackinawite) has... 

As mentioned above, the mackinawite-type FeS frequently forms with deviations from stoichiometry as is always the case for metallic [1013] Fci+gSe, the mineral achavalite, and the weakly ferro- or ferrimagnetic tellurides Fei.osTe—Fei.2Te. It was always assumed that the excess cations were inserted in the octahedral holes, in other words, that these phases were representatives of the defect Cu2Sb type. From a structure refinement on mackinawite, Taylor and Finger [337] concluded, on the contrary, that there is a slight deficiency of sulfur in the structure and not a metal excess, so that the formula should be written as FeSi-. According to [1043] FeSe is metallic and Pauli paramagnetic. Compare [1044]. 

The stoichiometric monotelluride CuTe, the yellow-bronze colored mineral vulcanite, on the other hand, does crystallize in a true layer structure which, however, represents a fairly distorted version of the mackinawite type (Figure 74). The formerly planar square net of the tetrahedrally coordinated metal atoms is now a slightly puckered Cu plane. Moreover, the lattice is compressed along [100] which leads to the formation of straight Te chains along the a-axis with Te—Te =... 

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