Crystal Structure of Cronstedtite-2H₂

Abstract

The crystal structure of a magnesian cronstedtite-2H2 from P~l'bram, Czechoslovakia, was refined in space group C 1 to a residual of 5.4% with 1832 independent reflections. Tetrahedral ordering between Fe a+ and Si is judged to be complete on the basis of electron density maps, the first confirmation of such ordering in a layer silicate. Octahedral cations are disordered on the M sites. Mean T-O bond lengths do not confirm the tetrahedral ordering, perhaps due to tetrahedral distortion in order to relieve the extreme corrugation of the sheet that would arise from ordering of such different size cations.In the initial stages of refinement the structure of the crystal under study could not be described adequately on the basis of a single 2H2 polytype. The structure contains two kinds of 2Hz domains that appear shifted by b/3 relative to one another due to a mistake in the interlayer stacking sequence. This mistake of zero shift (the normal 2H~ polytype consists of alternating -b/3 and +b/3 shifts) affects only the tetrahedral sheets. It creates adjacent enantiomorphic domains of 2H z packets such that domains with small Si tetrahedra sit above larger FeZ+-rich tetrahedra and vice versa. This mechanism to relieve strain may indeed be the cause of the stacking mistakes. A third kind of domain contains regions in which the sense of tetrahedral rotation is reversed, giving rise to split basal oxygens on electron density maps. This multiple domain model best explains extra atomic positions observed on Fourier maps, lack of streaking of k ~ 3n reflections, and possible nonintegral "satellitic" reflections observed on Weissenberg films.

Refinement of the Crystal Structure of Cronstedtite-1T

“…Both values are close to those reported for the 3T and 2H2 polytypes (Star, ok et al, 1994;Hybler, 1997) and also for cronstedtite-2H2 with a considerable content of Mg and Mn (Geiger et al, 1983). The differences in average cation-to-anion distances are <0.01 ,~.…”

“…The size of the T1 tetrahedron is close to values reported for previously refined polytypes of cronstedtite (Geiger et al, 1983;Smr~ok et al, 1994;Hybler, 1997). The tetrahedron is relatively regular, although the apical bond length is longer than the basal bond lengths.…”

“…I3urovi~ also presented models for the diffraction patterns for the 1M and 3T polytypes (of the same subfamily A) coexisting in one crystal. A refinement of another polytype, 2H2 (subfamily D), in hexagonal symmetry unique from the triclinic cell of Geiger et al (1983) is in progress (Hybler, 1997;Hybler et aL, unpubl. data).…”

A HRTEM Study of Cronstedtite: Determination of Polytypes and Layer Polarity in Trioctahedral 1:1 Phyllosilicates

“…reported to be in the range of 0.5-0.8 in previous works (Geiger et al, 1983;Smr6ok et al, 1994;Hybler et aL, 2000). It was found in the present investigation that this mineral is resistant to beam damage during HRTEM examination.…”

Polytypism of cronstedtite from Chyňava, Czech Republic

“…The structure is composed of edge-sharing octahedral and adjacent corner-sharing tetrahedral sheets, forming together 1 : 1 layer (Bailey 1988 2+ and/or Mg 2+ has been reported in some occurrences, e.g. in Příbram (Steinmann 1821;Damour 1860;Janovský 1875;Geiger et al 1983). A structural analogue of cronstedtite with Mn and Mg contents up to 1.86 and 0.54 apfu, respectively, was approved as a new distinct mineral species guidottiite (Wahle et al 2010).…”