Abstract
Franklinfurnaceite, ideally Ca2Fe3+Mn32+Mn3+ [Zn2Si2O10](OH)8, has space group C2, with a = 5.483(7), b = 9.39(3), c = 14.51(1) Å, β = 97.04(8)°, and Z = 2. The structure has been determined and refined to an unweighted R factor of 3.5%. It is a derivative of the structure of chlorite via substitution of Ca in sites that are ordinarily unoccupied between 2:1 layers and octahedral interlayers. Franklinfurnaceite represents the first example of (1) a type IIa structure (specifically type IIa-1), (2) a tri-dioctahedral chlorite and (3) a structure with Ca occupying octahedral sites between tetrahedral sheets and octahedral interlayers. The structure is intermediate between those of the brittle micas and chlorite, in that it has both interlayer cations and an octahedral interlayer. Alternating Zn and Si are ordered in the tetrahedral sheets.
The octahedral sheet of the 2:1 layer is trioctahedral with ordered Mn3+ and divalent cations. The interlayer dioctahedral sheet has ordered divalent cations (principally Mn2+), Fe3+, and vacancies, and it has a net negative charge. Optimum local charge balance is maintained by sandwiching of interlayer octahedral vacancies between opposing Ca sites, Mn2+ between Si4+ atoms, and Fe3+ between two Zn2+ atoms. The large tetrahedral Zn permits a large tetrahedral rotation angle, α, of 23.5°, thereby allowing the anions to form a nearly ideal, three-dimensional closest-packing sequence … CABABC…. Consequently, an octahedrally coordinated site suitable for Ca is formed, and a IIa structure results. Ca repels the H protons so that there is no interlayer hydrogen-bonding system. These structural features imply that Ca substitution in common rock-forming chlorites is unlikely and may occur only if the structure type is IIa, the interlayer is dioctahedral, and if substitutions of substantial Al and/or Fe3+ occur in tetrahedral sheets and in the octahedral sheet of the 2:1 layer.
