Refinement of the Crystal Structure of A Monoclinic Ferroan Clinochlore

Abstract

Abstract--A monoclinic IIb-2 clinochlore from Washington, D.C., contrary to previous studies, is primarily a ferroan rather than a ferrian chlorite. Disorder of tetrahedral Si,A1 cations is indicated because of unsuccessful structural refinements in subgroup symmetries. The true space group is C2/m. Slight ordering of Mg, Fe 2+, and Fe 3+ over octahedra M(1) and M(2) within the 2:1 layer (mean M-O,OH = 2.092 and 2.084 A, respectively), complete ordering of trivalent A1 into the centrosymmetric octahedron M(4) of the interlayer sheet (M-OH = 1.929 ,~), and ordering of primarily divalent cations (Mg and Fe) into the two interlayer M(3) octahedra (M--OH = 2.117 ~) exist. The excess of negative charge above unity due to tetrahedral substitution of A1 for Si (1.378 atoms) is compensated entirely within the octahedral sheet of the 2:1 layer.Ordering of a trivalent cation into one octahedron in the interlayer should be universal for all stable trioctahedral chlorites. In this specimen the ordering is due to (1) minimization of cation-cation repulsion by layer offsets which provide more space around the trivalent element, and (2) energy minimization by localization of the source of positive charge on the interlayer sheet in one octahedron rather than two. In other structures or for different compositions additional factors can be important also. Most chlorites of the IIb and Ib (fl = 90 ~ types are expected to show disorder of the tetrahedral cations. The b positioning of interlayer and layer provides no preferential driving force for concentration of Si and A1 in any tetrahedron as a consequence of the expected ordering of the interlayer cations. The monoclinic IIb-2 polytype is less abundant in nature than the triclinic IIb-4 and IIb-6 structures, because only half as many possible superpositions of layers exist that will produce monoclinic symmetry. Crystallization factors must also be important, because the IIb-2 chlorite is much less abundant than predicted by this purely geometrical argument.

Structures of Intergrown Triclinic and Monoclinic IIb Chlorites from Kenya

“…Bond lengths in the ideal space groups indicated disorder of the tetrahedral Si,A1 cations (Table 3), in accord with the finding of Phillips et al (1980) and Rule and Bailey (1987) that little or no driving force exists in the IIb structure to favor localization of A1 in any of the tetrahedral sites. Major interest thus focused on ordering of octahedrally coordinated cations in subgroups C1 for IIb-4 and C2 or Cm for IIb-2.…”

“…the validity of the predictions of Rule and Bailey (1987) as to cation ordering in chlorite (see below). The Kenyan chlorite is anomalous also in having 1.7% Cr20 3 and a green color rather than the usual purple color of Cr-bearing chlorite.…”

“…They are not in accord with the higher A1TM value of 1.4 reported by Steinfink (1958b) for the same specimen. According to the results of Rule and Bailey (1987), a tetrahedral A1 content of 1.4 corresponds to a mean T-O bond length!of 1.668 ,h,.…”

“…The atomic coordinates of Phillips et al (1980) for the Day Book Body chlorite were used as a starting point for refinement of the triclinic IIb-4 polytype assunaing the ideal C1 space group and those of Rule and Bailey (1987) for the monoclinic IIb-2 polytype assuming the ideal C2/m space group. Scattering factors from Cromer and Mann (1968) corresponding to 50% ionization were used in the refinement.…”

“…The monoclinic IIb-2 polytype has been reported previously only from a waterworks tunnel excavation in Washington, D.C. (Steinfink, 1958a(Steinfink, , 1961Rule and Bailey, 1987) and from the Achmatov mine, Ural Mountains, U.S.S.R. (Joswig et al, 1989), although its apparent rarity may be in part related to the paucity of studies of chlorites by single crystal methods. We have also found this polytype in colorless crystals from Wood's mine, Lancaster County, Pennsylvania (University of Wisconsin Museum #6002/1).…”