The crystal chemistry of the humite minerals: spectroscopic studies and structure refinement of an unusual iron-rich clinohumite

Abstract

Abstract An orange-red mineral with strong pleochroism from magnesian skarns of the Salminski rapakivi massif which was originally described as an olivine,is identified as an unusually iron-rich clinohumite.The composition of the mineral is,as obtained by combining data of electron microprobe analyses,Mössbauer analyses and unconstrained structure refinements: [Mg5.2(Fe3.42 2+Mn0.07Ni0.01Zn0.04Ca0.01Fe0.11 3+ Ti0.04 4+)Σ=3.70] × [Si4.03O16[F1.21(OH,O)0.79]Σ=2.00]. Thus,the Fe-end member fraction is X Fe = 0.42, i.e. unusually high when compared with those of the usual magnesian clinohumites, e.g. = 0.06 in the crystal studied structurally by Robinson et al.(1973). Fe is the sumof octahedral,Mg-substituting cations which could - due to the similarity of their scattering curves and the very low fractions except for that of iron - not be discriminated in the structure refinement. Such refinements yield the following Fe-fractionations, f Fe, between the different octahedral sites of the structure: f Fe(M1 c ) =0.17, f Fe(M1 n ) = 0.33, f Fe(M26) = 0.32, f Fe(M25) = 0.13, f Fe(M3) = 0.05 (ΣFe = 1.00). The analysis of the Mössbauer spectra yields a fractionation pattern very close to that obtained by XRD. The Fe-fractioning in the ferrous clinohumite with X Fe = 0.42 is nearly the same as that in magnesian clinohumite with X Fe = 0.06, such similarities being obviously related to the fact that a slightly distorted hexagonally close anion package forms the basis of all the humite mineral structures. The structural changes on iron incorporation pertain predominantely changes in the individual and mean M–O distances and O–M–O angles. Polyhedral distortions Δ oct and σ oct 2 increase most drastically, compared to magnesian clinohumite with X Fe = 0.06, in the M26 octahedra. The polarized electronic absorption spectra are dominated by two features: an intense, broad band centered at 22800 cm-1 strongly polarized E ∥ X (nα ∢ b ca. 10°), which was identified as Fe2+(M25)-Ti4+(M3) charge-transfer transition in the M25M3M3M25 octahedral chain fragments of the structure. The strong polarization of this band together with the position of the UV absorption edge,causes the pleochroismof the mineral.The second prominent feature in the polarized single crystal spectra is a complex band system in the NIR which is assigned to dd-transitions of Fe2+(M1 c + M1 n ), bands at 10800 and 7600 cm-1, and of Fe2+(M26), bands at 9400 and 8800 cm-1. Such transitions are in analogy with the behaviour of Fe2+ in the M1 and M2 sites of the olivine structure.