Crystal chemistry and defect structure of ekmanite: new data from transmission electron microscopy and MÖssbauer spectroscopy
Ferrow, Embaie A.; Wallenberg, L. Rhine; Holtstam, Dan
European Journal of Mineralogy Volume 11 Number 2 (1999), p. 299 - 308
published: Apr 19, 1999
manuscript accepted: Nov 22, 1998
manuscript received: Oct 3, 1997
ArtNo. ESP147051102014, Price: 29.00 €
Abstract Ekmanite is a member of the 2:1 type modulated layer silicates such as ganophyllite and bannisterite. Its chemical composition can be represented by K(M20)[Si32O76](OH)16 (M = Fe2+, Mg2+, Mn2+) where three of eight Si-tetrahedra are inverted across the interlayer sharing an apical oxygen with similarly inverted tetrahedra from the adjacent layers. The arrangement of the regular, D, and inverted, R, tetrahedra has the configuration 4D2R1D1R4D... along the b-axis. In local regions of possibly high Mg/(Mn+Fe) ratio, this configuration is interrupted by the absence of inverted tetrahedra. Ekmanite typically contains a multitude of defects; twinning and both regular and irregular stacking arrangements are the most common ones. Ekmanite is sensitive to oxidation and an unidentified phase, chemically related to it, is always found lining its grain boundaries. This texture results in intergrown rods defining polygons where the central parts do not contain ekmanite. The Mössbauer spectrum consists of Fe3+ and Fe3+ absorption doublets with the Fe3+ absorption composed of two Gaussian line pairs and the Fe3+ absorption of a single but broad line pair.