Polytype Diversity of the Hydrotalcite-Like Minerals II. Determination of the Polytypes of Experimentally Studied Varieties

A. S. Bookin, V. I. Cherkashin and V. A. Drits
Geological Institute of the Russian Academy of Sciences 109017 Moscow, Phyzevsky, 7 Russia

Abstract: Polytype diversity of hydrotalcite-like minerals is mainly a function of the nature of the interlayer anion. Among the varieties with CO32− anions, only two- and three-layer polylypes having the same structure as manasseite and hydrotalcite have been confirmed. Stichtite and reevesite, which have been previously identified as six-layer polytypes, are in fact three-layer polytypes.

Among SO42− varieties, one-layer and three-layer polytypes have been identified, but the one-layer types are only present in more hydrated minerals with larger interlayer spacings. The three-layer varieties are of three different polytypes, with both P- and O-types of interlayers. Both rhombohedral and hexagonal varieties exist. Interlayer type may change during hydration-dehydration or anion exchange. Thus, in contrast with the CO32−-bearing minerals, a complete description of the polytype of the SO42−-bearing minerals cannot be made by simply indicating the number of the brucite-like layers in the unit cell.

The two-layer unit cell seen in refined crystal structures of some minerals with SO42− interlayers is not due to a doubled periodicity of alternation of brucite-like layers but to periodicity of interlayer anions, or layer cations.

Key Words: Hydrotalcite-like group • Polytype • X-ray diffraction criteria

Clays and Clay Minerals; October 1993 v. 41; no. 5; p. 558-564; DOI: 10.1346/CCMN.1993.0410505
© 1993, The Clay Minerals Society
Clay Minerals Society (www.clays.org)