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

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 mi neral is, as obtained by combining data of electron microprobe analyses, Mo ssbauer analyses and unconstrained structure refinements: [Mg-5.2(Fe3.422+M n0.07Ni0.01Zn0.04Ca0.01Fe0.113+Ti0.044+)(Sigma =3.70)] x [Si4.03O16[F-1.21( OH,O)(0.79)](Sigma =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 clino humites, e.g. = 0.06 in the crystal studied structurally by Robinson rt al. (1973). Fe is the sum of octahedral, Mg-substituting cations which could - due to the similarity of their scattering curves and the very low fraction s except for that of iron - not be discriminated in the structure refinemen t. Such refinements yield the following Fe-fractionations, f(Fe), between t he different octahedral sites of the structure: f(Fe)(Ml(c))=0.17, f(Fe)(Ml (n))=0.33, f(Fe)(M2(6))=0.32, f(Fe)(M2(5))= 0.13, f(Fe)(M3) = 0.05 (Sigma F e = 1.00). The analysis of the Mossbauer spectra yields a fractionation pat tern 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 clinohum ite with X-Fe=0.06, such similarities being obviously related to the fact t hat a slightly distorted hexagonally close anion package forms the basis of all the humite mineral structures. The structural changes on iron incorpor ation pertain predominantely changes in the individual and mean M-O distanc es and O-M-O angles. Polyhedral distortions Delta (oct) and sigma (2)(oct) increase most drastically, compared to magnesian clinohumite with X-Fe=0.06 , in the M2(6) octahedra. The polarized electronic absorption spectra are d ominated by two features: an intense, broad band centered at 22800 cm(-1) s trongly polarized E \ \X(n(a) < b ca. 10<degrees>), which was identified as Fe2+(M2(5))-Ti4+(M3) charge-transfer transition in the M2(5)-M3M3M2(5) oct ahedral chain frag- ments of the structure. The strong polarization of this band together with the position of the UV absorption edge, causes the pleo chroism of the mineral. The second prominent;feature in the polarized singl e crystal spectra is a complex band system in the NIR which is assigned to dd-transitions of Fe2+(Ml(c) + Ml(n)), bands at 10800 and 7600cm(-1), and o f Fe2+(M2(6)), bands at 9400 and 8800 cm(-1). Such transitions are in analo gy with the behaviour of Fe2+ in the M1 and M2 sites of the olivine structu re.