Minerals of the System Stichtite–Pyroaurite–Iowaite–Woodallite from Serpentinites of the Terekta Ridge (Gorny Altai, Russia)

Abstract

Abstract —Hydrotalcite supergroup minerals stichtite, pyroaurite, iowaite, and woodallite form a complex solid-solution system at the Kyzyl-Uyuk locality (Terekta Ridge, Gorny Altai, Russia). The diversity of these minerals is due to: (1) subdivision by anionic interlayer composition into carbonate (stichtite and pyroaurite) and chloride (iowaite and woodallite) species and (2) isomorphism of M3+ cations, mainly between Cr- (stichtite and woodallite) and Fe3+-dominant species (pyroaurite and iowaite), with a quantitative predominance of stichtite and iowaite. Most of the studied samples correspond to stichtite and woodallite with high Fe3+ contents or pyroaurite and iowaite with high Cr3+ contents. According to vibrational (IR and Raman) spectroscopy data, the interlayer Cl– is partially substituted by OH– rather than CO32- groups. We suppose that the presence/absence of a band in the region 1400–1350 cm–1 in the Raman spectra of stichtite can be explained by the local distortion of triangular CO3 groups. Stichtite and iowaite occur here in polytypic modifications 3R and 2H that are the most widespread for the hydrotalcite supergroup iowaite and stichtite is 2:1. For both minerals, the polytype 3R strongly dominates over 2H. The lowest 3R/2H ratio determined for the Terekta iowaite and stichtite is 2:1. The Altai stichtite is close in 3R/2H to the stichtite from Tasmania (Australia) and differs significantly from that in Transvaal samples (South Africa).